When you suffer an attack of nerves you’re being attacked by the nervous system.
What chance has a man got against a system?
Russell Hoban (1925-2011), American writer and illustrator
The spine functions to protect the delicate sensory and motor nerves that allow for peripheral sensations and body movement. Sensory or neurologic loss can be a result of injury or pathologic abnormalities of any of the many areas that constitute the normal anatomy of this region (Fig. 4.1).
FIG. 4.1 Posttraumatic fractures of the thoracic spine.
OBJECTIVES
• Identify the structures of a typical vertebra.
• Identify the atypical structures of the atlas and axis, thoracic vertebrae, sacrum, and coccyx.
• Identify and explain the function of the spinal ligaments.
• Define the action of and identify the muscle groups of the spine.
• Describe the components of the spinal cord and spinal nerves.
• Describe the four plexuses of the spinal cord, and list the structures they innervate.
• Identify the vasculature of the spine and spinal cord.
VERTEBRAL COLUMN
The vertebral column is a remarkable structure that supports the weight of the body, helps maintain posture, and protects the delicate spinal cord and nerves. It is made up of 33 vertebrae, which can be separated into cervical, thoracic, lumbar, sacral, and coccygeal sections. Curvatures associated with the vertebral column provide spinal flexibility and distribute compressive forces over the spine. The cervical and lumbar sections convex forward, creating lordotic curves, and the thoracic and sacral sections convex backward, creating kyphotic curves (Fig. 4.2).
Kyphosis is a spinal disorder in which an excessive convex curvature or forward rounding of the thoracic spine occurs. In more severe situations, the kyphosis can progress over time, causing an exaggerated hunchback, which can lead to compression of the spinal cord and resulting neurologic symptoms. Kyphosis can occur at any age but most commonly occurs in older women as a result of osteoporosis. Other causes of kyphosis in the elderly include degenerative arthritis and ankylosing spondylitis that can lead to a collapse of the anterior vertebral bodies. Three types of kyphosis found in children are postural kyphosis, Scheuermann's kyphosis, and congenital kyphosis. Causes include weakening of the paraspinous muscles and ligaments, abnormal wedging of the vertebral bodies, and abnormal development of the vertebrae during development before birth.
An abnormal lateral curvature of the spine is termed scoliosis. Scoliosis can occur from congenital bony abnormalities of the spine present at birth, growth abnormalities during adolescence, and degenerative spinal changes or injury that occurs during adulthood. Scoliosis is rarely painful, and small curves do not require treatment; however, larger degrees of scoliosis may require the wearing of a brace or surgical interventions to restore normal posture.
Vertebrae vary in size and shape from section to section, but a typical vertebra consists of two main parts: the body (anterior element) and the vertebral arch (posterior element). The cylindrical body is anteriorly located and functions to support body weight (Figs. 4.3 and 4.4). The size of the vertebral bodies progressively increases from the superior to the inferior portion of the spine. The compact bone on the superior and inferior surfaces of the body is called the vertebral end plate (Figs. 4.5 and 4.6). Located posteriorly is the ringlike arch that attaches to the sides of the body, creating a space called the vertebral foramen (Figs. 4.3 and 4.4). The succession of the vertebral foramina forms the vertebral canal, which contains and protects the spinal cord. The vertebral arch is formed by pedicles (2), laminae (2), the spinous process (1), transverse processes (2), and superior (2) and inferior (2) articular processes (Figs. 4.3-4.6). The two pedicles project from the body to meet with two laminae, which continue posteriorly and medially to form a spinous process. The transverse processes project laterally from the approximate junction of the pedicle and lamina (Figs. 4.3 and 4.4). On the upper and lower surfaces of the pedicles is a concave surface termed the vertebral notch (Figs. 4.5 and 4.6). When the superior and inferior notches of adjacent vertebrae meet, they form intervertebral foramina, which allow for the transmission of spinal nerves and blood vessels (Fig. 4.6). Four articular processes, two superior and two inferior, arise from the junctions of the pedicles and laminae to articulate with adjacent vertebrae and form the zygapophyseal joints (facet joints). These joints give additional support and allow movement of the vertebral column (Figs. 4.7-4.10). The pars interarticularis is the small bony segment that joins the superior and inferior facet joints. It is the weakest portion of the vertebrae, making it the most vulnerable to injury (Figs. 4.21, 4.31, and 4.32).
