Chapter 3. Functional Regional Anesthesia Anatomy

It is often said that the practice of regional anesthesia is the practice of applied anatomy. Indeed, the practice of regional anesthesia is inconceivable without a sound knowledge of the basic anatomic facts that pertain to the individual anesthesia techniques. However, just as surgeons rely on surgical anatomy or pathologists rely on pathologic anatomy, the anatomic information necessary for the practice of regional anesthesia must be specific to this application. In the past, many new nerve block techniques and “me-too” approaches were devised by academicians merely relying on idealized anatomic diagrams and schematics, rather then on functional anatomy. Ultimately, many of these techniques have only introduced unnecessary confusion in the field and been of negligible relevance to clinical practice. Indeed, once the anatomic layers and tissues sheets are dissected, the fully exposed nerve structures are almost irrelevant to the practice of regional anesthesia. This is because accurate placement of the needle and the spread of the local anesthetic after an injection depends 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. However, much research by regional anesthesiologists has been done in the past 10–15 years on this subject, and many myths of the past have been dispelled. The reader should note that specific anatomic discussions pertaining to individual regional anesthesia techniques are detailed in their respective chapters. The purpose of this chapter is to provide a generalized and rather concise overview of anatomy relevant to the practice of regional anesthesia. The reader is referred to Figure 3–1 for an easier orientation of the body planes discussed throughout the book.

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 a very 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 very long and slender. They are also called nerve fibers. The peripheral nerve (PN) 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 a peripheral nerve, individual axons are enveloped in a loose connective tissue, the endoneurium. The endoneurium is a delicate layer of connective tissue around each nerve that is embedded within the perineurium. Small groups of axons are closely associated within a bundle called a nerve fascicle, 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. Several fascicles together form fascicular bundles within an extensive multilaminated perineurium. The perineurium surrounds each fasciculus and splits with it at each branching point. The fascicular bundles in turn, collectively form the peripheral nerve that is embedded in loose connective tissue called the epineurium. The epineurium surrounds the entire nerve and holds it loosely to the connective tissue through which it runs. This layer also sends septa into the nerve that divide the nerve fibers into bundles (fasciculi or funiculi) of varying sizes.

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, the probability is good that the distal segment of nerves synapsing with the muscles will be sutured to the central stump of motor axons; the same is true for sensory axons. In such cases, good functional recovery is possible. 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 result in disastrous consequences as opposed to clean needle nerve cuts which tend to heal much more readily and regularly.

The connective tissue of a nerve is tough, compared with the nerve fibers themselves. The connective tissue of a nerve permits a certain amount of stretch without damage to the nerve fibers. The nerve fibers are somewhat “wavy,” and when they are stretched, the connective tissue around them is also stretched—giving it some protection. This feature, perhaps, plays a “safety” role in nerve blockade by allowing the nerves to be “pushed” rather than pierced by the advancing needle during nerve localization. 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).

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 & Peripheral Nervous Systems

The functional boundary between the central (CNS) and the peripheral nervous system (PNS) lies at the junction where oligodenodrocytes meet Schwann cells along the axons that form the cranial and spinal nerve. The PNS, as opposed to the CNS, is not surrounded by bone and is therefore accessible for neural blockade, but it is also quite susceptible to physical injury. 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. 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 through 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 (Figure 3–4).

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

Dermatomal, myotomal, and osteotomal innervation are often emphasized in regional anesthesiology texts as important for the application of nerve blocks. However, it is more practical to think in terms of which block techniques provide adequate analgesia and anesthesia for specific surgical procedures, rather than trying to match nerves and spinal segments to the dermatomal, myotomal, and osteotomal territory. Nevertheless, their description 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 3–5 and 3–6). In the head and 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 (osteotome) often does not follow the same segmental pattern as the innervation of the muscles and other soft tissues (Figure 3–8).

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. This promotes 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, 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 result in phrenic blockade (Figure 3–11).

