Chapter 1. Back

The Big Picture

The skin of the back is thick, with increasing thickness toward the nape of the neck. The cutaneous innervation of the back is segmentally innervated through the dorsal rami of spinal nerves. The spinous processess of the vertebrae and other osteologic landmarks are palpable, which enable localization of spinal levels through surface landmarks.

Cutaneous Innervation

The skin of the back is segmentally innervated by cutaneous nerves that originate from the dorsal rami (Figure 1-1A). Dorsal rami contain both motor and sensory neurons as they branch from each level of the spinal cord and course posteriorly in the trunk. The motor neurons terminate in the deep back muscles (e.g., erector spinae muscles), where they cause muscle contraction. The sensory neurons, however, continue on and terminate in the skin where they provide cutaneous sensations such as pain, touch, and temperature at each dermatomal level of the back (Figure 1-1B). There is some segmental overlap of the peripheral sensory fields from adjacent dermatomes.

It should be noted that

• The dorsal ramus of C1 carries only motor neurons to the suboccipital muscles.
• The dorsal ramus of C2 carries only sensory neurons to the back of the scalp.
• The lateral aspects of the back are innervated by lateral cutaneous nerves, which are derived from the ventral rami segments of spinal nerves at each level (Figure 1-1A).
• S5 and Co 1 only carry sensory neurans.

Osteology and Surface Anatomy

The following structures are easily palpated through the skin (Figure 1-1A):

• Cervical vertebrae. The first prominent spinous process that is palpable is C7 (vertebra prominens). Cervical spines 1 to 6 are covered by the ligamentum nuchae, a large ligament that courses down the back of the neck and connects the skull to the spinous processes of the cervical vertebrae.
• Thoracic vertebrae. The most prominent spine is the T1 vertebra; other vertebrae can be easily recognized when the trunk is flexed anteriorly. Thoracic vertebrae have long spines that point downward so that each spinous process is level with the body of the inferior vertebra. The spines can be counted downward from C7 and T1, or from a line joining the iliac crests at the L4 vertebral level and then counting upward from that site.
• Lumbar vertebrae. The body of L3 is approximately at the center of the body from superior to inferior and medial to lateral. When a person is in an upright posture, most vertebral spines are not obvious because they are covered by the erector spinae muscles in the cervical and lumbar regions.
• Sacrum. The spines of the sacrum are fused together in the midline to form the median sacral crest. The crest can be felt beneath the skin in the uppermost part of the gluteal cleft between the buttocks. The sacral hiatus is situated on the posteroinferior aspect of the sacrum and is the location where the extradural space terminates. The hiatus lies approximately 5 cm above the tip of the coccyx and beneath the skin of the natal cleft.
• Coccyx. The tip of the coccyx is located in the upper part of the natal cleft and can be palpated approximately 2.5 cm posterior to the anus. The anterior surface of the coccyx can be palpated via the anal canal.
• Ilium. The crest of the ilium is located at the L4 vertebral level and is easily palpated. Regardless of the size or weight of a person, the posterior iliac spines can be palpated because they lie beneath a skin dimple at the S2 vertebral level and the middle of the sacroiliac joint.
• Scapulae. The scapulae overlie the posterior portion of the thoracic wall, covering the upper seven ribs. The superior angle can be palpated at the T1 vertebral level, the spine of the scapula at T3, and the inferior angel at T7. The acromion forms the lateral end of the scapular spine and is easily palpated.

Vertebral Curvatures

In utero, the entire vertebral column has a primary or kyphotic curvature (concave anteriorly) (Figure 1-1C). After birth, with the demands of walking, weightbearing, and gravity, the cervical and lumbar regions form secondary or lordotic curvatures (concave posteriorly) (Figure 1-1D). Primary (kyphotic) curvatures occur in the thoracic and sacral regions, where the vertebrae curve posteriorly. This allows increased space for the heart and lungs in the thorax and birth canal in the sacral region. Lordotic or secondary curvatures occur in the cervical and lumbar regions, where the vertebrae curve anteriorly.

