Section 1: Anatomy and Histology of Peripheral Nervous System and Neuraxis

INTRODUCTION

A thorough understanding of the underlying anatomy is fundamental to a logical approach to the techniques used in regional anesthesia. An appreciation for the embryologic development of tissues and structures can significantly add to the understanding of functional anatomy as it relates to regional anesthesia. In this chapter, I emphasize the embryologic development of the brain, spinal cord, peripheral and autonomic nervous systems, as well as the musculoskeletal system as it pertains to regional anesthesia. Many excellent comprehensive texts on embryology are available. For this chapter, I have relied heavily on information from primary texts and refer the reader to them for a complete discussion of all embryologic development. The first two, Langman’s Medical Embryology¹ and Basic Concepts in Embryology,² are valuable for their ease of readability, clarity of figures, and clinical correlations. Human Embryology and Developmental Biology³ is a good contemporary, comprehensive explanation of the molecular genetics of embryologic development. The Developing Human: Clinically Oriented Embryology⁴ is the time-tested standard text of embryology.

GENERAL EMBRYOLOGY

The prenatal period is divided into two major periods: the embryonic period, from fertilization through 2 months, and the fetal period, from the third month through birth.

Embryonic Period

The embryonic period is the time when all tissues are formed and, particularly during the second month, all organs are formed. The fetal period is a time of organ growth.² Embryologic development begins with fertilization, the process by which the male and female gametes unite to give rise to a zygote. Approximately 3 days after fertilization, cells of the compacted embryo divide to form a morula, which is composed of an inner and outer cell mass. The inner cell mass gives rise to the tissues of the embryoblast, and the outer cell mass forms the trophoblast, which later contributes to the placenta. After a period of cell division, during which time the morula enters the uterine cavity, the blastocoele forms, and the embryo is known as a blastocyst (Figure 2–1).

By the eighth day of development, the blastocyst is partially embedded in the endometrium. At this time, the trophoblast differentiates into an inner and an outer layer. Lacunae develop in the outer layer, maternal sinusoids are eroded, and by the end of the second week a primitive uteroplacental circulation begins to develop.

The inner cell mass, or embryoblast, differentiates into two layers, the epiblast and the hypoblast, which together form the bilaminar germ disk.

The most characteristic event occurring during the third week of gestation is gastrulation. This is the process that establishes all three germ layers: endoderm, ectoderm, and mesoderm in the embryo (Figure 2–2). Gastrulation begins with the formation of the primitive streak on the surface of the epiblast portion of the bilaminar germ disk. Cells migrate toward the primitive streak, detach from the epiblast, and slip beneath it. This inward movement is known as invagination. Once the cells have invaginated, some displace the hypoblast, creating the new embryonic endoderm. Other cells come to lie between the epiblast and the newly created endoderm to form the mesoderm. Cells remaining in the epiblast then form the ectoderm. Through the process of gastrulation, the epiblast therefore becomes the source of all the germ layers in the embryo.¹ Developments during the first 3 weeks of the embryonic period produce an embryo with one germ layer (week 1), two germ layers (week 2), and three germ layers with a recognizable three-dimensional body form (week 3)² (Figure 2–3).

In general terms, the ectoderm germ layer gives rise to organs and structures that allow us to maintain contact with the outside world.⁴ It gives rise to the central and peripheral nervous systems; the sensory epithelium of the eye, ear, and nose; the epidermis and its appendages; hair and nails, mammary glands; the hypophysis, subcutaneous glands; and the enamel of the teeth.

The mesoderm gives rise to supporting structures of the body, such as cartilage, bone, and connective tissue; striated and smooth muscle; the heart, blood, lymph vessels, and cells; and the kidneys, gonads, and serous membranes lining the body cavities, spleen, and the cortex of the adrenal gland.

The endoderm produces the epithelial lining of the gastrointestinal and respiratory tracts, as well as the epithelial lining of the bladder and urethra, tympanic cavity, antrum, and auditory tube. It also engenders the parenchyma of the tonsils, thyroid, parathyroid thymus, liver, and pancreas.

As the embryo forms, it rapidly develops along several axes,² the first of which is the craniocaudal axis. It is established while the embryo is still a flat disk or sheet of cells. This axis runs from the future head to the future tail of the body form. The dorsoventral axis is the next to be established. This occurs as the body folds and defines the future front and back sides of the body form.

Establishment of the body axes takes place prior to and during the period of gastrulation. Cells at the posterior margin of the embryonic disk signal the craniocaudal axis. The dorsoventral orientation of tissue is controlled by a complex interaction of proteins and growth factors.

This early orientation of cells in the body is a result of the expression of Hox genes. There are four Hox gene complexes in vertebrates: Hox a, b, c, and d. Each consists of a group of between 9 and 11 genes arranged sequentially along a particular chromosome. A cascade of genes producing signaling factors also orchestrates left-right asymmetry, which is established early in development. As a result of complex interactions, for example, the heart and spleen lie on the left side of the body and the main lobe of the liver lies on the right.

Regions of the epiblast that migrate and ingress through the primitive streak have been mapped and their ultimate fates determined. Mesoderm cells that ingress through the cranial region of the primitive node become the notochord; those migrating at the lateral edges of the primitive node and from the cranial end of the primitive streak become the paraxial mesoderm. Cells migrating through the midstreak region become the intermediate mesoderm, and those migrating through the caudal part of the streak form the lateral plate mesoderm. This orientation of the mesoderm is important in understanding limb development.

The embryonic disk, initially flat and almost round, gradually becomes elongated, with a broad cephalic and a narrow caudal end. Expansion of the embryonic disk occurs mainly in the cephalic region. This growth in elongation is caused by continuous migration of cells from the primitive streak region in a cephalad direction. Invagination and migration forward and laterally of surface cells in the primitive streak continue until the end of the fourth week. Germ cell layers in the cephalic region begin their specific differentiation by the middle of the third week, whereas those in the caudal part differentiate beginning by the end of the fourth week. This causes the embryo to develop in a cephalocaudal direction (Figure 2–4).

At the beginning of the third week of development, the ectodermal germ layer has the shape of the disk. The ectoderm gives rise to two subdivisions: neuroectoderm, which forms all neural tissue, and the epidermal covering of the body.² Appearance of the notochord and prechordal mesoderm induces the overlying ectoderm to thicken and form the neural plate (Figure 2–5). Cells of the neural plate make up the neuroectoderm, and their induction represents the initial event in the process of neurulation. By the end of the third week, the lateral edges of the neural plate become elevated to form neural folds, and the depressed midregion forms the neural groove. Gradually, the neural folds approach each other in the midline, where they fuse, resulting in formation of the neural tube. Neurulation is then complete, and the central nervous system (CNS) is represented by a closed tubular structure with a narrow caudal portion, the spinal cord, and a much broader cephalic portion characterized by a number of dilations, the brain vesicles. As the neural folds elevate and fuse, cells at the lateral border begin to dissociate from their neighbors. This cell population, called the neural crest, will undergo a transition as they leave the neuroectoderm to enter the underlying mesoderm.

During the body-folding process, the endoderm is formed into an epithelial tube, which runs the length of the body. The derivatives of the endoderm tube are all epithelial tissues (Figure 2–6).

Initially, cells of the mesoderm germ layer form a thin sheet of tissue on each side of the midline.¹ These cells proliferate and form a thick end plate of tissue known as paraxial mesoderm (Figure 2–7). More laterally, the mesoderm remains thin and is known as the lateral plate. This lateral plate divides into two layers: a somatic, or parietal, mesoderm layer and a splanchnic, or visceral, mesoderm layer. Together, these layers form the intraembryonic cavity. Intermediate mesoderm connects the paraxial and lateral plate mesoderm.

By the beginning of the third week, paraxial mesoderm is organized into segments. These segments are known as somitomeres. They first appear in the cephalic region of the embryo, and their formation proceeds in a craniocaudal direction. In the head region, somitomeres transition, in association with segmentation of the neural plate, into neuromeres. From the occipital region caudally, somitomeres further organize into somites. Somites give rise to the myotome (ultimately muscle tissue), sclerotome (ultimately cartilage and bone), and dermatome (ultimately subcutaneous tissue of the skin). Collectively, these are all supporting tissues of the body (Figure 2–8).

Signaling for this somite differentiation arises from surrounding structures, including the notochord, neural tube, epidermis, and lateral plate mesoderm. By the beginning of the fourth week, cells forming the ventral and medial walls of the somite lose their compact organization and shift their position to surround the notochord (the dense cord of mesoderm that induces neuroectoderm). These cells, collectively known as the sclerotome, form a loosely woven tissue called the mesenchyme. They will surround the spinal cord and notochord to form the vertebral column. Cells at the dorsolateral portion of the somite also migrate as precursors of limb and body wall structures. Following migration of these muscle cells and cells of the sclerotome, cells at the dorsomedial portion of the somite proliferate and migrate down the ventral side of the remaining dorsal epithelium of the somite to form a new layer, the myotome. The remaining dorsal epithelium forms the dermatome, and together these layers constitute the dermomyotome. Each segmentally arranged myotome contributes to muscles of the back, whereas the dermatomes disperse to form the dermis and subcutaneous tissue of the skin. Each myotome and dermatome retains its innervation from the segment of origin, no matter where the cells migrate. Therefore, each somite forms its own sclerotome, the cartilage and bone component; its own myotome, providing the segmental muscle component; and its own dermatome, the segmental skin component. Each myotome and dermatome also has its owns segmental nerve component (Figure 2–9).

During the second month, the external appearance of the embryo is changed greatly by the enormous size of the head and formation of the limbs, face, ears, nose, and eyes.¹ By the beginning of the fifth week, forelimbs and hind limbs appear as paddle-shaped buds (Figure 2–10). The forelimbs are located dorsal to the pericardial swelling at the level of the fourth cervical to first thoracic somites, which explains their innervation by the brachial plexus. Hind-limb buds appear slightly later just caudal to the attachment of the umbilical stock at the level of the lumbar and uppers sacral somites. With further growth, the terminal portions of the buds flatten, and a circular constriction separates them from the proximal, more cylindrical segment. Soon, four radial grooves separating five slightly thicker areas appear on the distal portion of the buds. This development foreshadows formation of the digits. These grooves, known as rays, appear in the hand region first and, shortly afterward, in the foot because the upper limb is slightly more advanced in development than the lower limb. While fingers and toes are being formed, a second constriction divides the proximal portion of the buds into two segments, and the three parts characteristic of the adult extremities can be recognized.

Fetal Period

The period from the beginning of the ninth week to the end of the intrauterine life is known as the fetal period.¹ It is characterized by maturation of tissues and organs and rapid growth of the body in length. This is particularly striking during the third, fourth, and fifth months, and increasing weight is most striking during the last 2 months of gestation. During the third month, the face becomes more human looking, and the limbs reach their relative length in comparison with the rest of the body, although the lower limbs are still a little shorter and less well developed than the upper extremities. Primary ossification centers are present in the long bones and the skull by the 12th week.

Skeletal System: Limb Growth and Development

The skeletal system develops from paraxial and lateral plate mesoderm as well as neural crest tissue.¹ The somites (as previously described) differentiate into a ventromedial component called the sclerotome and a dorsolateral component called the dermomyotome. This organization of cells forms a loosely woven tissue called the mesenchyme. The mesenchyme migrates and differentiates into fibroblasts, chondroblasts, and osteoblasts.

At the end of the fourth week of development, limb buds become visible as outpocketings of the ventrolateral body wall.¹ Initially, they consist of a mesenchymal core derived from the somatic layer of lateral plate mesoderm that will form the bones and connective tissue of the limb, covered by a layer of ectoderm. Ectoderm at the distal border of the limb thickens and forms a specialized inducing tissue known as the apical ectodermal ridge (AER). The AER exerts an inductive influence on the adjacent mesenchyme, causing it to remain as a population of undifferentiated, rapidly proliferating cells called the progress zone. As the limb grows, cells farther from the influence of the AER begin to differentiate into cartilage and muscle. In this manner of development, the limb proceeds proximodistally. In 6-week-old embryos, the terminal portion of the limb buds becomes flattened to form hand and footplates and is separated from the proximal segment by a circular constriction. Later, a second constriction divides the proximal portions into two segments, and the main parts of the extremities can be recognized.¹ Fingers and toes are formed when programmed cell death in the AER separates this ridge into five parts. Further formation of the digits depends on their continued outgrowth under the influence of the five remaining segments of ridge ectoderm. This results in condensation of the mesenchyme to form cartilaginous digital rays. Development of the upper and lower limbs is similar except that morphogenesis of the lower limb is approximately 1–2 days behind that of the upper limb.

During the seventh week of gestation, a key event occurs that is critical in understanding the final orientation and innervation of the limbs. The limbs rotate in opposite directions. The upper limb rotates 90° laterally so that the extensor muscles lie on the lateral and posterior surface and the thumbs lie laterally. The lower limb rotates approximately 90° medially, placing the extensor muscles on the anterior surface and the great toe medially.¹ This explains why homologous joints of the upper and lower extremities (knees and elbows) point in opposite directions. This limb rotation results in the following²:

1. The final orientation of the limbs.

2. The final location and orientation of muscle groups (because the muscles are connected to the limb bones prior to rotation).

3. The patterns of sensory innervation of the skin (also because nerve fibers are connected with the dermis layer of the skin prior to rotation and are pulled along).

While the external shape is being established, mesenchyme in the buds begins to condense, and by the sixth week of development, the first hyaline cartilage models can be recognized. Ossification of the bones of the extremities begins by the end of the embryonic period. Primary ossification centers are present in all long bones of the limbs by the 12th week of development.

Molecular Regulation of Limb Development

Positioning of the limbs along the craniocaudal axis in the flank regions of the embryo is regulated by the Hox genes expressed along this axis.¹ Once positioning along this axis is determined, growth must also be regulated along the proximodistal, anteroposterior, and dorsoventral axes. Patterning of the anteroposterior axis of the limb is regulated by the zone of polarizing activity (ZPA), a cluster of cells at the posterior border of the limb near the flank. These cells produce retinoic acid, which initiates expression of sonic hedgehog (Shh), a secreted factor that regulates development along this axis.¹ This regulation results in digits appearing in the proper order, with the thumb on the radial (anterior) side. As the limb grows, the ZPA moves distally to remain in proximity to the posterior border of the AER. The dorsoventral axis is patterned in a similar fashion by the dorsal ectoderm of the limb.