The vertebral bodies are separated by shock-absorbing cartilaginous intervertebral disks. These disks consist of a central mass of soft, semigelatinous material called the nucleus pulposus and a firm outer portion termed the annulus fibrosus. The nucleus pulposus contains up to 85% water at birth, which gradually decreases as a person ages. The fibers of the annulus fibrosis surround and help contain the nucleus pulposus. They attach to the anterior and posterior longitudinal ligaments, which help increase the stability of the spine (Figs. 4.9-4.12).
Cervical Vertebrae
There are seven cervical vertebrae that vary in size and shape. The cervical vertebrae are the most mobile section of the spine. Within the transverse process of each cervical vertebra is a transverse foramen (Figs. 4.13 and 4.14). These foramina allow passage of the vertebral arteries and veins as they ascend to and descend from the head. The first cervical vertebra is termed the atlas because it supports the head; its large superior articular processes articulate with the occipital condyles of the cranium to form the atlantooccipital joint. The atlas is a ringlike structure that has no body and no spinous process. It consists of an anterior arch, posterior arch, and two large lateral masses (Figs. 4.13-4.15). The lateral masses provide the only weight-bearing articulation between the cranium and vertebral column.
The second cervical vertebra, the axis, has a large odontoid process (dens) that projects upward from the superior surface of the vertebral body. The odontoid process projects into the anterior vertebral foramen and articulates with the anterior arch of the atlas to act as a pivot for rotational movement of the atlas (Figs. 4.144.19). Lateral to the odontoid process on the upper surface of the body are the superior articular processes, on which the atlas articulates at the atlantoaxial joint (Figs. 4.15-4.17 and 4.19-4.21). The spinous process of the axis is the first projection to be felt in the posterior groove of the neck.
The cervical vertebrae C3-C6 have a unique configuration with their bifid spinous processes (Figs. 4.22 and 4.23). The seventh cervical vertebra (vertebra prominens) has a long spinous process that is typically not bifid. This spinous process is easily palpable posteriorly at the base of the neck (Figs. 4.24-4.26). Also unique to the C3-C7 vertebrae are the uncinate processes. These hook-shaped projections are located bilaterally on the superior end plates of the vertebral bodies where the lateral edges curve upward. The uncinate processes help to prevent lateral movement of the cervical vertebrae (Figs. 4.15, 4.17, 4.20, and 4.22).
FIG. 4.20 3D CT of cervical vertebrae, anterior view.
FIG. 4.21 Lateral view of C2 (axis).
FIG. 4.22 Superior view of cervical vertebra with bifid spinous process.
Thoracic Vertebrae
Twelve vertebrae make up the thoracic section. They have typical vertebral configurations except for their four characteristic costal facets (demi-facets), two located on the body and two located on the transverse processes, that articulate with the ribs. The head of the rib articulates with the vertebral bodies at the costovertebral joints, whereas the tubercle of the ribs articulates with the transverse processes at the costotransverse joints. The spinous processes of the thoracic vertebrae are typically long and slender, projecting inferiorly over the vertebral arches of the vertebrae below (Figs. 4.7, 4.25, 4.27-4.29).
Lumbar Vertebrae
The lumbar section typically consists of five vertebrae. Their massive bodies increase in size from superior to inferior (Fig. 4.30). The largest of the lumbar vertebrae, L5, is characterized by its massive transverse processes. The entire weight of the upper body is transferred from the fifth lumbar vertebra to the base of the sacrum across the L5-S1 intervertebral disk (Figs. 4.9, 4.10, and 4.314.35).
A unilateral or bilateral stress fracture or defect of the pars interarticularis is termed spondylolysis. Spondylolysis is most common at the L5 level and is a frequent cause of low back pain in adolescent athletes. Bilateral spondylolysis may result in spondylolisthesis, which is an anterior slippage of one vertebra over another. When the pars interarticularis of C2 is fractured, it is usually the result of hyperextension of the head and is commonly called a hangman's fracture.