The Big Picture

The superficial back muscles consist of the trapezius, levator scapulae, rhomboid major, rhomboid minor, and latissimus dorsi muscles (Figure 1-2A; Table 1-1). Although these muscles are located in the back, they are considered to be muscles of the upper limbs because they connect the upper limbs to the trunk and assist in upper limb movements directly or indirectly via the scapula or humerus. The superficial back muscles are located in the back region and receive most of their nerve supply from the ventral rami of spinal nerves (primarily the brachial plexus) and act on the upper limbs. These muscles are discussed in much greater detail in Section VI, Upper Limb, but are included here because a brief discussion must precede the primary discussion of the deeper muscles and structures of the back.

Trapezius Muscle

The trapezius muscle is the most superficial back muscle. It attaches to the occipital bone, nuchal ligament, spinous processes of C7–T12, scapular spine, acromion, and clavicle. The trapezius muscle has a triangular shape and has the following muscle fiber orientations:

• Superior fibers. Course obliquely from the occipital bone and upper nuchal ligament to the scapula, causing scapular elevation and upward rotation.
• Middle fibers. Course horizontally from the lower nuchal ligament and thoracic vertebrae to the scapula, causing scapular retraction.
• Inferior fibers. Course superiorly from the lower thoracic vertebrae to the scapula, causing scapular depression and upward rotation.

In addition, the multiple fiber orientation of the trapezius muscle fixes the scapula to the posterior wall of the thorax during upper limb movement. It is innervated by the spinal accessory nerve (CN XI). The superficial branch of the transverse cervical artery supplies the trapezius muscle, whereas the deep branch of the transverse cervical artery supplies the levator scapulae and rhomboid muscles. In some instances, the dorsal scapular artery will replace the deep branch of the transverse cervical artery.

Levator Scapulae Muscle

The levator scapula muscle is located deep to the trapezius muscle and superior to the rhomboids. The levator scapula muscle attaches to the cervical vertebrae and the superior angle of the scapula, causing elevation and downward rotation of the scapula. The nerve supply is from branches of ventral rami from spinal nerves C3–C4 and occasionally from C5, via the dorsal scapular nerve, and vascular supply is from the deep branch of the transverse cervical artery.

Rhomboid Major and Minor Muscles

The rhomboid minor is superior to the rhomboid major, with both positioned deep to the trapezius muscle. The rhomboid muscles attach to the spinous processes of C7–T5 and the medial border of the scapula, resulting in scapular retraction. They are innervated by the dorsal scapular nerve (i.e., the ventral ramus of C5) and the vascular supply from the deep branch of the transverse cervical artery.

Latissimus Dorsi Muscle

The latissimus dorsi is a broad, flat muscle of the lower region of the back. It attaches to the spinous processes of T7, inferior to the sacrum via the thoracolumbar fascia, and inserts laterally into the intertubercular groove of the humerus. The latissimus dorsi acts on the humerus (arm) causing powerful adduction, extension, and medial rotation of the arm. It is innervated by the thoracodorsal nerve (ventral rami of C6–C8) and receives its blood supply from the thoracodorsal artery (branch off the axillary artery).

Scapular Movements

The scapula glides over the thoracic wall, but there are no distinct anatomic joints (Figure 1-2B). Instead, muscles pull the scapula forward and backward (protraction and retraction, respectively) and up and down (elevation and depression, respectively) and rotate it so that the glenoid fossa moves superiorly (upward rotation) or inferiorly (downward rotation). Rotation of the scapula is defined by the direction that the glenoid fossa faces. For more details describing the upper limb and scapular muscles, see Section VI.

The Big Picture

The deep back muscles consist of the splenius, erector spinae, transversospinalis, and suboccipital muscles (Table 1-2). These deep back muscles are segmentally innervated by the dorsal rami of spinal nerves at each vertebral level where they attach. It is not important to know every detailed attachment for the deep back muscles; however, you should realize that these muscles are responsible for maintaining posture and are in constant use during locomotion. The erector spinae muscles course medially to superolaterally, and they extend the vertebral column and rotate the trunk ipsilaterally. The transversospinalis group course laterally to superomedially and extend the vertebral column and rotate the trunk contralaterally.