Although patterning genes for the limb axes have been predetermined, it is the Hox genes that regulate the types and shapes of the bones of the limbs. These Hox genes are nested in overlapping patterns of expression that somehow regulate patterning.¹ As a result, variations in their combinations create patterns of expression that may account for differences in fore- and hind-limb structures.

Vertebral Column

During the fourth week of development, cells of the sclerotome shift their position to surround both the spinal cord and the notochord.¹ During further development, the caudal portion of each sclerotome segment proliferates extensively and condenses. This proliferation is so extensive that it proceeds into the subjacent intersegmental tissue and binds the caudal half of one sclerotome to the cephalic half of the adjacent sclerotome. By incorporation of this intersegmental tissue into the precartilaginous vertebral body, the body of the vertebrae becomes intersegmental (Figure 2–11). Hox genes also control this patterning. Mesenchymal cells between cephalic and caudal parts of the original sclerotome form the intervertebral disk. Although the notochord regresses entirely in the region of the vertebral bodies, it persists and enlarges in the region of the intervertebral disk. Here, it contributes to the nucleus pulposus, which is later surrounded by circular fibers of the anulus fibrosus. Together, these structures form the intervertebral disk.¹

Muscular and Peripheral Nervous Systems

With the exception of some smooth muscle tissue, the muscular system develops from the mesoderm germ layer.¹ Skeletal muscle is derived from paraxial mesoderm, which forms somites from the occipital to sacral regions and somitomeres in the head. The somites and somitomeres form the musculature of the axial skeleton body wall, limbs, and head. From the occipital region caudally, somites form and differentiate into this sclerotome, dermatome, and two muscle-forming regions. One of these is in the dorsolateral region of the somite and provides progenitor cells (myoblasts) for the limb and body wall musculature. The other region lies dorsomedially, migrates ventrally to cells that form the dermatome, and forms the myotome. Patterns of muscle formation are under the influence of the surrounding connective tissue into which myoblasts migrate. In the head region, this connective tissue is derived from neural crest cells. In cervical and occipital regions, muscles differentiate from somitic mesoderm, whereas in the body wall and limbs, they originate from the somatic mesoderm. By the end of the fifth week, prospective muscle cells are collected into two parts: a small dorsal portion, the epimere, and a larger ventral part called the hypomere.¹ Nerves innervating segmental muscles are also divided into a dorsal primary ramus for the epimere and a ventral primary ramus for the hypomere. These nerves remain with their original muscle segment throughout its migration. Myoblasts of the epimere form extensor muscles of the vertebral column, and those of the hypomere give rise to the muscles of the limbs and body wall. The first indication of limb musculature is observed in the seventh week of development as a condensation of mesenchyme near the base of the limb buds. This mesenchyme is derived from dorsolateral cells of the somites that migrate into the limb bud to form the muscles. This connective tissue dictates the pattern of muscle formation and is derived from somatic mesoderm, which also gives rise to the bones of the limb.¹

With elongation of the limb buds, the muscle tissue splits into flexor and extensor components. The upper limb bud lies opposite the lower five cervical and upper two thoracic segments. The lower limb buds lie opposite the lower four lumbar and upper two sacral segments. As soon as the buds form, ventral primary rami from the appropriate spinal nerves penetrate into the mesenchyme (Figure 2–12). At first, each ventral ramus enters with isolated dorsal and ventral branches, but soon these branches unite to form large dorsal and ventral nerves. Thus, in the upper extremity, the radial nerve supplies all the extensor musculature and is formed by a combination of dorsal segmental branches. The ulnar and median nerves, which supply all the flexor muscles, are formed by a combination of the ventral branches¹ (Figure 2–13). Immediately after the nerves have entered the limb buds, they establish contact with the differentiating mesoderm condensations. This early contact is a prerequisite for their complete functional differentiation. Spinal nerves therefore play an important role in differentiation and motor innervation of the limb musculature, as well as providing sensory innervation for the dermatomes. Although the original dermatomal pattern changes with growth of the extremities, an orderly sequence can still be recognized in the adult.

Central Nervous System

The CNS originates in the ectoderm and appears as the neural plate at the middle of the third week¹ (Figure 2–14). After the edges of the plate fold, the neural folds approach each other in the midline to fuse, forming the neural tube. The CNS then forms as a tubular structure with a broad cephalic portion (the brain) and a long caudal portion (the spinal cord). A basal plate, containing the motor neurons, and an alar plate, containing the sensory neurons, characterize the spinal cord, which forms the caudal end of the CNS.

The walls of the recently closed neural tube consist of neuroepithelial cells. These cells give rise to another type of cell, the neuroblasts, which are primitive nerve cells. They form a mantle layer around the neuroepithelial layer. This mantle layer later forms the gray matter of the spinal cord. The outermost layer of the spinal cord, called the marginal layer, contains nerve fibers emerging from neuroblasts in the mantle layer. As a result of myelination, this layer takes on a white appearance and is called the white matter of the spinal cord.¹ Because of the continuous addition of neuroblasts to the mantle layer, each side of the neural tube shows ventral and dorsal thickening. The ventral thickening (the basal plates) contains ventral motor horn cells. The dorsal thickening (the alar plates) forms the sensory areas. A group of neurons accumulate between these two areas, forming a small intermediate horn, which contains neurons of the sympathetic portion of the autonomic nervous system and is present only at thoracic and upper lumbar levels of the spinal cord¹ (Figure 2–15).

Spinal Nerves

Motor nerve fibers begin to appear in the fourth week, arising from nerve cells in the basal plates of the spinal cord.⁴ They collect into bundles known as ventral nerve roots. Likewise, dorsal nerve roots form as collections of fibers originating from cells in dorsal root ganglia. Central processes from these ganglia form processes that grow into the spinal cord opposite the dorsal horns, and distal processes join the ventral nerve roots to form a spinal nerve. Spinal nerves divide into dorsal and ventral primary rami. Dorsal rami innervate dorsal axial musculature, vertebral joints, and the skin of the back. Ventral primary rami innervate the limbs and ventral body wall and form the major nerve plexuses.¹

In the third month of development, the spinal cord extends the entire length of the embryo, and spinal nerves pass through the intervertebral foramina at their level of origin. With increasing age, the vertebral column and dura lengthen more rapidly than the neural tube, resulting in the terminal end of the spinal cord gradually shifting to a higher level. At birth, this level is at the third lumbar vertebra. In the adult, the spinal cord terminates at the level of L2 to L3, whereas the dural sac and subarachnoid space extend to S2. As a result of this disproportionate growth, spinal nerves run obliquely from their segment of origin in the spinal cord to the corresponding level of the vertebral column. Below L2 to L3, a thread-like extension of the pia mater forms the filum terminale, which is attached to the periosteum of the first coccygeal vertebra and marks the tract of regression of the spinal cord.¹

The Brain

The brain consists originally of three vesicles: the rhombencephalon (hindbrain), mesencephalon (midbrain), and prosencephalon (forebrain). The rhombencephalon will ultimately form the medulla oblongata, pons, and cerebellum. The mesencephalon resembles the spinal cord with its basal and alar plates. It will contain the anterior and posterior colliculi, forming the relay stations for visual and auditory reflex centers. The prosencephalon will ultimately give rise to the thalamus and hypothalamus as well as the cerebral hemispheres.¹

Autonomic Nervous System

Functionally, the autonomic nervous system can be divided into two parts: a sympathetic portion in the thoracolumbar region and a parasympathetic portion in the cephalic and sacral regions. Both components of the autonomic nervous system consist of two tiers of neurons: pre- and postganglionic (Figure 2–16).

Sympathetic Nervous System

Preganglionic neurons of the sympathetic nervous system arise from the intermediate horn of the gray matter in the spinal cord.³ At levels from T1 to L2, their myelinated axons grow from the cord through the ventral roots, paralleling the motor axons that supply the skeletal musculature. Shortly after the dorsal and ventral roots of the spinal nerve join, the preganglionic sympathetic axons, derived from the neuroepithelium of the neural tube, leave the spinal nerve via a white communicating ramus. They soon enter one of a series of sympathetic ganglia to synapse with neural crest-derived postganglionic neurons. The sympathetic ganglia, the bulk of which are organized as two chains running ventrolateral to the vertebral bodies, are laid down by neural crest cells that migrate from the closing neural tube along a special pathway. Once the migrating sympathetic neuroblasts have reached the site at which the sympathetic chain ganglia form, they spread both cranially and caudally until the extent of the chains approximates that seen in the adult. Some of the sympathetic neuroblasts migrate farther ventrally than the level of the chain ganglion to form a variety of other collateral ganglia. The adrenal medulla can be broadly viewed as a highly modified sympathetic ganglion. The outgrowing preganglionic sympathetic neurons either terminate within the chain ganglia or pass through on their way to more distant sympathetic ganglia to form synapses with the cell bodies of the second-order, or postganglionic, sympathetic neuroblasts. Axons of some postganglionic neuroblasts, which are unmyelinated, leave the chain ganglion as a parallel group and reenter the nearest spinal nerve through the gray communicating ramus. Once in the spinal nerve, these axons continue to grow until they reach their peripheral targets.

During the fifth week of development, cells originating in the neural crest of the thoracic region migrate on each side of the spinal cord toward the region immediately behind the dorsal aorta.¹ Here, they form a bilateral chain of segmentally arranged sympathetic ganglia interconnected by longitudinal nerve fibers. Together, they form the sympathetic chains on each side of the vertebral column. From their position in the thorax, neuroblasts migrate toward the cervical and lumbosacral regions, extending the sympathetic chains to their full length. Some sympathetic neuroblasts migrate in front of the aorta to form preaortic ganglia, such as the celiac and mesenteric ganglia. Other sympathetic cells migrate to the heart, lungs, and gastrointestinal tract, where they give rise to sympathetic organ plexuses. Once the sympathetic chains have been established, nerve fibers originating in the intermediate horn of the thoracolumbar segments of the spinal cord penetrate the ganglia of the chain. They are known as preganglionic fibers, have a myelin sheath, and stimulate sympathetic ganglion cells. Passing from spinal nerves to the sympathetic ganglia, they form the white communicating rami. Axons of the sympathetic ganglion cells, the postganglionic fibers, have no myelin sheath. They either pass to other levels of the sympathetic chain or extend to the heart, lungs, and intestinal tract. Other fibers, the gray communicating rami, which are found at all levels of the spinal cord, pass from the sympathetic chain to spinal nerves and from there to peripheral blood vessels, hair, and sweat glands.

Parasympathetic Nervous System

Neurons in the brainstem and the sacral region of the spinal cord give rise to preganglionic parasympathetic fibers.¹ Although also organized on a preganglionic and postganglionic basis, the parasympathetic nervous system has a distribution quite different from that of the sympathetic system. Like those of the sympathetic nervous system, preganglionic parasympathetic neurons originate in the intermediate column of the CNS. However, the levels of origin of these neuroblasts are the mid- and hindbrain and in the second to fourth sacral segments of the developing spinal cord. Axons from these preganglionic neuroblasts grow long distances before they meet the neural crest–derived postganglionic neurons. These are typically embedded in small, scattered ganglia or plexuses in the walls of the organs they innervate.

CLINICAL RELEVANCE

Functional Analysis of the Brachial Plexus

The separation of the trunks of the brachial plexus into anterior and posterior divisions places the nerves into relation with muscles formed from primitive anterior and posterior mesoderm masses during the development of the limbs. Once these relationships are established, they are never reversed, and the same fundamental correlation exists in the adult.^5,6 The anterior divisions of the trunks unite to form the lateral and medial chords of the brachial plexus, and the posterior divisions join to form the single posterior cord. Thus, all branches of the lateral and medial chords carry nerve bundles derived from the anterior divisions of the trunks, and the branches of the posterior cord conduct exclusively fibers from the posterior divisions. The discrete compartments of the upper extremity contain muscle groups of similar and related functions as well as the blood vessels and nerves that supply them. This concept is emphasized in the word preaxial to designate the component and structures anterior to the bone and fascial plane or axis of the limb and the word postaxial to designate structures behind the bony and fascial axis. With the limb in the anatomical position, those parts anterior to the bony axis are all in a continuous plane down the front of the limb, with the ventral axial line extending along the anterior surface of the arm and forearm. The postaxial parts are positioned continuously down the back of the limb.

The branches of the lateral and medial chords are all preaxial and innervate the preaxial muscles of the limb, whereas the posterior cord branches are postaxial and innervate the postaxial musculature. The radial nerve, being the only postaxial nerve below the shoulder, supplies all the postaxial muscles in the remainder of the limb. The median, musculocutaneous, and ulnar nerves share preaxial innervation.

The musculocutaneous nerve is muscular in the arm and cutaneous in the forearm, and it is the sole preaxial muscular nerve in the arm. The preaxial median and ulnar nerves are nerves of passage in the arm, but in the forearm and hand, each contributes to innervation, the median nerve more heavily in the forearm, the ulnar nerve more heavily in the hand.

In the shoulder region, many of the supra- and infraclavicular branches of the brachial plexus arise from recognizable pre- and postaxial cords or divisions. Their origins have the same significance as the major anterior and posterior divisions of the trunks. An example of this is the clavicle and the scapula. The clavicle is an anterior and the scapula a posterior bone. An exception to this designation involves the scapula because its coracoid process has a phylogenetic history as a separate bone and fuses with the scapula. Therefore, all muscles arising from the scapula, exclusive of the coracoid process (pectoralis minor, coracobrachialis, and short head of biceps), belong to a postaxial group at the shoulder, and those from the clavicle and coracoid belong to a preaxial group. The nerve-muscle correlation is maintained in that all muscles of scapular origin are supplied by postaxial branches of the brachial plexus, and all muscles derived from the clavicle and coracoid are supplied by preaxial branches.