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 (Figure 3–12). 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 innervate the region covering the clavicle. Furthermore, the supraclavicular nerves provide articular branches to the sternoclavicular and acromioclavicular joints.4

Brachial Plexus

The brachial plexus innervates muscles, joints, and the skin of the upper extremity (Table 3–2). It is formed by ventral rami of C5 to T1 (Figure 3–13). 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 subclavius.5–7

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 form 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 glenohumeral 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.

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 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. 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 it coarse, 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 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 (Figure 3–14) 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 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 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 glenohumeral 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 3–14). 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 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 3–14). 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 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 nailbeds. In addition to muscle and skin, the median nerve innervates the 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, both of which muscles it innervates. 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 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 innerve 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 Figure 3–14), 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, both of which muscles it innerves. 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 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 ring and little fingers. The innervation continues onto the nailbeds 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 the 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

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 3–15). 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 coarse. 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 intercostals 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 form the ventral rami of T1 through T11. They travel along the inferior margin of the rib of 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) (Figure 3–16). 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 (Figure 3–17). 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 coarse.

Lumbosacral Plexus

The lumbosacral plexus innervates the muscles, joints, skin, and peritoneal lining of the abdominopelvic wall11,12 (Tables 3–3 and 3–4).

It also innervates the inferior extremities. It is formed by the ventral rami of L1 to S5 (Figure 3–18). 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 (Figure 3–19).

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 (Figures 3–18 and 3–20). It innervates the muscle, skin, and parietal peritoneum along its coarse.

Ilioinguinal Nerve

The ilioinguinal nerve (see Figures 3–18 and 3–20) 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 coarse.

Genitofemoral Nerve

The genitofemoral nerve arises from the ventral rami from L1 and L2. 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.

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 form 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 obturator internus to terminate in the inferior gemellus and quadratus femoris muscles. As indicated by its name, the nerve innervates the inferior gemellus and quadratus femoris muscles.

Lateral Femoral Cutaneous Nerve

The lateral femoral cutaneous nerve arises form the posterior divisions of the ventral rami at L2 to L3. It descends the posterior abdominal wall, 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 (see Figure 3–20).

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 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 (see Box 3–6) arises from the anterior division of the ventral rami at L2 to L4 (see Figure 3–18). It descends through the pelvis medial to the psoas major muscle, crosses the superior pubic ramus inferiorly, passes through the obturator foramen into the medial compartment of the thigh where it divides into posterior and anterior branches (Figure 3–21). 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 crease-inguinal area, the nerve is positioned lateral to the femoral artery and vein (mnemonic: NAVEL) (Figure 3–22). 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 bellow the inguinal ligament consists of an anterior and a posterior part. The anterior part contains branches to the sartorius muscle 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 knee to the arch of the foot (Figure 3–23). En route, the nerve is accompanied by the saphenous vein, so it is called the saphenous branch of the femoral nerve. 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 3–18). 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 3–24). 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 (Figure 3–25).15 Posterior to the knee the sciatic nerve descends into the popliteal fossa, where it diverges into the tibial and common peroneal nerves (Figure 3–26).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 surface of the foot. The deep peroneal nerve enters the anterior compartment of the leg, where is 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

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 to the better understanding of 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 extremity. Tables 3–7 and 3–8 summarize the innervation and kinetic function of the major muscle groups of the upper (see Table 3–7) and lower (see Table 3–8) extremity.

Shoulder Joint

Innervation to the shoulder joints stems mostly from the axillary and suprascapular nerves. The skin over most medial parts of shoulder receives nerves from the cervical plexus. 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 3–27).

Elbow Joint

Nerve supply to the elbow joint includes branches of all major nerves of the brachial plexus: musculocutaneous, radial, median, and ulnar nerves (Figure 3–28).

Wrist Joint

The wrist is innervated by the radial, ulnar and median nerves (Figures 3–29 and 3–30).

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 3–31).

Knee Joint

Knee innervation is obtained from branches from the femoral, obturator, and sciatic nerves (Figure 3–32). Articular branches from the tibial and common peroneal divisions of the sciatic nerve, 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 quite 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 3–33).

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 3–34).

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 viscerae.20,21