Splenius Capitis and Cervicis Muscles

The splenius capitis and splenius cervicis muscles lie superficial to the erector spinae and deep to the levator scapulae and rhomboid muscles. The splenius capitis extends and rotates the head, whereas the splenius cervicis rotates and extends the cervical spine. Both muscles, acting unilaterally (only one side contracts while the other side relaxes), produce lateral flexion of the head and neck.

Erector Spinae Muscles

From lateral to medial, the erector spinae muscles include the iliocostalis, the longissimus, and the spinalis muscles (Figure 1-3A and B).

• Acting bilaterally, the erector spinae extend the vertebral column.
• Acting unilaterally, the erector spinae laterally flex the vertebral column and rotate to the ipsilateral side.

The deep muscles of the back are involved in the control of posture; they contract intermittently during swaying movements, such as walking.

The erector spinae muscles ascend throughout the length of the back as columns composed of fascicles of shorter length. Each column is composed of a rope-like series of fascicles, with various bundles arising as others are inserting; each fascicle spans from 6 to 10 segments between bony attachments. The erector spinae muscles are segmentally innervated by dorsal rami. The motor neurons in the dorsal rami terminate in and innervate the erector spinae muscles (Figure 1-3B).

Transversospinalis Muscles

The transversospinalis muscles are deep to the erector spinae muscles. From superficial to deep, the transversospinalis muscles include the semispinalis, multifidus, and rotatores. A defining characteristic of this muscle group is that their muscle fibers ascend superomedially, crossing from one to six vertebral levels, and attach inferiorly from a transverse process and superiorly to the spinous process of neighboring vertebrae.

• Acting bilaterally, the transversospinalis group extends the vertebral column.
• Acting unilaterally, they rotate the vertebral column to the opposite side on which the transversospinalis muscle is located.

Suboccipital Muscles

The suboccipital muscles are inferior to the occipital bone and deep to the semispinalis capitis muscle (Figure 1-3C). The suboccipital muscles are as follows:

• Rectus capitis posterior major muscle
• Rectus capitis posterior minor muscle
• Obliquus capitis superior muscle
• Obliquus capitis inferior muscle

These muscles attach to the occipital bone and C1 and C2 vertebrae, and are innervated by the dorsal ramus of C1 (also known as the suboccipital nerve). The suboccipital nerve carries only motor neurons. These suboccipital muscles are mainly postural muscles, but they may also extend and rotate the head.

• The suboccipital triangle is the area in the suboccipital region between the rectus capitis posterior major and the obliquus capitis superior and inferior muscles. It is covered by a layer of dense fibro-fatty tissue, deep to the semispinalis capitis muscle. The floor is formed by the posterior occipitoatlantal membrane and the posterior arch of the C1 vertebrae (atlas). Contents of the suboccipital triangle include the following structures:
  • Vertebral artery. Exits the transverse foramen and courses along the top surface of the posterior arch of C1, prior to entering the skull via the foramen magnum. Once inside the skull, the paired vertebral arteries join to form the basilar artery and contribute to the collateral arterial supply of the brain.
  • Suboccipital nerve. Emerges between the occipital bone and the atlas, by the vertebral artery, and branches to provide motor innervation to the suboccipital muscles.
  • Greater occipital nerve (dorsal ramus of C2 nerve). Carries only sensory neurons as it emerges below the obliquus capitis inferior muscle and courses upward, piercing the semispinalis capitis and trapezius muscles to supply sensory innervation to the back of the scalp.

The Big Picture

The vertebral column consists of 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 3–4 coccygeal). These vertebrae, along with their ligaments and intervertebral discs, form the flexible, protective, and supportive vertebral column that contains the spinal cord.

Vertebral Column

The vertebral column consists of 33 vertebrae, which are divided into cervical (C), thoracic (T), lumbar (L), sacral (S), and coccygeal (Co) regions (Figure 1-4A; Table 1-3). To simplify their descriptions, each vertebra is referred to by the first letter of its region. For example, the fourth cervical vertebra is referred to as the C4 vertebra.