Functional Analysis of the Lumbosacral Plexus

The innervation of the lower extremity follows the same pattern of pre- and postaxial orientation as described for the upper extremity. The primordial dermatomal pattern has disappeared, but an orderly sequence of dermatomes can still be recognized. Most of the original ventral surface of the lower limb lies on the back of the adult limb. The ventral axial line extends along the medial side of the thigh and knee to the posteromedial aspect of the leg to the heel.

As previously described, this results from the medial rotation of the lower limb at the end of the embryonic period. The lumbar plexuses are formed by the ventral rami of the first three lumbar nerves and part of the fourth lumbar nerve. The primary preaxial components are the genitofemoral nerve, which derives from L1 and L2, and obturator and accessory obturator nerves, which are derived from L2 and L3, respectively. The primary postaxial components are the lateral femoral cutaneous nerve, derived from L2 and L3, and the femoral nerve, derived from L2 through L4. The sacral plexus combines the ventral rami of part of L4 and L5 and S1 through S3, as well as part of S4. All these nerves, except S4, divide into anterior and posterior branches. Like the brachial plexus, the anterior branches form preaxial nerves related to preaxial muscle masses in skin areas, and the posterior branches form similarly related postaxial nerves. The principal nerve of the sacral plexus is the sciatic, which is composed of a preaxial nerve (the tibial nerve) and a postaxial nerve (the common peroneal), which is enclosed within a single sheath.

SUMMARY

In summary, regional anesthesia may be considered a practice of applied anatomy. Successful neuraxial and peripheral nerve blockade alike require a logical approach to the anatomic principles. Knowledge of the embryologic development of tissues and neural structures can significantly add to the understanding of functional anatomy as it relates to regional anesthesia.

REFERENCES

INTRODUCTION

The practice of regional anesthesia is inconceivable without sound knowledge of the functional regional anesthesia anatomy. Just as surgical technique relies on surgical anatomy or pathology leans on pathologic anatomy, the anatomic information necessary for the practice of regional anesthesia must be specific to this application. In the past, many nerve block techniques and approaches were devised by academicians merely relying on idealized anatomic diagrams and schematics, rather than on functional anatomy. However, once the anatomic layers and tissue sheets are dissected, the anatomy of nerve structures without the tissue sheaths around them is of little relevance to the clinical practice of regional anesthesia. This is because accurate placement of the needle and the spread of the local anesthetic after an injection depend on the interplay between neurologic structures and the neighboring tissues where local anesthetic pools and accumulates, rather than on the mere anatomic organization of the nerves and plexuses. Much research on functional regional anesthesia, a term introduced by Dr. Jerry Vloka in the 1990s, has contributed to better understanding of the anatomy of regional nerve blockade. Moreover, the introduction of ultrasound in the practice of regional anesthesia has further clarified the relationship of the needle and the nerve and the dynamics of local anesthetic spread.

The goal of this chapter is to provide a generalized and rather concise overview of anatomy relevant to the practice of regional anesthesia; more specific anatomic discussions pertaining to individual regional anesthesia techniques are detailed in their respective chapters. The reader is referred to Figure 3–1 for an easier orientation of the body planes discussed throughout the book.

ANATOMY OF PERIPHERAL NERVES

All peripheral nerves are similar in structure. The neuron is the basic functional neuronal unit responsible for the conduction of nerve impulses. Neurons are the longest cells in the body, many reaching a meter in length. Most neurons are incapable of dividing under normal circumstances and have limited ability to repair themselves after injury. A typical neuron consists of a cell body (soma) that contains a large nucleus. The cell body is attached to several branching processes, called dendrites, and a single axon. Dendrites receive incoming messages; axons conduct outgoing messages. Axons vary in length, and there is one only per neuron. In peripheral nerves, axons are long and slender. They are also called nerve fibers. The peripheral nerve is composed of three parts: (1) somatosensory or afferent neurons, (2) motor or efferent neurons, and (3) autonomic neurons.

Individual nerve fibers bind together, somewhat like individual wires in an electric cable (Figure 3–2). In the peripheral nerve, individual axons are eveloped by the endoneurium, which is a delicate layer of loose connective tissue around each axon. Groups of axons are closely associated within a bundle called a nerve fascicle that is surrounded by the perineurium, which imparts mechanical strength to the peripheral nerve. In surgical procedures, the perineurium holds sutures without tearing. In addition to its mechanical strength, the perineurium functions as a diffusion barrier to the fascicle, isolating the endoneural space around the axon from the surrounding tissue.¹ This barrier helps to preserve the ionic milieu of the axon and functions as a blood–nerve barrier. The perineurium surrounds each fasciculus and splits with it at each branching point. The fascicular bundles in turn are embedded in loose connective tissue called the interfascicular epineurium, which contains adipose tissue, fibroblasts, mastocytes, blood vessels (with small nerve fibers innervating these vessels), and lymphatics. In contrast, a more dense collagenous tissue forms the epineurium that surrounds the entire nerve and holds it loosely to the connective tissue through which it travels.

Of note, the fascicular bundles are not continuous throughout the peripheral nerve. They divide and anastomose with one another as frequently as every few millimeters.¹ However, the axons within a small set of adjacent bundles redistribute themselves so that the axons remain in approximately the same quadrant of the nerve for several centimeters. This arrangement is a practical concern to the surgeons trying to repair a severed nerve. If the cut is clean, it may be possible to suture individual fascicular bundles together. In such a scenario, there is a greater chance that the distal segment of nerves synapsing with the muscles will be sutured to the central stump of motor or sensory axons. In such cases, good functional recovery is more likely. If a short segment of the nerve is missing, however, the fascicles in the various quadrants of the stump may no longer correspond with one another, good axial alignment may not be possible, and functional recovery is greatly compromised or improbable.¹ This arrangement of the peripheral nerve helps explain why intraneural injections may result in disastrous consequences.

The connective tissue of a nerve is tougher, compared to the nerve fibers themselves, and allows a certain amount of “stretch” without damage to the nerve fibers. For instance, the axons are somewhat “wavy,” and when stretched, the connective tissue around them is also stretched—giving it some protection. This feature plays a “safety” role in nerve blockade by allowing the nerves to be “pushed” rather than pierced by the advancing needle, as often seen on ultrasound. For this reason, it is prudent to avoid stretching the nerves and nerve plexuses during nerve blockade (eg, in axillary brachial plexus blocks and some approaches to the sciatic block).

The paraneurium consists of loose connective tissue that holds a stable relationship between adjacent structures filling the space in between them, such as the neurovascular bundles of intermuscular septae. This tissue contributes to functional mobility of nerves during joint and muscular movement.

Nerves receive blood from the adjacent blood vessels running along their course. These feeding branches to larger nerves are macroscopic in size and irregularly arranged, forming anastomoses to become longitudinally running vessel(s) that supply the nerve and give off subsidiary branches. Although the connective tissue sheath enveloping nerves serves to protect the nerves from stretching, it is also believed that neuronal injury after nerve blockade may be due, at least partly, to the pressure or stretch within connective sheaths that do not stretch well and the consequent interference with the vascular supply to the nerve.

Communication Between the Central and Peripheral Nervous Systems

The functional boundary between the central nervous system (CNS) and the peripheral nervous system (PNS) lies at the junction where oligodendrocytes meet Schwann cells along the axons that form the cranial and spinal nerve. The CNS communicates with the body through spinal nerves. Spinal nerves have both sensory and motor components (Figure 3–3). The sensory fibers arise from neurons in the dorsal root ganglia. Fibers enter the dorsolateral aspect of the spinal cord to form the dorsal root. The motor fibers arise from neurons in the ventral horn of the spinal cord. The fibers pass through the ventrolateral aspect of the spinal cord and form the ventral root. The dorsal and ventral roots converge in the intervertebral foramen to form a spinal nerve. After passing through the intervertebral foramen, the spinal nerve divides into dorsal and ventral rami. The dorsal ramus innervates muscle, bones, joints, and the skin of the back. The ventral ramus innervates muscle, bones, joints, and the skin of the anterior neck, thorax, abdomen, pelvis, and the extremities.

Spinal Nerves

There are 31 pairs of spinal nerves (Figure 3-4). The spinal nerves are enumerated by region: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Spinal nerves pass through the vertebral column at the intervertebral foramina. The first cervical nerve (C1) passes superior to the C1 vertebra (atlas). The second cervical nerve (C2) passes between the C1 (atlas) and C2 (axis) vertebrae. This pattern continues down the cervical spine. A shift in pattern occurs at the C8 nerve because there is no C8 vertebra. The C8 nerve passes between the C7 and T1 vertebrae. The T1 nerve passes between the T1 and T2 vertebrae. This pattern continues down the remainder of the spine. The vertebral arch of the fifth sacral and first coccygeal vertebrae is rudimentary. Because of this, the vertebral canal opens inferiorly at the sacral hiatus. The fifth sacral and first coccygeal nerves pass through the sacral hiatus. Because the inferior end of the spinal cord (conus medullaris) in adults is located at the L1 to L2 vertebral level, roots of spinal nerves must descend through the vertebral canal before exiting the vertebral column through the appropriate intervertebral foramen. Collectively, these roots are called the cauda equina.

Outside the vertebral column, ventral rami from different spinal levels coalesce to form intricate networks called plexuses. From the plexuses, nerves extend into the neck, the arms, and the legs.^1,2

DERMATOMES, MYOTOMES, AND OSTEOTOMES

Dermatomal, myotomal, and osteotomal innervations are often emphasized in regional anesthesiology texts as important for the application of nerve blocks. In clinical practice of regional anesthesia, however, it is more practical to think in terms of which block techniques provide adequate analgesia and anesthesia for specific surgical procedures, rather than attempt to match nerves and spinal segments to the relevant dermatomal, myotomal, and osteotomal territory. Nevertheless, the description of dermatomal, myotomal, and osteotomal innervation is of didactic importance in regional anesthesia and is briefly presented here.

A dermatome is an area of the skin supplied by the dorsal (sensory) root of the spinal nerve (Figures 3–5a, 3–5b, and 3–6). In the trunk, each segment is horizontally disposed, except C1, which does not have a sensory component. The dermatomes of the limbs from the fifth cervical to the first thoracic nerve and from the third lumbar to the second sacral vertebrae extend as a series of bands from the midline of the trunk posteriorly into the limbs. It should be noted that considerable overlapping occurs between adjacent dermatomes; that is, each segmental nerve overlaps the territories of its neighbors.³

A myotome is the segmental innervation of skeletal muscle by the ventral (motor) root(s) of the spinal nerve(s). Major myotomes, their function, and corresponding spinal levels are represented in Figure 3–7. The innervation of the bones and joints (osteotome) often does not follow the same segmental pattern as the innervation of the muscles and other soft tissues (Figure 3–8).

ANATOMY OF PLEXUSES AND PERIPHERAL NERVES

Cervical Plexus

The cervical plexus innervates muscles, joints, and skin in the anterior neck (Table 3–1). It is formed by the ventral rami of C1 through C4 (Figures 3–9 and 3–10). The rami form a loop called the ansa cervicalis that sends branches to the infrahyoid muscles. In addition, the rami form nerves that pass directly to several structures in the neck and thorax, including the scalene muscles, diaphragm, clavicular joints, and skin covering the anterior neck.

Ansa Cervicalis

The ventral ramus of C1 attaches to the ventral rami of C2 to C3. The attachment forms a loop called the ansa cervicalis, which sends branches to the infrahyoid muscles. The infrahyoid muscles consist of the omohyoid, sternohyoid, and sternothyroid muscles. They attach to the anterior surface of the hyoid bone or to the thyroid cartilage. Contraction of these muscles moves the hyoid bone or thyroid cartilage downward, effectively opening the laryngeal aditus, promoting inspiration. The C1 component also sends fibers to the thyrohyoid and geniohyoid muscles. Contraction of these muscles moves the anterior hyoid bone superiorly, closing the laryngeal aditus. Closure of the laryngeal aditus is necessary for swallowing to occur safely. This is one of the reasons why high levels of spinal anesthesia result in airway compromise and the risk of aspiration.

Nerves to Scalene Muscles

The ventral rami of C2 to C4 send branches directly to the scalene muscles, which attach between the cervical spine and ribs. When the cervical spine is stabilized, contraction elevates the ribs. This promotes inspiration. Interscalene block may result in block of the scalene muscles in addition to the phrenic block. This is typically asymptomatic in healthy patients but may result in acute respiratory insufficiency in patients with borderline pulmonary function or in those with an exacerbation of asthma or chronic obstructive bronchitis. It is recommended that more distal approaches to a brachial plexus block and smaller injection volumes be used to limit the cephalad extension of the block, as well as shorter-acting local anesthetics to avoid prolonged blockade in case of respiratory insufficiency.

Phrenic Nerve

The phrenic nerve is formed by junction of fibers from C3 to C5, (Figure 3-10) and it innervates the diaphragm. The phrenic nerve descends through the neck on the anterior surface of the anterior scalene muscle, passing through the superior thoracic aperture and descending on the walls of the mediastinum to the diaphragm. In addition to muscular fibers, the phrenic nerve transmits sensory fibers to the superior and inferior surfaces of the diaphragm. All approaches to the block of the brachial plexus above the clavicle with high volumes result in phrenic blockade (Figure 3-12).

Cutaneous Nerves of the Anterior Neck

Cutaneous sensory nerves arise from the cervical plexus, pass around the posterior margin of sternocleidomastoid, and terminate in the scalp and anterior neck. The minor occipital nerve passes to the posterior auricular region of the scalp. The major auricular nerve passes to the auricle of the ear and to the region of the face anterior to the tragus. The transverse cervical nerve supplies the anterior neck. A series of supraclavicular nerves innervates the region covering the clavicle. Furthermore, the supraclavicular nerves may provide articular branches to the sternoclavicular and acromioclavicular joints.⁴

Brachial Plexus

The brachial plexus innervates bones, joints, muscles, and the skin of the upper extremity (Table 3–2). It is formed by ventral rami of C5 to T1 (Figures 3–11 and 3–12). In the posterior cervical triangle between the anterior and middle scalene muscles, the ventral rami join to form trunks. C5 and C6 join to form the superior trunk. C7 forms the middle trunk. C8 and T1 join to form the inferior trunk. All trunks branch into anterior and posterior divisions. All the posterior divisions join to form the posterior cord. The anterior divisions of the superior and middle trunks join to form the lateral cord. The anterior division of the inferior trunk forms the medial cord. Several terminal nerves arise within the posterior cervical triangle. Because they arise superior to the clavicle, they are called supraclavicular branches. The supraclavicular branches include the dorsal scapular nerve, the long thoracic nerve, the suprascapular nerve, and the nerve to the subclavius.^5,6,7

Supraclavicular Branches

Dorsal Scapular Nerve

The dorsal scapular nerve arises from the ventral ramus of C5. It follows the levator scapula muscle to the scapula and descends the medial border of the scapula on the deep surface of the rhomboid muscles. In its route, the dorsal scapular nerve innervates the levator scapula and rhomboid muscles.

Long Thoracic Nerve

The long thoracic nerve arises from the ventral rami of C5 to C7. It descends along the anterior surface of the middle scalene to the first rib and then transfers onto the serratus anterior muscle, which it innervates.

Suprascapular Nerve

The suprascapular nerve arises from the superior trunk. It follows the inferior belly of the omohyoid muscle to the scapula; passes through the superior notch into the supraspinatus fossa, where it innervates the supraspinatus muscle; and continues around the scapular notch (lateral margin of the scapular spine) to the infraspinatus fossa, where it innervates the infraspinatus muscle. In addition to muscle, the suprascapular nerve innervates the posterior aspect of the glenohumeral joint, subacromial bursa, and acromioclavicular joint.

Nerve to Subclavius

The nerve to subclavius arises from the superior trunk. It passes anteriorly a short distance to innervate the subclavius muscle and the sternoclavicular joint.

The cords of the brachial plexus leave the posterior cervical triangle and enter the axilla through the axillary inlet. The remainder of the terminal branches arise within the axilla from the cords (Figure 3-12).

Posterior Cord Branches

The posterior cord forms the upper and lower subscapular nerves, thoracodorsal nerve, axillary nerve, and radial nerve.

Subscapular Nerves

The subscapular nerves are formed by fibers from C5 to C6. The upper subscapular nerve is the first nerve to arise from the posterior cord. It passes onto the anterior surface of the subscapularis muscle, which it innervates. The lower subscapular nerve arises more distally. It descends across the anterior surface of the subscapularis muscle to the teres major muscle and innervates both the subscapularis and teres major muscles.

Thoracodorsal Nerve

The thoracodorsal nerve is formed by fibers from C5 to C7. It arises from the posterior cord, usually between the subscapular nerves, and descends across the subscapularis and teres major muscle to the latissimus dorsi muscle. It innervates the latissimus dorsi.

Axillary Nerve

The axillary nerve is formed by fibers from C5 to C6 (Box 3–1). It passes from the axilla into the shoulder between the teres major and minor muscles, the long head of triceps and humerus quadrangular space of Velpeau. It innervates the teres minor. The nerve continues posterior to the surgical neck of the humerus to innervate the deltoid muscle. The superior lateral brachial cutaneous branch of the axillary nerve passes around the posterior margin of the deltoid to innervate the skin covering the deltoid. In addition to muscle and skin, the axillary nerve innervates the glenohumeral and acromioclavicular joints. Throughout its course, the nerve is associated with the posterior circumflex humeral artery and its branches.

Radial Nerve

The radial nerve is formed by fibers from C5 to T1 (Box 3–2). It passes from the axilla into the arm through the triangular space. The triangular space is located inferior to the teres major between the long head of the triceps brachii and the humerus. The radial nerve innervates the long head of the triceps muscle and sends a posterior brachial cutaneous branch to the skin covering this muscle. It descends along the shaft of the humerus in the spiral groove in association with the deep radial artery. In the spiral groove, the radial nerve innervates the medial and lateral heads of the triceps brachii as well as the anconeus muscles. In addition to innervating these muscles, it sends an inferior lateral brachial cutaneous nerve to the skin covering the posterior arm and a posterior antebrachial cutaneous branch to the skin covering the posterior surface of the forearm. The radial nerve pierces the lateral intermuscular septum and crosses the elbow anterior to the lateral epicondyle between the brachialis and brachioradialis muscles. Here, it divides into a superficial and deep branch. The superficial branch descends the forearm on the deep surface of the brachioradialis. Proximal to the wrist, it enters the skin, providing innervation over the dorsum of the hand onto the thumb, index, middle, and ring fingers to the level of the distal interphalangeal joint. The deep branch pierces the supinator muscle and descends the forearm along the interosseous membrane as the posterior interosseous nerve. En route, it innervates the brachioradialis, extensor carpi radialis longus and brevis, supinator, extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, extensor indicis, extensor pollicis longus and brevis, and abductor pollicis muscles. In addition, it innervates the humerus, elbow, radioulnar, and wrist joints.⁸

Branches From the Lateral Cord

The lateral cord forms the lateral pectoral nerve, musculocutaneous nerve, and part of the median nerve.

Lateral Pectoral Nerve

The lateral pectoral nerve is formed by fibers from C5 to C7. It crosses the axilla deep to the pectoralis minor muscle and penetrates the deep surface of pectoralis major muscle, which it innervates. In addition, it innervates the acromioclavicular joint.

Musculocutaneous Nerve

The musculocutaneous nerve is formed by fibers from C5 to C7 (Box 3–3). It pierces the coracobrachialis muscle and descends between the brachialis and biceps brachii muscles (see Figure 3–12). En route, it innervates all of these muscles. At the elbow, the musculocutaneous nerve becomes the lateral antebrachial cutaneous nerve and descends along the superficial surface of the brachioradialis muscle, innervating the skin covering that muscle. In addition to muscle and skin, the musculocutaneous nerve innervates the humerus elbow and proximal radioulnar joints.

Median Nerve

The median nerve is formed by junction of branches from the lateral and medial cords (Box 3–4). It descends the arm in association with the brachial artery and crosses the cubital fossa medial to the artery (see Figure 3–12). At the elbow, it innervates the pronator teres, flexor carpi radialis, and palmaris longus muscles. It passes into the forearm between the humeral and radial heads of the pronator teres muscle and descends in the space between the flexor digitorum superficialis and profundus muscles. En route, it innervates the flexor digitorum superficialis, the lateral part of the flexor digitorum profundus (fibers to the index and middle fingers), the flexor pollicis longus, and the pronator quadratus muscles. In addition, the median nerve sends a palmar cutaneous branch to the skin covering the thenar eminence. At the wrist, the median nerve passes through the carpal tunnel deep to the flexor retinaculum. In the hand, the median nerve sends branches to the thenar muscles, which are the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. The median nerve divides into three common palmar digital branches, which innerve the lateral two lumbrical muscles. The common palmar branches divide into proper palmar branches that innervate the skin of the thumb, index, middle, and ring (lateral half) fingers. The innervation covers the palmar surface and the nail beds. In addition to muscle and skin, the median nerve innervates the diaphysis of the radius and ulna, and the anterior elbow and all joints distal to it.^9,10

Medial Cord Branches

The medial cord forms the medial pectoral nerve, medial brachial cutaneous nerve, medial antebrachial cutaneous nerve, and ulnar nerve and sends fibers to the median nerve.

Medial Pectoral Nerve

The medial pectoral is formed by fibers from C8 to T1. It pierces the pectoralis minor and ends by branching on the deep surface of the pectoralis major, innervating both muscles. Contraction of the pectoralis minor in conjunction with the serratus anterior and rhomboid muscles pulls the pectoral girdle (clavicle and scapula) against the chest wall when load is applied to the upper extremity. Without this stabilization of the proximal joints, movement of the distal joint in the upper extremity would collapse.

Medial Brachial and Antebrachial Cutaneous Nerves

Both medial brachial and antebrachial cutaneous nerves descend in the arm associated with the brachial artery. The medial brachial cutaneous nerve distributes fibers to the skin covering the medial surface of the arm. Occasionally, the medial brachial nerve joins the lateral cutaneous branch of the second intercostal nerve to form the intercostobrachial nerve. The medial antebrachial cutaneous nerve crosses the cubital fossa and enters the skin to innervate the medial aspect of the forearm.⁴

Ulnar Nerve

The ulnar nerve is formed by fibers from C8 to T1 (Box 3–5). It descends the arm in association with the brachial artery (see Figures 3-11 and 3-12), pierces the medial intermuscular septum, and crosses the elbow posterior to the medial epicondyle. After crossing the elbow, the ulnar nerve descends the forearm between the flexor carpi ulnaris and flexor digitorum profundus, innervating both muscles. The ulnar innervation of the flexor digitorum is limited to fibers affecting the ring and little fingers. Proximal to the wrist, the ulnar nerve sends a palmar branch to the skin covering the hypothenar eminence and a dorsal branch to the skin covering the dorsal and medial surface of the hand and the skin covering the dorsal surface of the ring and little fingers. The ulnar nerve passes through Guyon’s canal (deep to the transverse carpal ligament) to enter the hand. It divides into a superficial and a deep branch. The superficial branch sends branches to all muscles of the hypothenar eminence, including the abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi. Then, it divides into common palmar digital branches, which in turn divide into proper palmar digital branches. These branches innervate the skin covering the palmar surface of the ring and little fingers. The innervation continues on to the nail beds of these fingers. The deep branch of the ulnar nerve passes beneath the adductor pollicis muscle, which it innervates. The ulnar nerve sends fibers to all interosseous muscles in the hand and to the lumbrical muscles affecting the ring and little fingers. The ulnar nerve ends by innervating the deep head of the flexor pollicis brevis muscle.^9,10

Along its course, the ulnar nerve supplies the medial aspect of the elbow joint, the ulna and all joints of the medial aspect of the wrist, hand, and ring and little fingers.

Thoracic Spinal Nerves

Thoracic spinal nerves innervate the muscles, joints, skin, and pleuroperitoneal lining of the thoracic and abdominal walls. Because the nerves travel within the intercostal spaces, they are called intercostal nerves. The intercostal nerves comprise the anterior rami of the upper 11 thoracic spinal nerves. Each intercostal nerve enters the neurovascular plane posteriorly and gives a collateral branch that supplies the intercostal muscles of the space. Except for the first, each intercostal nerve gives off a lateral cutaneous branch that pierces the overlying muscle near the midaxillary line. This cutaneous nerve divides into anterior and posterior branches, which supply the adjacent skin (Figure 3–13). The intercostal nerves of the second to the sixth spaces enter the superficial fascia near the lateral border of the sternum and divide into medial and lateral cutaneous branches. Most of the fibers of the anterior ramus of the first thoracic spinal nerve join the brachial plexus for distribution to the upper limb. The small first intercostal nerve is the collateral branch and supplies only the muscles of the intercostal space, not the overlying skin.

The intercostal nerves can be divided into two groups. One group is formed by nerves arising from T1 through T5. These nerves remain in the intercostal spaces throughout their course. The second group is formed by nerves arising from T6 to T12. These nerves initially travel in the intercostal spaces, but then cross the costal margin and terminate in the abdominal wall. This subgroup of intercostal nerves is called the thoracoabdominal nerves. The ventral ramus of T12 forms the subcostal nerve. This nerve travels entirely in the abdominal wall.

Intercostal Nerves

The intercostal nerves arise from the ventral rami of T1 through T11. They travel along the inferior margin of the rib of the corresponding number (eg, T1 nerve travels along the inferior margin of rib 1). En route, the nerve is located between the deepest (transverse thoracis muscle) and intermediate layer (internal intercostals muscle) of muscle. It is associated with the intercostal arteries and veins. From the top to the bottom, the neurovascular bundle is arranged as vein, artery, and nerve (mnemonic VAN). The intercostal nerves send branches to the transverse thoracis, internal intercostals, and external intercostal muscles. They innervate the costal joints. Through lateral and anterior cutaneous branches, they innervate the skin covering the respective intercostal spaces as well as the parietal pleura lining the intercostal spaces.

Thoracoabdominal (Intercostals T6 to T11) Nerves

The T6 through T11 intercostal (thoracoabdominal) nerves begin as typical intercostal nerves but then send branches across the costal margin into the muscles of the anterior abdominal wall. These branches innervate the transverse abdominis, internal abdominal oblique, external abdominal oblique, and rectus abdominis muscles. In addition, they innervate the skin of the anterior wall in a metameric manner from the xiphoid process to the umbilicus.

Subcostal Nerve

The T12, or subcostal, nerve never enters an intercostal space. It travels through the abdominal wall, terminating between the umbilicus and the pubic symphysis. It innervates muscle and skin along its course.

Lumbosacral Plexus

The lumbosacral plexus innervates the muscles, joints, skin, and peritoneal lining of the abdominopelvic wall^11,12 (Tables 3–3 and 3–4). It also innervates the inferior extremities. It is formed by the ventral rami of L1 to S5 (Figures 3–14 and 3–15). The ventral rami join to form the terminal nerves. Between the L2 and S3 levels, the plexus is more complex. The ventral rami divide into anterior and posterior divisions that join to form the terminal nerves. The plexus is located in the posterior abdominal wall between the psoas major and quadratus lumborum muscles (see Figure 3-15).

Iliohypogastric Nerve

The iliohypogastric nerve arises from the ventral ramus of L1 and travels in the abdominal wall to the level of the pubic symphysis (see Figures 3-15 and 3-16). It innervates the muscle, skin, and parietal peritoneum along its course.

Ilioinguinal Nerve

The ilioinguinal nerve (see Figures 3-15 and 3-16) arises from the ventral rami of L1, travels in the abdominal wall, pierces in the posterior wall of the inguinal canal, passes through the superficial inguinal ring, and terminates on the anterior scrotum or labia majora. It innervates the muscle, skin, and parietal peritoneum along its course.