• Seven cervical vertebrae. The C1 vertebra, also called the “atlas,” articulates with the skull. The C2 vertebra is also called the “axis” because, through its synovial joint articulation with C1, it provides a side-to-side rotation, as in the head movement indicating “no.” A unique characteristic of cervical vertebrae is that each transverse process has its own transverse foramen for the transmission of the vertebral arteries from the subclavian artery to the brain.
• Twelve thoracic vertebrae. On each side of a thoracic vertebra, there is a rib that articulates with the vertebral body and transverse process. Thus, there are 12 pairs of ribs articulating with 12 thoracic vertebrae.
• Five lumbar vertebrae. These massive vertebrae form the support to the lower part of the back.
• Five sacral vertebrae. These vertebrae are fused into one bone called the sacrum.
• Coccygeal vertebrae. There are three to four fused coccygeal vertebrae that form the coccyx, or “tailbone.”

A Typical Vertebra

A typical vertebra consists of the following (Figure 1-4B):

• Body. The anterior vertebral region that is the primary weight-bearing component of the vertebrae.
• Vertebral arch. Arches posteriorly from the vertebral body and forms the vertebral foreman, which contains the spinal cord.
• Pedicle. Part of the vertebral arch that joins the vertebral body to the transverse process.
• Transverse processes. Extend laterally from the junction of the pedicle and lamina.
• Superior and inferior articular facets. Form synovial zygapophyseal (facet) joints with the vertebrae above and below.
• Laminae. The paired posterior segments of the vertebral arch that connect the transverse processes to the spinous process.
• Spinous processes. The posteriorly projecting tip of the vertebral arch. These processes are easily palpated beneath the skin.
• Vertebral foramen. The space in which the spinal cord, its coverings, and the rootlets of the spinal nerves lie. The series of vertebral foramina form the vertebral canal.
• Intervertebral foramina. Bilateral foramina that form the space between pedicles of adjacent vertebrae for passage of spinal nerves.

Vertebral Articulations

There are two types of articulations that enable the 33 vertebrae to not only provide support, protection, and absorb shock, but also to provide flexibility. These two articulations are the intervertebral discs and the zygapophyseal joints.

• Intervertebral discs. Located between vertebrae, from the second cervical disc down to the first sacral vertebra (Figure 1-1D). The discs permit a limited amount of movement between adjacent vertebrae. Intervertebral discs are composed of an annulus fibrosus and a nucleus pulposus. The anulus fibrosus is the tough hyaline cartilaginous rim, whereas the nucleus pulposus is the softer fibrocartilaginous core. The disc can bear weight because spread of the nucleus pulposus is constrained by the anulus fibrosus.

• Zygapophyseal joints. Synovial joints located between the superior and inferior articular facets of adjacent vertebrae. These joints provide varying amounts of flexibility.

Vertebral Ligaments and Joints

The vertebral column is stabilized by ligaments that run between vertebrae (Figure 1-4C). Some of the ligaments that limit vertebral flexion (bending forward) include the following:

• Ligamentum flavum. Connects paired laminae of adjacent vertebrae.
• Supraspinous ligament. Connects the apices of the spinous processes from C7 to the sacrum.
• Interspinous ligament. Connects adjoining spinous processes.
• Nuchal ligament. Extends from the external occipital protuberance along the spinous processes of C1–C7.
• Posterior longitudinal ligament. Courses longitudinally, down the posterior surface of the vertebral bodies within the vertebral canal. It supports the intervertebral disc posteriorly, thus reducing the incidence of herniations that may compress the spinal cord and cauda equina.
• Anterior longitudinal ligament. Courses longitudinally along the anterior surface of the vertebral bodies limiting vertebral extension.

Big Picture

The brain and spinal cord are surrounded and protected by three layers of connective tissue meninges called the dura mater, the arachnoid mater, and the pia mater (Figure 1-5A and B). The meninges are supplied by the general sensory branches of the cranial and spinal sensory nerves.

Dura Mater

The dura mater is the most superficial layer of the meninges and is composed of dense fibrous connective tissue. It forms a sheath around the spinal cord that extends from the internal surface of the skull to the S2 vertebral level where the dura mater forms the coccygeal ligament, which covers the pial filum terminale. The dura mater evaginates into each intervertebral foramen and becomes continuous with the connective tissue covering (epineurium) around each spinal nerve. The nerve roots in the subarachnoid space lack this connective tissue covering and, therefore, are more fragile than spinal nerves. The dura mater defines the epidural space and the subdural space.

Arachnoid Mater

The arachnoid mater is the intermediate meningeal layer. It is attached to the underlying pia by numerous arachnoid trabeculae. The subarachnoid space is the cavity between the arachnoid and pial layers and contains cerebrospinal fluid (CSF). CSF suspends the spinal cord, brain, and nerve roots. In addition, large blood vessels course within the subarachnoid space. The dural sac is that portion of the subarachnoid space between the conus medullaris (approximately the L1 vertebral level) and the point at which the coccygeal ligament begins (approximately the S2 vertebral level). The dural sac contains only spinal roots and the filum terminale, which are suspended in CSF (Figure 1-5C).

Pia Mater

The pia mater is the deepest meningeal layer and is inseparable from the surface of the spinal cord. It contains a plexus of small blood vessels that supply the spinal cord.

• Denticulate ligaments. Lateral extensions of the pia mater that support the entire spinal cord by attaching to the arachoid and dura mater and maintaining a centralized location of the spinal cord in the subarachnoid space. These ligaments are located in the coronal plane, between the ventral and dorsal roots, and project through the arachnoid mater to attach to the dura mater in a series of sawtooth projections. In this manner, the spinal cord is tethered to the dura mater while floating within the CSF.
• Filum terminale. An extension of the pia mater beyond the tip of the spinal cord (conus medullaris) that attaches to the coccyx in the vertebral canal. It is as if the prolongation of the pia mater were occupied by the spinal cord in utero but, as the spinal cord withdraws upward during development and maturation, the sleeve of the pia mater that surrounded the distal end of the spinal cord collapsed upon itself becoming the filum terminale.

The Big Picture

The spinal cord is a part of the central nervous system (CNS) and receives sensory input from the body tissues via spinal nerves. These messages are processed within the CNS and appropriate motor responses are sent out through spinal nerves. The spinal cord consists of both white and gray matter. The spinal cord is located within the vertebral (spinal) canal, where it is protected by bones, ligaments, and the three layers of connective tissue known as the meninges.

Topography and Overview

The spinal cord is located within the vertebral (spinal) canal and extends from the medulla oblongata at the C1 vertebral level and terminates at the conus medullaris at the L1 and L2 vertebral level (Figure 1-6A). In a newborn, the spinal cord terminates at the L3 and L4 vertebral level; in a fetus, it continues on to the sacrum. The vertebral canal serves as a physical protector to the delicate spinal cord.

In cross-section, the spinal cord consists of white matter surrounding gray matter (Figure 1-6B). The white matter consists of neuronal axons, and the myelin appears white. The gray matter consists of aggregates of neuronal cell bodies, which do not contain myelin and thus appears gray.

White Matter of the Spinal Cord

White matter is composed of vertical columns of axons arranged so that those that perform similar functions are grouped together to form a tract. These tracts are not sharply demarcated from each other and, therefore, there may be some overlap between them.

Bundles of axons in the white matter carry impulses up to the brain from sensory tracts and, conversely, bundles of axons carry impulses in the white matter down from the brain to neurons in the gray matter of the spinal cord from motor tracts.

The amount of white matter increases at each successive higher spinal segment. Cervical spinal cord levels, therefore, contain more white matter because all neurons descending from the brain inferiorly or from body tissues to the brain pass through the cervical spinal cord. As a result, the sacral spinal cord has the least amount of white matter because most ascending or descending fibers have entered or exited the spinal cord superior to the caudal spinal cord region (compare Figures 1-6B–E).

Gray Matter of the Spinal Cord

Spinal cord gray matter forms a letter “H” in cross-section and consists of the following regions:

• Ventral horn. Contains cell bodies of motor neurons innervating skeletal muscle. The ventral horns are largest in those parts of the spinal cord that serve regions of the body with many muscles. For example, the spinal cord segments C5–T1, which serve the upper limbs (brachial plexus), have a cervical spinal cord swelling, whereas L4–S3 spinal cord segments serving the lower limbs (lumbosacral plexus) form a lumbar spinal cord swelling.