Genitofemoral Nerve

The genitofemoral nerve arises from the ventral rami from L1 and L2 (see Figure 3–15). It travels in the abdominal wall and passes through the deep inguinal ring into the inguinal canal. A femoral branch pierces the anterior wall of the canal and innervates the skin covering the femoral hiatus in the crural fascia. The genital branch passes through the superficial inguinal ring to innervate the skin on the scrotum or labia majora. En route, it innervates the cremaster muscle. Contraction of the cremaster elevates the scrotum.

Nerve to the Coccygeus and Levator Ani

The nerve to the coccygeus and levator ani muscles arises from the posterior division of the ventral rami at S3 to S4. It travels anteriorly onto the superior surface of the coccygeus and levator ani.

Pudendal Nerve

The pudendal nerve arises from the anterior division of the ventral rami from S2 to S4. It passes from the pelvis through the greater sciatic foramen into the gluteal region. It enters the gluteal region inferior to the piriformis muscle, passes posterior to the ischial spine, then enters the perineum by passing through the lesser sciatic foramen. It innervates the muscle and skin of the perineum, anal canal and external anal sphincter.

Superior Gluteal Nerve

The superior gluteal nerve arises from the posterior division of the ventral rami at L4 to S1. It passes from the pelvis through the greater sciatic foramen to the gluteal region. It enters the gluteal region superior to the piriformis muscle, passes in the plane between gluteal medius and minimus muscles, and terminates in the tensor fascia lata muscle. En route, it innervates the gluteus medius and minimus muscles as well as the tensor fascia lata.

Inferior Gluteal Nerve

The inferior gluteal nerve arises from the posterior division of the ventral rami at L5 to S2. It passes from the pelvis through the greater sciatic foramen into the gluteal region. It enters the gluteal region inferior to the piriformis muscle and terminates on the deep surface of the gluteal maximus muscle, which it innervates.

Nerve to Piriformis

The nerve to piriformis arises from the posterior division of the ventral rami at S1 to S2 and passes onto the deep surface of the piriformis muscle, which it innervates.

Nerve to Obturator Internus and Superior Gemellus

The nerve to the obturator internus and superior gemellus muscles arises from the anterior division of the ventral rami at L5 and S1. It passes from the pelvis through the greater sciatic foramen into the gluteal region. In enters the gluteal region inferior to the piriformis muscle and passes along the deep surface of the superior gemellus to the obturator internus, innervating these last two muscles.

Nerve to the Quadratus Femoris and Inferior Gemellus

The nerve to the quadratus femoris and inferior gemellus muscles arises from the anterior division of the ventral rami at L4 to L5. It passes from the pelvis through the greater sciatic foramen to the gluteal region and enters the gluteal region inferior to piriformis, passing deep to the obturator internus to terminate innervating the inferior gemellus and quadratus fermoris, as indicated by its name.

Lateral Femoral Cutaneous Nerve

The lateral femoral cutaneous nerve arises from the posterior divisions of the ventral rami at L2 to L3. It descends the posterior abdominal wall and crosses the iliac crest into the pelvis, where it descends on the iliacus muscle, passes deep to the inguinal ligament at the anterior iliac spine, and distributes cutaneous innervation on the lateral aspect of the thigh to the level of the knee (Figure 3–17).

Posterior Femoral Cutaneous Nerve

The posterior femoral cutaneous nerve arises from the anterior and posterior divisions of the ventral rami at S1 to S3. It passes from the pelvis through the greater sciatic foramen into the gluteal region. It enters the gluteal region inferior to the piriformis muscle, descends in the muscle plane between the gluteus maximus posteriorly and oburator internus anteriorly, and passes into the posterior thigh, where it supplies cutaneous innervation from the hip to the midcalf.⁴

Obturator Nerve

The obturator nerve (Box 3–6) arises from the anterior division of the ventral rami at L2 to L4 (Figure 3–18). It descends through the pelvis medial to the psoas major muscle, crosses the superior pubic ramus inferiorly, and passes through the obturator foramen into the medial compartment of the thigh, where it divides into posterior and anterior branches (see Figure 3–18). The posterior branch descends superficial to the adductor magnus muscle, which it innervates. The anterior branch passes superficial to the obturator externus muscle, descends the thigh in the muscle plane between the adductor brevis and adductor longus, and terminates in the gracilis muscle. En route, it innervates all of these muscles. Furthermore, it provides articular branches to the hip and cutaneous branches to the skin covering the medial thigh.

Femoral Nerve

The femoral nerve arises from the posterior division of the ventral rami at L2 to L4 (Box 3–7). It descends through the pelvis lateral to the psoas major muscle, passes deep to the inguinal ligament, and enters the anterior compartment of the thigh, where it divides into multiple branches supplying the muscle, joints, and skin in that region. In the femoral triangle–inguinal crease, the nerve is positioned lateral to the femoral artery and vein (mnemonic: NAVEL) (Figure 3–19). Muscular branches innervate the iliacus, psoas major, pectineus, rectus femoris, vastus lateralis, vastus intermedius, vastus medialis, and sartorius muscles. Articular branches innervate the hip and knee.¹³ Of note, the femoral nerve below the inguinal ligament consists of an anterior and a posterior part. The anterior part contains branches to the sartorius muscle and cutaneous branches of the anterior thigh, and the posterior contains the saphenous nerve (most medial part) and branches to the individual heads of the quadriceps muscle.¹⁴

Saphenous Nerve and Other Cutaneous Branches of the Femoral Nerve

The superficial branches of the femoral nerve supply the skin covering the anterior thigh. One cutaneous branch follows the deep surface of the sartorius muscle to its attachment on the tibia. Here, it passes onto the skin, providing innervation of the medial leg from the knee to the arch of the foot. En route, the nerve is accompanied by the saphenous vein, so it is called the saphenous branch of the femoral nerve (see Figure 3–19). As previously mentioned, the saphenous nerve is the most medial part of the femoral nerve at the inguinal (femoral) crease.¹⁴

Sciatic Nerve

The sciatic nerve is formed by the junction of the tibial and common peroneal nerves (Box 3–8). The tibial nerve arises from the anterior division of the ventral rami at L4 to S3 (see Figure 3–14). The common peroneal nerve arises from the posterior division of the ventral rami at L4 to S2. The sciatic nerve passes from the pelvis through the greater sciatic foramen into the gluteal region. It enters the gluteal region inferior to the piriformis muscle, descends in the muscle plane between the gluteus maximus posteriorly and the obturator internus anteriorly, and passes lateral to the ischial tuberosity to enter the posterior thigh (Figure 3–20). In the posterior thigh, it passes between the adductor magnus and the long head of the biceps femoris. It descends in the groove between the biceps femoris medially and the semitendinosus and semimembranosus laterally. En route, it innervates the adductor magnus, biceps femoris, semitendinosus, and semimembranosus muscles.¹⁵ Posterior to the knee, the sciatic nerve descends into the popliteal fossa, where it diverges into the tibial and common peroneal nerves (Figure 3–21).¹⁶

Of note, these two branches are distinct from the onset and travel together enveloped in the same tissue sheath.¹⁷ The tibial nerves exits the popliteal fossa passing between the heads of the gastrocnemius muscle into the superficial posterior compartment of the leg. Here, it descends deep to the plantaris and superficial to popliteus muscles. It passes between the tibial and fibular heads of the soleus muscle to enter the deep posterior compartment. The nerve passes posterior to the medial malleolus, where it enters the foot and divides into medial and lateral plantar nerves that innervate the muscle and skin on the plantar surface of the foot. The common peroneal nerve follows the tendon of the biceps femoris to its attachment on the fibula. The nerve passes inferior to the neck of the fibula and divides into superficial and deep branches. The superficial branch enters the lateral compartment of the leg, where it innervates the peroneus longus and brevis muscles. The nerve terminates as cutaneous fibers on the dorsal and lateral surfaces of the foot. The deep peroneal nerve enters the anterior compartment of the leg, where it innervates the tibialis anterior, extensor digitorum longus, and extensor hallucis longus muscles. It crosses the anterior surface of the ankle into the foot, where it innervates the extensor digitorum brevis and extensor hallucis brevis muscles. It terminates as cutaneous fibers supplying skin between the hallux and second toe.¹⁸

SENSORY INNERVATION OF THE MAJOR JOINTS

Much of the practice of peripheral nerve blocks involves orthopedic and other joint surgery. Consequently, knowledge of the sensory innervation of the major joints is important for better understanding the neuronal components that need to be anesthetized to achieve anesthesia for, or analgesia, after joint surgery. Tables 3–5 and 3–6 summarize the sensory innervation of the major joints of the upper and lower extremities, respectively. Tables 3–7 and 3–8 summarize the innervation and kinetic function of the major muscle groups of the upper and lower extremities respectively.

Shoulder Joint

Innervation to the shoulder joints stems mostly from the axillary and suprascapular nerves (C5–C7). The skin over most medial parts of the shoulder receives nerves from the cervical plexus (see Figure 3-10). Such an arrangement explains why a brachial plexus block at the interscalene level is the most appropriate technique to achieve anesthesia to the shoulder (Figure 3–22).

Elbow Joint

Nerve supply to the elbow joint includes branches of all major nerves of the brachial plexus that cross the joint: musculocutaneous, radial, median, and ulnar nerves (Figure 3–23).

Wrist Joint

The wrist joint is supplied by the radial, ulnar, and median nerves (Figure 3–24), including interosseous branches of the radial and median nerves diverging at the proximal forearm. The cutaneous branches of these nerves, in addition to the antebrachial medial cutaneous and lateral cutaneous nerves, display frequent variations and connections among them at different levels, resulting in overlapping innervation areas.

Hip Joint

Nerves to the hip joint include the nerve to the rectus femoris from the femoral nerve, branches from the anterior division of the obturator nerve, and the nerve to the quadratus femoris from the sacral plexus (Figure 3–25).

Knee Joint

Knee innervation is obtained from branches from the femoral, obturator, and sciatic nerves (Figure 3–26). The femoral nerve suplies the anterior aspect of the joint. Articular branches from the tibial and common peroneal divisions of the sciatic nerve innervate the posterior aspect, while fibers from the posterior division of the obturator nerve may contribute to the innervation of the medial aspect of the joint, together with fibers from the posterior division of the obturator nerve, may also contribute to the innervation of the joint.

Ankle Joint

The innervation of the ankle joint is complex and involves the terminal branches of the peroneal (deep and superficial peroneal nerves), tibial (posterior tibial nerve), and femoral nerves (saphenous nerve). A more simplistic view is that the entire innervation of the ankle joint stems from the sciatic nerve, with the exception of the skin on the medial aspect around the medial malleolus (saphenous nerve, a branch of the femoral nerve; Figure 3–27).

AUTONOMIC COMPONENT OF SPINAL NERVES

All spinal nerves transmit autonomic fibers to glands and smooth muscle in the region they innervate. The autonomic fibers are sympathetic. There are no parasympathetic fibers in spinal nerves. Sympathetic fibers originate in the spinal cord between T1 and L2. They pass from the spinal cord through the ventral roots of the T1 to L2 spinal nerves. They depart from the spinal nerve through white rami communicans to enter the sympathetic trunk. The sympathetic trunk is formed by a series of interconnected paravertebral ganglia, which are adjacent to the vertebral bodies and extend from the axis (C2 vertebra) to the sacrum. The preganglionic fibers synapse on cell bodies of neurons forming the paravertebral ganglia. The axons of the paravertebral ganglia (postganglionic fibers) can remain at the same level or they can change level by ascending or descending the trunk. The fibers pass from the trunk through gray rami communicans to spinal nerves. The sympathetic trunk sends a gray ramus to every spinal nerve. The sympathetic nerves travel along branches of the spinal nerve to the target destination¹⁹ (Figure 3–28).

Parasympathetic fibers arise from the lumbosacral plexus. They originate in the spinal cord between S2 and S4, pass through the ventral roots, and enter the ventral rami of the S2 to S4 spinal nerves. The parasympathetic fibers separate from the ventral rami and form the pelvic splanchnic nerve. This nerve travels across the pelvic diaphragm (formed by levator ani and coccygeus muscles) to synapse on intramural ganglia in the wall of the pelvic viscera.^20,21

REFERENCES

INTRODUCTION

Microscopic anatomy that emphasizes structure-function relations is important to the clinical practice of regional anesthesia. This chapter provides basis for understanding of the structure, classification, and organization of the peripheral nerves and insight into how the characteristics of the peripheral nerves (Figure 4–1) relate to the clinical practice of regional anesthesia.

ORGANIZATION OF THE PERIPHERAL NERVOUS SYSTEM

The nervous system enables the body to respond to continuous changes in its external and internal environments. It controls and integrates the functional activities of the organs and organ systems.

Nervous system cells consist of neurons and neuroglia. Neurons transmit nerve impulses to and from the central nervous system (CNS), thereby integrating motor and sensory functions. Neuroglial cells support and protect the neurons. In the CNS, myelin is produced by oligodendrocytes and in the peripheral nervous system (PNS) by the Schwann cells. Although both Schwann cells and oligodendrocytes are in charge of axon myelination, they have distinct morphological and molecular properties and different embryonic origins, the neural crest and the neural tube, respectively.¹

The PNS consists of peripheral nerves (craniospinal, somatic, autonomic) with their associated ganglia and connective tissue investments. All lie peripheral to the pial covering of the CNS.^2,3,4,5,6

Peripheral nerves contain fascicles of nerve fibers consisting of axons. In peripheral nerve fibers, axons are ensheathed by Schwann cells, which may or may not form myelin around the axons, depending on their diameter. Nerve fibers are grouped into fascicles of variable numbers. The size, number, and pattern of fascicles vary in different nerves and at different levels along their paths. Generally, their number increases and their size decreases at some distance proximal to the branching point.

NEURONS

A neuron is the structural and functional unit of the nervous system. It includes the cell body, dendrites, and axon.