• Lateral horn. The lateral horn is only present in the spinal cord between the T1 and L2 vertebral levels and contains cell bodies of preganglionic sympathetic motor neurons of the autonomic nervous system.
• Dorsal horn. Receives sensory impulses entering via the dorsal root. The thoracic and upper lumbar levels have relatively small amounts of gray matter because these vertebral levels only innervate the thoracic and abdominal regions.

The Big Picture

At each spinal cord segment, ventral and dorsal spinal roots join to form spinal nerves that bifurcate into ventral and dorsal rami. Spinal roots carry sensory (dorsal root) or motor (ventral root) neurons, whereas the spinal nerves and rami contain a mixture of sensory and motor neurons. The dorsal rami segmentally innervate deep back muscles (motor) and the skin of the back (sensory). This section of the chapter helps to differentiate among vertebral levels, spinal cord levels, and spinal nerve levels.

Spinal Roots

At each spinal cord segment, paired dorsal and ventral roots exit the lateral sides of the cord to form left and right spinal nerves. As a result of unequal growth between the vertebral canal and the spinal cord (the vertebral canal is longer than the spinal cord in adults), the nerve roots follow an oblique course from superior to inferior (Figure 1-7A).

• Only in the cervical region are the segments of the spinal cord at the same level with the corresponding cervical vertebrae. Inferior to the cervical region, each spinal nerve from the thoracic, lumbar, and sacral spinal cord segments exits inferior to its similarly numbered vertebra.
• Dorsal roots convey sensory (afferent) information from body tissues to the spinal cord (i.e., skin to spinal cord) (Figure 1-7B). The dorsal root ganglion is a swelling in the dorsal root and houses the cell bodies of all sensory neurons entering the spinal cord for that specific body segment.
• Ventral roots convey motor (efferent) information away from the spinal cord to the body tissues (i.e., spinal cord to the biceps brachii muscle).
• The spinal cord terminates at the L1 vertebral level in adults. Therefore, the lumbar and sacral nerve roots descending in the vertebral canal below the L1 vertebral level form a mass of nerve roots that resembles the tail of a horse; hence, the name cauda equina (Figure 1-7C). Because the cauda equina floats in the CSF, a needle introduced into the subarachnoid space will displace the roots with little possibility of puncture damage.

Spinal Nerves

The spinal roots unite in or near the intervertebral foramen to form a spinal nerve (Figure 1-7B and C). The 31 pairs of spinal nerves formed by the dorsal and ventral roots are organized as follows:

• Eight cervical spinal nerves. The first seven cervical spinal nerves, C1–C7, exit the vertebral canal superior to each respective cervical vertebra. The last cervical nerve, C8, exits inferior to the seventh cervical vertebra.

All of the remaining spinal nerves segmentally exit the spinal cord inferior to their respective vertebra, as follows:

• Twelve thoracic spinal nerves. Exit inferior to the 12 thoracic vertebrae.
• Five lumbar spinal nerves. Exit inferior to the five lumbar vertebrae.
• Five sacral spinal nerves. Exit inferiorly through the dorsal sacral foramina of the sacrum.
• One coccyx spinal nerve. Exits by the coccyx bone.

Rami

The spinal nerve exits the vertebral canal through the intervertebral foramen. Each spinal nerve bifurcates into a dorsal primary ramus and a ventral primary ramus (Figure 1-7B).

• Dorsal rami. Segmentally supply the skin of the back (in a dermatomal pattern) as well as provide motor innervation to the deep vertebral muscles of the median portion of the back (e.g., the erector spinae and transversospinalis muscles). The dorsal rami do not contribute to the innervation of the limbs or face.
• Ventral primary rami. Supply the dermatomes and myotomes of the anterolateral portions of the torso as well as the upper and lower limbs.
  • Note: Dorsal and ventral roots are not the same as dorsal and ventral rami. Dorsal roots convey sensory impulses, whereas ventral roots convey motor impulses. Once these roots unite to form the spinal nerve, the majority of subsequent branches, including the rami, convey both sensory and motor impulses (mixed nerves).