The cell body (perykarion) is the dilated region of the neuron that contains a large, euchromatic nucleus with a prominent nucleolus and surrounding perinuclear cytoplasm (Figure 4–2). The perinuclear cytoplasm contains abundant rough-surfaced endoplasmic reticulum and free ribosomes. On light microscopy, the rough endoplasmic reticulum with rosettes of free ribosomes appears as small bodies, called Nissl bodies. The perinuclear cytoplasm contains numerous mitochondria, a large perinuclear Golgi apparatus, liposomes, microtubules, neurofilaments, transport vesicles, and inclusions. The presence of the euchromatic nucleus, large nucleolus, prominent Golgi apparatus, and Nissl bodies indicates the high level of anabolic activity needed to maintain these large cells.

Dendrites are elaborations of the receptive plasma membrane of the neuron. Most neurons possess multiple dendrites that typically arise from the cell body as single short trunks that ramify into smaller branches that taper at the ends. Dendrite-branching patterns are characteristic of each kind of neuron. The base of the dendrite contains the same organelles as the cell body, except the Golgi apparatus. Many organelles become sparse or absent toward the distal end of the dendrite. Dendrite branching results in several synaptic terminals and allows a neuron to receive and integrate multiple impulses.

The axon arises from the cell body as a single thin process, much longer than the dendrites. Its thickness is directly related to conduction velocity, which increases with axonal diameter. Some axons possess collateral branches. The portion of the axon between the cell body and the beginning of the myelin sheath is the initial segment.⁷ At the end of the axon, the ramifications may form many small branches. The axonal cytoplasm is called axoplasm.

Almost all of the structural and functional protein molecules are synthesized in the cell body and are transported to distant locations within a neuron in a process known as axonal transport. Crucial to the trophic relations within the axon, axonal transport serves as a mode of intracellular communication carrying molecules and information along microtubules and intermediate filaments from the neuronal cell body to the axon terminal (anterograde transport) or from the axon terminal to the neuronal cell body (retrograde transport). Neurons communicate with other neurons and with effector cells by synapses. These special junctions between neurons and effector cells facilitate the transmission of nerve impulses from one (presynaptic) neuron to another (postsynaptic) neuron or from axons to effector (target) cells, such as muscle and gland cells.

Neurons have greater variation in size and shape than any other group of cells in the body. They are classified morphologically into three major types according to their shape and the arrangement of their processes. The most common neuron type, multipolar, possesses a single axon with various arrangements of multiple dendrites emanating from the cell body. The majority of multipolar neurons (Figure 4–2 and Figure 4–3) are motor neurons. A second type of neuron, unipolar or pseudounipolar (Figure 4–3), possesses only one process, the axon emanating from the cell body and opening up into the peripheral and central branches shortly after leaving the cell body. The central branch enters the CNS, while the peripheral branch proceeds to its corresponding receptor in the body. Each of the two branches is morphologically axonal and can propagate nerve impulses, although the very distal part of the peripheral branch arborizes, indicating its receptor function. The majority of unipolar neurons are sensory neurons, whose cell bodies are situated in the dorsal root ganglia of spinal nerves and in the sensory ganglia of cranial nerves. The third type of neuron, bipolar, possesses two processes emanating from the cell body: a single dendrite and a single axon. They can only be found in some cranial nerves.

Functionally, the nervous system has somatic and autonomic components. Nerve fibers innervating tissues derived from somites (muscles and skin) are described as somatic; nerve fibers innervating endodermal or other mesodermal derivatives (internal organs) are visceral. The somatic nervous system controls functions that are under conscious voluntary control with the exception of the reflex arch. It provides sensory and motor innervation to all parts of the body except the viscera, smooth muscles, cardiac muscle, and glands. The autonomic nervous system provides efferent involuntary innervation to smooth and cardiac muscles and glands. It also provides the afferent sensory innervation of the viscera (pain and autonomic reflexes).

Efferent Axons

Efferent axons arise from either the somatic or the autonomic nervous system. Somatic efferent (motor) neurons innervate skeletal muscle and have cell bodies located in somatic motor nuclei of the brainstem (cranial nerves) or in the ventral horns of the spinal cord (spinal nerves).

Preganglionic visceral efferent neurons of the sympathetic part of the autonomic nervous system arise from the intermediolateral column of the spinal cord between levels T1 and L2 and synapse on paravertebral or prevertebral (preaortic) ganglia. Peripheral nerves thus contain both preganglionic and postganglionic sympathetic fibers. Preganglionic visceral efferent neurons of the parasympathetic part of the autonomic nervous system arise from the parasympathetic nuclei within the brainstem (cranial part of parasympathetic nervous system) or sacral spinal cord between the S2 and S4 segments (sacral part of the parasympathetic nervous system). Only preganglionic parasympathetic fibers travel along peripheral nerves to synapse on intramural ganglia in the wall of target organs.

Afferent Axons

Afferent axons are either somatic or visceral and have cell bodies either in the dorsal root ganglia of the spinal nerves or in the sensory ganglia of the cranial nerves. Somatic afferent (sensory) neurons transmit impulses from the receptors for touch, temperature, or pain (nociceptors) located in the body wall (skin) and from the proprioceptors in the skeletal muscles and joints. Visceral afferent neurons transmit information from viscera (interoceptors and nociceptors). The visceral afferent axons travel along the visceral efferent fibers and pass through the communicating branches and dorsal roots of the spinal nerves or along the vagus nerve to enter the CNS.

SCHWANN CELLS

Axons of peripheral nerves are ensheathed by Schwann cells. Their myelin sheath (modified plasmalemma) separates the axons from the endoneurium. Schwann cells are distributed along the axons in longitudinal chains depending on myelination along the axon. The coordinated differentiation of the axons and their myelinating cells requires close communication between neurons and glia.⁸ Signals provided by the axons regulate the proliferation, survival, and differentiation of glial cells.^9,10 On the other hand, reciprocal glial signals affect axonal cytoskeleton and transport¹¹ and are required for axonal survival^12,13 and regeneration.¹⁴ Schwann cells also have a guiding function for growing axons,¹⁵ indicating that glia do more than provide support to the axon.

Schwann cell phenotypes are characterized by distinct morphologies⁵ and differential expression of myelin proteins, cell adhesion molecules, receptors, enzymes, intermediate filament proteins, ion channels, and extracellular matrix proteins.⁶ All Schwann cells are surrounded by basal lamina, whose extracellular matrix molecules, such as laminin, regulate key aspects of Schwann cell development (for review, see Reference 16).

CLASSIFICATION OF NERVE FIBERS

Nerve fibers are classified according to axonal diameter, conduction velocity, type of receptor, and myelin sheath thickness (Table 4–1).¹⁷ Conduction velocity is related to axonal diameter; that is, the larger the fiber is, the faster the conduction will be.

MYELINATED NERVE FIBERS

Myelinated nerve fibers are ensheathed by myelin, greatly extended and modified plasmalemma of the Schwann cells (Figures 4–4 and 4–5). Myelin formation begins with extension of the Schwann cell cytoplasm and development of the inner mesaxon, which wraps around the axon several times. During the wrapping process, the cytoplasm is nearly extruded between the plasmalemma. Apposing extracellular faces of plasmalemma become “the major dense line,” and apposing cytoplasmic faces form an “intraperiod line” of myelin. The proposed molecular structure of myelin fits the concept of plasmalemma as a lipid bilayer with integral and peripheral membrane proteins attached to the extracellular or to the cytoplasmic side of plasmalemma. In contrast to most biological membranes, myelin has a high ratio of lipid to protein (70%–85% lipid, 15%–30% protein), where the latter serve as structural proteins, enzymes, voltage channels, and signal transducers.¹⁸

Myelin sheath wraps the axon in segments. Areas of the axon covered by concentric lamellae of myelin and a single myelin-producing Schwann cell are called internodes and range in length from 200 to 1000 µm. Interruptions, which occur in the myelin sheath at regular intervals along the length of axons and expose the axon, are called nodes of Ranvier (Figure 4–6). Each node indicates an interface between the myelin sheaths of two different Schwann cells located along the axon.

The nodal region and its surroundings can be further subdivided into several domains (Figure 4–6) that contain a unique set of ion channels, cell adhesion molecules, and cytoplasmic adaptor proteins (for review, see Reference 8). In the PNS, the node is in contact with Schwann cell microvilli and covered by its basal lamina (Figure 4–6). An important characteristic of the nodal axolemma is its high density of voltage-gated Na⁺ channels¹⁹ as compared to the juxtaparanodal axolemma, which typically contains a high density of K⁺ channels.²⁰ Na⁺ channels potentiate the nerve impulse in a saltatory manner (Figure 4–7) along the myelinated fibers.^21,22 When the membrane at the node is excited, the local circuit that is generated cannot flow through the high-resistance myelin sheath. It therefore flows out and depolarizes the membrane at the next node, which may be 1 mm or farther away. The low capacitance of the sheath means that little energy is required to depolarize the remaining membrane between the nodes, resulting in increased speed of local circuit spreading.¹⁸

Myelination is an example of cell-to-cell communication in which axons interact with Schwann cells. The number of myelin layers is determined by the axon and not by the Schwann cell. Myelin sheath thickness is regulated by a growth factor called neuregulin 1 (Nrg1). The compaction of myelin sheath is associated with the expression of transmembrane myelin-specific proteins such as protein 0 (P0), a peripheral myelin protein of 22 kilodaltons (PMP22), and a myelin basic protein (MBP). The absence of proteins that regulate myelin sheath formation might result in severe hypomyelination or dismyelination in humans and experimental animals.¹⁸

UNMYELINATED NERVE FIBERS

Unmyelinated axons are also enveloped by Schwann cells and their basal lamina. An individual Schwann cell can ensheath a single or several unmyelinated axons (Figures 4–8 and 4–9). Unmyelinated fibers predominate in human cutaneous spinal nerves, where the average ratio of unmyelinated to myelinated fiber density is 3.7:1.²³ In unmyelinated fibers, conduction velocity is proportional to the square root of fiber diameter and is much slower compared to saltatory conduction in myelinated fibers (Table 4–1).

CONNECTIVE TISSUE INVESTMENTS OF PERIPHERAL NERVES

In a peripheral nerve, nerve fibers and their supporting Schwann cells are held together by connective tissue organized into three distinctive components that have specific morphological and functional characteristics. The epineurium forms the outermost connective tissue of the peripheral nerve, the perineurium surrounds each nerve fascicle separately, while the individual nerve fibers are embedded in the endoneurium² (Figures 4–10, 4–11, 4–12, 4–13).

Epineurium

The epineurium is a condensation of a loose areolar connective tissue that surrounds a peripheral nerve and binds its fascicles into a common bundle (Figure 4–10 and Figure 4–11). Epineurium that extends between the fascicles is the interfascicular or inner epineurium, while epineurium that surrounds the entire nerve trunk is the epifascicular or external epineurium^24,25 called the epineurium comprises 30%–75% of the nerve cross-sectional area²⁶ but varies along the nerve. It is the thickest where continuous with the dura covering the CNS and more abundant in nerves adjacent to the joints, where nerves are subject to pressure.²⁷ Susceptibility to compression injury is therefore likely to be greater in unifascicular than in multifascicular nerves because the latter have greater amount of epineurium. As the peripheral nerve divides and the number of fascicles is reduced, the epineurium becomes progressively thinner and eventually disappears around monofascicular nerves.

The epineurium contains collagen, fibroblasts, mast cells, and fat cells. Collagen bundles have a predominant longitudinal orientation; however, an electron microscopy study found epineural collagen in bundles 10–20 µm in width are arrayed obliquely around the circumference of the nerve.²⁸ Elastic fibers are also present, particularly adjacent to the perineurium,^29,30 which are mainly oriented longitudinally. Collagen and elastic fibers are aligned and oriented to prevent damage by overstretching of the nerve bundle, suggesting that the epineurium is designed to accommodate the stretch.

Human epineurium is constructed predominantly of type I and type III collagen, with the type I predominating.³¹ The diameter of the collagen fibrils averages 60–110 nm.²

Adipose tissue inside a nerve surrounds the fascicles and forms adipose sheaths that separate the fascicles from each other. The thickness of adipose sheaths varies from one fascicle to another and is greater in larger nerve trunks, highlighting its protective function in cushioning the fascicles against damage by compression.²⁷ Loss of epineural fat may present a risk factor for pressure-caused palsies in emaciated, bedridden patients.² In contrast, excessive adipose tissue can also delay the diffusion of local anesthetic injected near a nerve, thus interfering with the anesthetic blockade.³² Epineurium is continuous with the connective tissue called adventitia or mesoneurium that surrounds the nerve when passing through, underneath, or between the muscle fascia, serving as (1) a conduit for the injected local anesthetic, (2) a path allowing for nerve gliding, and (3) a layer of protection against nerve trauma.²⁵ Because their attachment is loose, nerves are relatively mobile except where tethered by entering vessels or exiting nerve branches.²⁴

Perineurium

The perineurium is a specialized connective tissue surrounding individual nerve fascicles (Figures 4–10 and 4–12). This protective cellular layer is thinner than the epineurium and separates the endoneurium from the epineurium.² The perineurium consists of alternating layers of flattened polygonal cells, which are thought to be derived from fibroblasts, and collagenous connective tissue,²⁹ the formation of which is controlled by the Schwann cells.³³ The flattened polygonal cells, which constitute the lamellae, are specialized to function as a diffusion barrier.³⁴ The number of lamellae varies, depending mainly on the diameter of the fascicle; the larger the fascicle is, the greater the number of lamellae.³⁵ In mammalian nerve trunks, the perineurium contains 15–20 cell layers.⁵ Contiguous cells in each layer interdigitate along extensive tight junctions.³⁰ The cells may branch and give rise to processes and contribute to the adjacent lamellae. Each layer of cells, enclosed by basal lamina, can reach a thickness of up to 0.5 µm in human nerves.^2,5

Collagen fibers originate in a lattice-like arrangement, in which bundles are circular, longitudinal, and obliquely arranged. The innermost perineural cell layer adheres to a distinct boundary layer of densely woven collagen fibers and subperineurial fibroblasts that mechanically links the perineurium to the endoneurial contents.²⁸ Collagen fibers are predominantly type III, although type I collagen fibers are also present.³¹ The diameter of the collagen fibrils is substantially smaller than that of the epineural fibrils, with an average of 52 nm in the rat sural nerve.³⁰ The basal lamina of polygonal cells is composed of collagens IV and V, fibronectin, heparan sulfate proteoglycan,³⁶ and laminin.³⁷ The ubiquitous presence of pinocytotic vesicles rich in phosphorylating enzymes underlie the assumption that the perineurium functions as a metabolically active diffusion barrier, playing an essential role in maintaining the osmotic milieu and fluid pressure within the endoneurium.³⁴ For instance, in one of our studies, inflammatory cells accumulated between the nerve fascicles in piglets after exposure of the nerve to ultrasound gel did not penetrate the perineurium.³⁸ Because of its tightly adherent cellular structure and more longitudinally oriented collagen, the perineurium is less tolerant to elongation than the epineurium. In the rabbit, mechanical failure during elongation coincided with a disruption of the perineurium while the epineurium remained intact.³⁹ The integrity of the diffusion barrier was maintained after 2 hours of 15% elongation, while 27% elongation caused acute perineural disruption.³⁹

Endoneurium

The endoneurium comprises loose intrafascicular connective tissue that does not include the perineural partitions that subdivide the fascicles and surrounds Schwann cells (Figure 4–12). Approximately 40%–50% of the intrafascicular space is occupied by nonneural elements (ie, other than axon and Schwann cells), of which the endoneurial fluid and connective tissue matrix occupy 20%–30%.⁴⁰ There are substantial variations among nerves in different species⁵ and age groups.⁴¹

The endoneurium is composed of collagen fibers (produced by the underlying Schwann cells and fibroblasts); cellular components are bathed in endoneurial fluid, contained in substantial intrafascicular spaces. The nerve fibers tend to be grouped into small bundles with intervening clefts. Endoneurial fluid pressure is slightly higher than that of the surrounding epineurium. It is believed that this pressure gradient minimizes endoneurial contamination by toxic substances external to the nerve bundle.⁴²

Endoneurial collagen fibrils are smaller than those in the epineurium and range between 30 and 65 nm in diameter in humans.⁴³ The fibrils run parallel to and around the nerve fibers, binding them into fascicles or bundles. They show condensations around capillaries and nerve fibers. Near the distal terminus of the axon, the endoneurium is reduced to a few reticular fibers surrounding the basal lamina of the Schwann cells. Types I, II, and III collagen are present in endoneurium.⁴⁴

Cellular constituents of endoneurium are fibroblasts, endothelial cells of capillaries, mast cells, and macrophages. Mast cells occur in varying numbers, being especially numerous along blood vessels. Macrophages account for 2%–4% of the intrafascicular nuclei in rat peripheral nerve⁴⁵ and are the primary antigen-presenting cells of peripheral nerve. They scavenge extracellular proteins and present them to T cells emerging from circulation. The macrophages mediate immunologic surveillance and participate in nerve tissue repair. Following nerve injury, they proliferate and actively phagocytose myelin debris.

The extracellular matrix is rich in glycoproteins, glycosaminoglycans, and proteoglycans. The best characterized of these include the glycoprotein fibronectin, tenascin C, trombospondin, and the chondroitin sulfate proteoglycans versican and decorin.⁴⁶ Expression of these molecules changes after nerve injury, so they are potentially relevant during nerve regeneration.⁴⁶

From a hydrodynamic point of view, the various tissues that comprise a peripheral nerve can be divided into the loose, high-compliance, expansible connective tissue of the epineurium and the low-compliance, disruptable fascicles and fascicular bundles, densely packed within the perineurium. These anatomical differences between connective tissues and fascicles or their bundles explain why an injection into fascicles requires more force (pressure) than injection into the loose connective tissue of the epineurium.^25,47

THE CENTRAL-PERIPHERAL TRANSITION REGION

The transition between the CNS and the PNS in cranial and spinal nerve roots is referred to as the central-peripheral transition region or CNS-PNS border (Figure 4–14). It represents an abrupt change in the type of myelin, supporting elements, and vascularization. The main glial components in the CNS are astrocytes and oligodendrocytes, while in the PNS the main components are the Schwann cells.⁵² The nerve roots of spinal nerves are bathed in cerebrospinal fluid. The transition region is the length of the rootlet that contains both central and peripheral nervous tissue. Transition details distinguishing ensheathment of the spinal roots with the meninges and connective tissue investments of the peripheral nerves have not been fully clarified.⁵ Their structural arrangements however, are well documented in electron microscopic studies.^{53,54,55,56,57}

The cellular components of the endoneurium in the spinal roots resemble those of peripheral nerves. The quantity of collagen is substantially less and is not organized in sheaths around the nerve fibers.⁴³ The region in which the spinal roots attach to the spinal cord is characterized by an irregularly designed transition from the peripheral nerve to the CNS, the Obersteiner-Redlich zone where Schwann cells are replaced by oligodendrocytes. The central portion of the root is limited at its periphery by marginal glia, composed of astrocytes covered by basal lamina.⁵

The spinal roots traverse the subarachnoid space covered by a multicellular root sheath and penetrate the dura at the subarachnoid angle (Figure 4–14). External to the subarachnoid angle, the nerve roots possess epineurium, perineurium, and endoneurium as in the peripheral nerve trunks. The epineurium is the continuation of the spinal dura, while the endoneurium is developed distal to the junction of the roots with the central nervous tissue. Perineurium ensheaths the spinal ganglia and is proximal to it. It is divided in the outer layers that pass between the dura and the arachnoid to form the “dural mesothelium,”⁵⁸ while the inner layers of perineurium continue over the roots as the “inner layer of the root sheath.”^5,54

The root sheath is composed of cellular and fibrous lamellae divided into two layers.⁵⁴ The outer layer consists of loosely associated cells bordering on the subarachnoid space. Where the roots become attached to the spinal cord, the cells of the outer layer of the root sheath become continuous with the pia.⁵⁹ At the subarachnoid angle, the outer layer becomes reflected to the external meningeal investments of the spinal cord (arachnoidea attached to the inner layer of spinal dura).⁵⁶ The inner layer of the root sheath consists of flattened cells that are closely associated with each other, are intermittently invested with a basal lamina, and resemble the perineurium but are not classifiable as perineural cells. It becomes continuous with the perineurium peripherally.

The subarachnoid space opens into a lateral recess that extends between the ventral and dorsal roots and may constitute a communication between the subarachnoid and endoneurial spaces.⁵⁴ This communication is clinically relevant because it allows inflammation to spread from the subarachnoid space to the endoneurium in the case of polyradiculoneuritides.

VASCULAR SUPPLY OF PERIPHERAL NERVES

The peripheral nerve is a well-vascularized structure,⁶² supplied by vessels that originate from the nearby large arteries and veins as well as from smaller adjacent muscular and periostal blood vessels (Figure 4–12). Peripheral nerves have two separate, functionally independent, vascular systems: an extrinsic system (regional nutritive vessels and epineural vessels) and an intrinsic system (microvessels in the endoneurium).⁶³ There are rich anastomoses between the two systems, resulting in considerable overlap between the territories of the segmental arteries.

The epineurium is characterized by a predominantly longitudinal vascular plexus. Transperineural arterioles, 10–25 µm in diameter, pass from the epineurium to the endoneurium through sleeves of perineural tissue.⁶⁴ Their course through the perineurium is oblique, rendering them potentially susceptible to changes in intra- or extrafascicular pressure.⁶² Epineurial and perineurial vessels have a rich perivascular plexus of peptidergic, serotoninergic, and adrenergic nerves that play an important role in the neurogenic control of endoneurial blood flow.⁶⁴

The endoneurial vasculature is noted for its anatomical dissimilarities from a conventional capillary bed, although, physiologically, it serves similar metabolic functions. Transperineurial arterioles gradually lose their continuous muscle coat and become postarteriolar capillaries. Endoneurial capillaries have atypically greater diameter and intercapillary distances than those in many other tissues.⁶⁵ Such angioarchitecture suggests a lower exchange capacity.⁶⁶ Endoneurial arterioles have a poorly developed smooth muscle layer and thus limited capacity for autoregulation.⁶³ The density of endoneurial microvessels varies significantly throughout peripheral nerves; these variations correlate with susceptibility to ischemic neuropathy.⁶⁷ This unique pattern of vessels, together with the high basal blood flow relative to metabolic requirements of the nerve, confer a high degree of resistance to ischemia so that nerve dysfunction does not occur during acute ischemia until blood flow is almost zero. The outstanding characteristic of the peripheral neurovascular system is its flexibility. Peripheral nerves may be surgically mobilized, severing their nutrition vessels without clinical consequences, to a surprising degree. However, the distribution of the circulation within the endoneurium is exquisitely sensitive to physical and chemical manipulation.⁶²

AGE-RELATED CHANGES IN PERIPHERAL NERVES

An intact aged PNS is characterized by several extensive structural, functional, and biochemical changes, which have been documented in both myelinated and unmyelinated fibers.^5,69 In the elderly, myelinated fiber density decreases.^{23,41,70,71,72} A regular relation between internodal length and fiber diameter becomes less precise with aging.^41,73 This is associated with segmental demyelination and remyelination and the axonal degeneration and regeneration clinically evident as mild peripheral neuropathy.⁵

In unmyelinated fibers, regressive changes attributed to aging have been reported.^74,75 In unmyelinated fiber complexes of aging nerves, the proportion of Schwann cell bands devoid of axons increases (so-called collagen pockets; see Figure 4–9). An early age-related change appears to be the budding of Schwann cell processes into numerous flattened tongues, which usually occur in groups.⁵ The perineurial index (ratio of perineurium thickness to fascicle diameter) shows a tendency to increase with age,⁷² most probably reflecting age-related loss of nerve fibers.

Aging is associated with a decreased number of endoneurial capillaries and an increase in the thickness of capillary walls and the perineurium.⁷⁰ The rate of axonal regeneration becomes slower as density and number of regenerating axons decreases. Aging also impairs terminal sprouting of regenerated axons and collateral sprouting of intact adjacent axons, further limiting target reinnervation and functional recovery.⁷⁶

The cause of changes related to aging is uncertain. It is not yet established whether they are the result of neuronal aging, giving rise to distal axonal degeneration and secondary demyelination, or local factors in the nerves, such as ischemia or the consequences of repeated minor trauma. Nevertheless, age-related changes in peripheral nerves probably result from the cumulative, lifelong effect of various pathogenic factors modified by genetic determinants and by a gradual decrease in regenerative capacity.⁷⁰

RESPONSE OF PERIPHERAL NERVE TO INJURY

Injuries to the peripheral nerves result in loss of motor, sensory, and autonomic functions in the denervated segments of the body due to the interruption of axons, degeneration of distal nerve fibers, and eventual death of axotomized neurons.⁷⁸ Functional deficits caused by nerve injuries can be compensated by reinnervation of denervated targets by regenerating injured axons or by collateral branching of undamaged axons and remodeling of nervous system circuitry related to the lost functions. Nerve regeneration is possible if the cut ends remain near each other⁷⁶ otherwise, regeneration may not be complete or successful.

After an injury, the neuron attempts to repair the damage, regenerate the process, and restore function by initiating a series of structural and metabolic events called axon reaction. The reactions to the trauma are localized in three regions of the neuron: at the site of damage (local changes), distal to the site of damage (anterograde changes), and proximal to the site of damage (retrograde changes). Local reaction to injury involves removal of debris by neuroglial cells. The portion of the axon distal to an injury undergoes degeneration and is phagocytosed. The proximal portion of the injured axon undergoes degeneration followed by sprouting of a new axon whose growth is directed by Schwann cells.

SUMMARY

The knowledge that neural anatomy is unique at different anatomical sites is essential for a safe and effective practice of regional anesthesia. Understanding the peripheral nerve structure and its implication while using state-of-the-art monitors, including ultrasonography, nerve stimulation, and injection pressure monitoring, is helpful in minimizing the potential for patient injury.

REFERENCES

INTRODUCTION

A better understanding of some features of the fine structure of peripheral nerves may provide us with essential information that can be helpful in anesthetic clinical practice. This chapter reviews the ultrastructure of connective tissues of peripheral nerves to facilitate understanding of its role as the perineurial diffusion barrier and implication in regional anesthesia.

FASCICLES

Nerves and their principal branches (Figures 5–1, 5–2, 5–3) consist of parallel bundles of nerve fibers (nerve fascicles, fasciculi). The size, number, and pattern of fasciculi vary among nerves and at different distances from their origin. When the connective tissue of the peripheral nerve is removed, 20 or more tubular structures or fascicles are typically seen.

Inside each nerve, the axons form an intraneural plexus in such a fashion that one axon can contribute to different fascicles along the nerve length (Figure 5–4). In other words, an axon can travel from a peripheral position to a more central position as well as swap the fascicles altogether along its descent more peripherally.^1,2,3,4 Indeed, cross-sectional anatomy of nerves at close distance from each other demonstrates that the location and number of fascicles within nerves are highly variable (see Figure 5–3) with the presence of intraneural plexuses (Figures 5–5 and 5–6). The number, size, and location of fascicles in peripheral nerves are also variable even within a single nerve and can vary as much as 23 times along a 4- to 5-cm length of nerve.^5,6,7,8

In a cross section of a sciatic nerve, fascicles comprise 25%–75% of the cross-sectional area (see Figures 5–1 and 5–3). This proportion varies in different nerves and at different levels of the same nerve. Up to 50% of the cross-sectional area is made up of non-neural tissue, including endoneural fluid and connective stroma. The number of fascicles increases at the level of nerve branching. In the proximity of joints, the fascicles are thinner, more numerous and have a thicker perineurium, which may confer better protection against pressure and stretching.⁸

CONNECTIVE TISSUE SHEATHS OF PERIPHERAL NERVES

The connective tissue inside nerves functions to support and protect nerves blood and lymphatic vessels (see Figure 5–1 and 5–2). The connective tissue of peripheral nerves takes different names according to its location.^1,2,3 On the outside of each peripheral nerve, there is collagenous tissue: epineurium. Surrounding every fascicle within the nerve is the perineurium. Individual nerve fibers within the fascicles are embedded in endoneurium, which fills the space bound by the perineurium. As the peripheral nerve divides and the number of fascicles decreases, the connective tissue sheaths become progressively thinner. For instance, in monofascicular nerves the epineurium is absent, distributed irregularly, or appears integrated with the perineurium. Connective tissue that connect nerves to the surrounding structures are thinner and disperse, often losing any distinguishable feature from that of general connective tissue.⁹

ENDONEURIUM

The endoneurium (Figures 5–7 and 5–8) intimately surrounds Schwann cells and fills the space bounded externally by the perineurium. Endoneurium contains collagen fibers, fibroblasts, capillaries, and a few mast cells and macrophages. Collagen fibers are permeable and concentrated in a zone beneath the perineurium and around nerve fibers and blood vessels. The collagen fibers surround both myelinated and unmyelinated nerve fibers. However, the endoneurial sheaths around smaller myelinated fibers and around some unmyelinated axons are less well organized (Figure 5–9).

Fibroblasts are among the most abundant cell types of the endoneurium. They are responsible for fiber formation and production of ground substance. When sectioned transversely, endoneurial fibroblasts have triangular or rectangular perikaria. The appearance of fibroblasts varies depending on their functional activity. When the cell is metabolically active, as is the case with growth and in tissue regeneration after injury, the nucleus is larger and nucleoli are more prominent. The cytoplasm also stains more deeply and is basophilic in contrast with the lightly staining, slightly acidophilic cytoplasm of a relatively inactive cell. Like those in the epineurium, fibroblasts in the endoneurium lack a basal lamina.

Mast cells are especially numerous along the course of blood vessels. Mast cell granules are soluble in water and are therefore not easily revealed in sections routinely prepared with hematoxylin and eosin stain. After adequate fixation, the granules stain with most basic dyes and become metachromatic after certain dyes, such as toluidine blue. Electron microphotographs show that the secretory granules are membrane bound, and the granule matrices have varying densities and characteristic helical-type patterns (Figure 5–10). Macrophages are also found frequently around perivascular endoneurium (Figure 5–11). The endoneurium contributes to the stability of the internal medium where the Schwann cells and axons are located.¹⁰ The endoneurium of cutaneous nerves contains more collagen fibers than deep nerves; this probably is related to its protective role. The endoneural collagen is believed to originate from the Schwann cells, which are 9:1 more prominent than fibroblasts. Schwann cells represent 90% of intrafascicular cells, whereas fibroblasts account for less than 5% of the remaining number. The endoneurium along with the epineurium and perineurium contribute to nerve protection against elongation under strain. The sinuous trajectories of axons confer additional protection on nerves. Endoneurial sheaths around axons is demonstrated in figures 5-7, 5-8 and 5-9. Instead of individually shaped endoneurial layers, the endoneurium appears rather as a continuum, forming several canaliculi into which axons are embedded.

PERINEURIUM

Each fascicle is surrounded by a connective tissue sheath, the perineurium.^1,2,3,11-14 The perineurium consists of concentric layers of flattened cells separated by layers of collagen (Figures 5–12, 5–13, 5–14, 5–15, 5–16). The number of perineurial cell layers depends on the size of the fascicle. As many as 8–16 concentric layers may be present around large nerve fascicles, but a single layer of perineurial cells surrounds small distal fascicles. In larger peripheral nerves, concentric cell layers alternate with layers of collagen fibers arranged longitudinally, similar to those of the epineurium. Collagen fibers are thinner than those of the epineurium, and only a few elastic fibers are scattered among them. Perineurial cells have a basal lamina on each side that may be considerably dense. At sites known as hemidesmosomes, the perineurial cell plasma membrane strongly adheres to the basal lamina.^3,11

With electron microscopy, perineurial cells are seen as thin sheets of cytoplasm containing small amounts of endoplasmic reticulum, filaments, and numerous endocytic vesicles. Tight junctions and gap junctions between adjacent cells within the same layer of perineurium are also observed.^3,11 Similar tight junctions may also appear between successive layers of the perineurium when their cells are in close proximity. Tight junctions in the inner layers of the perineurium and tight junctions in endoneurial capillaries form a blood-nerve barrier structure^14,15,16 (Figures 5–17 and 5–18). The blood-nerve barrier is not equivalent to the blood-brain barrier¹¹ as the blood-brain barrier astrocytes help regulate the flow of compounds between blood and the brain. The perineural cells are metabolically active, and their cytoplasms contain enzymes like ATPase (adenosine triphosphatase), 5-nucleotidase, and so on. These cells probably play a role in maintaining electrolyte and glucose balance around the nerve cells.¹⁷

The perineurium forms a tubular wrapping that allows some axonal movement inside the fascicles. The thickness of the epineurium varies between 1 and 100 μm. As the number of fascicles within a nerve increases, the thickness of the perineurium generally decreases. For instance, along the trajectory of the median nerve, the epineurium appears proportionally thicker in the wrist than in the axilla. There are three areas where the perineurium is absent and the epineurium becomes in contact with the endoneurium:nerve endings, around blood vessels, and in areas where reticular fibers penetrate the perineurium.

The role of the perineurium is to maintain intrafascicular pressure and to contribute to the barrier effect.^13,16,18 Pressure exerted on the perineurium is transmitted to the endoneurium and ultimately the nerve fibers (axons).¹⁹ The perineurium increases in thickness around points of nerve branching to provide additional protection.²⁰

The perineurium may be also protective in limiting the extension of infection and inflammatory reactions. For instance, when a nerve with intact perineurium crosses an infected area, the nerve responds generally by thickening of its perineurial layer. Conversely, when perineurium is not intact, the infection easily spreads across nerve fascicles.¹⁹ Injury to the epineurium, however, does not compromise the axonal safety to the same extent. Söderfelt demonstrated that the barrier effect of the epineurium is preserved for up to 22 hours postmortem in conditions of ischemia.¹⁶ Olsson has studied the loss of the nerve barrier effect in vivo after nerve lesion. He also has demonstrated recovery of the effect between 2 and 30 days postinjury.¹³

EPINEURIUM

The outermost sheath of the epineurium consists of moderately dense connective tissue that binds nerve fascicles (Figures 5–3, 5–19, and 5–20). Epineurium merges with adipose tissue surrounding peripheral nerves, particularly in subcutaneous tissue. The amount of epineurial tissue varies along a nerve and is more abundant around joints. The thickness of the epineurium varies in different nerves and in different locations of the same nerve. For instance, the average thickness of the epineurium is 22% of the ulnar nerve at the level of the elbow and 88% of the sciatic nerve at the gluteal level. In general, the epineurium represents between 30% and 75% of the cross-sectional area of a nerve.^8,20,21 The proportion of epineurium is higher in larger nerves with increasing numbers of nerve fascicles. However, the epineurium is absent around monofascicular nerves and at nerve endings.

The epineurium contains adipocytes, fibroblasts, connective tissue fibers, mast cells, small blood and lymph vessels, and small nerve fibers innervating the vessels.²⁰ Epineurium is a permeable structure,^20,21 and its fibroblasts are ultrastructurally identical to fibroblasts elsewhere in the body. Scattered throughout the epineurium, fibroblasts form the epineurial collagen, the most prominent component of this layer. As collagen is a protein stained by most acid dyes, collagen fibers turn weak pink wiht eosin in preparations stained with hematoxylin-eosin. Under the electron microscope, fibers of mature collagen have frecuent cross bandings. Elastic fibers are also present, and these are considerably more compact than collagen fibers. They stain weak pink in sections stained with hematoxylin and eosin, brown with orcein, and blue-purple with resorcin-fuchsin. In electron micrographs, elastin fibers typically appear more stained (darker) at the periphery and are embedded in a substance containing thinner elastin filaments.

The epineurium of some nerves contains a considerable amount of fat, as is the case with the sciatic nerve. The common peroneal and tibial nerves, however, contain less fat than the sciatic nerve, and usually the former contains less fat than the latter.⁸

Seen under the microscope, intraneural adipose cells resemble honeycombs, with empty vacuoles due to fat dissolution, during the fixation process²² (Figure 5–21).

Mast cells are distributed throughout connective tissue and are often located in the proximity of small blood vessels.

Vasa nervorum supplying peripheral nerves arise from branches of regional arteries. Branches from these arteries enter the epineurium to form an vascular plexus (Figures 5–22 and 5–23). From the plexus, vessels penetrate the perineurium and enter the endoneurium as arterioles and capillaries. In nerves consisting of several fascicles, arteries, veins, and lymphatics run longitudinally and parallel to nerve fascicles.

Epineurium also projects longitudinal “ondulations” along its trajectory, providing elasticity especially in nerves supplying innervation to the extremities.

PARANEURAL SHEATHS AND COMMON EPINEURAL SHEATH

Whereas gross anatomy descriptions of distal peripheral nerves accurately identify each connective layer enclosing axons (endoneurium), nerve fascicles or bundles of axons (perineurium), and single peripheral nerves (epineurium), this becomes more complex where the connective tissue binds more than one nerve. An example of this is the sciatic nerve at the popliteal fossa. Various terms, such as paraneurium, paraneural sheaths,^23,24 common epineural sheath,²⁵ conjuctiva nervorum,²⁶ or adventitia,²⁷ are interchangeably used to refer to the same connective tissue.

Small nerves formed by a single group of fascicles feature a layer of perineurium enclosing each fascicle along with scarce amounts of adipose tissue. A connective tissue known as epineurium composed of collagenous fibers encloses the nerve.³ Specific techniques enable identification of perineurium using staining methods positive for EMA (epithelial membrane antigen) and of collagen with Masson trichrome and EMA-negative stain.³

Similarly, staining techniques aid in the identification of structures in more complex nerves, such as the sciatic nerve, where groups of variable numbers of fascicles are present. In these types of nerves, EMA staining reveals that perineurium encloses each nerve fascicle, as opposed to connective tissue made up by collagen fibers typically present in epineurium that encloses groups of fascicles (detected with Masson trichromic stain).³ Microscopic analysis of complex nerve structures such as the sciatic nerve at increasingly proximal locations shows that nerve branches within these nerve structures appear divided by their respective epineurial layers even before physical branch division materializes.

Each peripheral nerve at both plexus and terminal sites is encircled by concentric clusters of fat tissue, which appear just prior to the division of the nerve (Figure 5–24 and 5–25). The adipose tissue extends alongside its collateral or terminal branches. The amount and shape of fat tissue vary along the nerve, progressively losing their concentric contour and becoming unevenly distributed. The collagen layer, similar to the epineurium, that wrapps together the nerve components and the adipose tissue in between has been refered to as paraneurium, paraneural sheath, common epineural sheath, conjuctiva nervorum or adventitia by different authors.

In clinical practice, ultrasound-guided injection of local anesthetics enables the indirect identification of paraneurium as the space between this layer and the nerve expands displaying a concentric shape. Neural layers surrounding fascicles, groups of fascicles, nerves, as well as more complex nerve structures have similar morphology, and they are mainly composed of collagen fibers. Therefore, it may seem reasonable to unify terminology noting that present denominations based on the anatomic location of each neural fascia seems rather confusing.^28,29 However, epineurium and paraneurium may best to avitted terms to avoid the present confusion. Both epineurium and paraneurium have similar functions, which include insulation and protection of nerves from injury. Paraneural compartments facilitate longitudinal displacement of nerves controlling body movement. This movement is necessary to neutralize lateral compression by changing their shape.³⁰ If the tissue is exposed to external irritation, it reacts, leading to interfascicular fibrosis.³⁰

In relation to the anatomic features of the sciatic nerves, Andersen et al²⁴ found the sheath surrounding the sciatic nerve to be a thin transparent structure that was clearly distinctive, both macroscopically and microscopically different from the epineurium. The sheath facilitated the spread of the injectate along the nerve. However, its projections did not completely encircle the nerve. The internal layers of the paraneurium around the sciatic nerve³¹ had a similar structure of that sheath. Vloka et al²⁵ used the term common epineural sheath. Tran et al³² compared the effectiveness of the sciatic nerve block in relation to what they called “subepineural” injection at the “bifurcation,” which fills a common paraneural compartment shared by tibial and peroneal nerves close to their macroscopic division.

Orebaugh et al³³ reported that needle-tip placement in the interscalene region and injection of anesthetic solution took place frequently inside the epineurium. This occurred in approximately 50% of the nerve blocks without evidence of fascicular or axonal damage and no trace of dye within fascicles to suggest that the needle had traversed them.³³

Spinner et al³⁴ demonstrated that intraepineurial injection of dye results in its dissection along paths of least resistance, suggesting the presence of anatomic constraints among epineurial compartments. When Spinner injected the inner epineurium, the dye expanded within the same compartment but did not cross over or extend to the common external epineurial space. Therefore, the concept of “intraneural injection” should be revised for each nerve examined, avoiding extrapolations based on studies of a single nerve^35,36,37 due to the considerable anatomic variability among peripheral nerves.

PERIPHERAL NERVE BLOCKS

Diffusion of anesthetic into the axons is influenced by the presence and characteristics of the connective tissue sheaths (eg, perineurium, myelin) and the size and location of the axons inside fascicles. During intravenous peripheral anesthesia (Bier block), the local anesthetic most likely reaches the peripheral nerve endings through the intraneural capillary network.

Perineurium and endoneural capillary endothelium protect the axons from foreign substances thanks to their tight junctions between endothelial cells and among perineural cells.

Local anesthetic injected outside the epineurium of a nerve must traverse both the epineurium and perineurium to reach the axons. Subsequently, only a small proportion of the injected anesthetic comes in direct contact with axons leading to delayed onset, incomplete or failed neural blockade. For instance, Popitz and collaborators³⁸ injected 1% lidocaine into the sciatic nerve of rats and found that when the block was complete, the intraneural amount of local anesthetic was around 1.6% of the injected dose.

SUMMARY

The composition and arrangement of the connective tissue of peripheral nerves play a major role in protection of the peripheral nerves and in the practice of regional anesthesia. Characteristics and variability of the connective tissue may substantially influence the spread of the local anesthetic during nerve block injection and therefore the dynamics and quality of the neural blockade.

REFERENCES