FIG. 4.28 Axial CT of thoracic vertebra.
Sacrum and Coccyx
The sacral section consists of five vertebrae that fuse to form the sacrum. Their transverse processes combine to form the lateral masses (alae), which articulate with the pelvic bones at the sacroiliac joints. Located within the lateral masses are the sacral foramina that allow for the passage of spinal nerves (Figs. 4.30, 4.36, and 4.37). The lateral masses provide the lateral boundaries of the sacral canal, which is a continuation of the vertebral canal. The triangular-shaped sacral canal terminates at the sacral hiatus and communicates with the first four pairs of sacral foramina for the passage of the S1-S4 nerve roots (Fig. 4.39). The first sacral segment has a prominent ridge located on the anterior surface of the body termed the sacral promontory (Figs. 4.38 and 4.39). This bony landmark is used to separate the abdominal cavity from the pelvic cavity. The spinous process of the fifth sacral segment is absent, leaving an opening termed the sacral hiatus (Fig. 4.2). Located at the sides of the sacral hiatus are the inferior articular processes of the fifth sacral segment, which project downward as the sacral cornu. Located inferior to the fifth sacral segment is the coccyx, which consists of three to five small fused bony segments (Figs. 4.36 and 4.38). Superior projections off the first coccygeal segment, called cornua, have ligamentous attachments to the sacral cornu that provide additional stability to the articulation between the sacrum and coccyx. The coccyx represents the most inferior portion of the vertebral column.
LIGAMENTS
There are several ligaments and membranes of the spine that serve to connect the cervical vertebrae and the cranium to provide mobility and protection for the head and neck. The apical ligament is a midline structure that connects the apex of the odontoid process to the inferior margin of the clivus (Figs. 4.40-4.42). The alar ligaments are two strong bands that extend obliquely from the sides of the odontoid process and upward to the lateral margins of the occipital condyles to limit rotation and flexion of the head (Figs. 4.43 and 4.44). The transverse ligament extends across the vertebral foramen of Cl to form a sling over the posterior surface of the odontoid process. It has a small band of longitudinal fibers that ascend to attach to the posteroinferior aspect of the clivus and inferiorly to attach to the body of the axis. The transverse ligament holds the odontoid process of C2 against the anterior arch of Cl (Figs. 4.43-4.46). The transverse ligament is sometimes called the cruciform ligament because of its crosslike appearance when viewed in the coronal plane.
In addition to the ligaments listed previously, the stability of the suboccipital region of the spine is reinforced with the atlantooccipital and tectorial membranes. The atlantooccipital membrane consists of an anterior and posterior portion, which serve to connect the arches of the atlas with the occipital bone. The anterior atlantooccipital membrane passes from the anterior arch of the atlas and connects to the base of the occipital bone at its anterior margin (Fig. 4.40). This ligament is the superior extension of the anterior longitudinal ligament. The posterior atlantooccipital membrane extends from the posterior arch of Cl to the occipital bone, closing the posterior portion of the vertebral canal between the cranium and C1 (Figs. 4.40-4.42). The tectorial membrane is a broad ligament that extends from the clivus of the occipital bone to the posterior body of the axis, covering the dens, transverse, apical, and alar ligaments. The tectorial membrane forms the anterior boundary of the vertebral canal and is continuous with the posterior longitudinal ligament (Figs. 4.40-4.42).
Another important ligament of the cervical region is the ligamentum nuchae, which serves as an attachment point for muscles in the posterior portion of the neck. This expansive ligament extends from the external occipital protuberance of the cranium to the spinous processes of the cervical vertebrae (Figs. 4.40-4.42). The ligamentum nuchae continues inferiorly as the supraspinous ligament. The supraspinous ligament is a narrow band of fibers that runs over and connects the tips of the spinous processes from the seventh cervical vertebra to the lower lumbar vertebrae. The interspinous ligaments extend between adjacent spinous processes throughout the spinal column (Figs. 4.47-4.49).
Several ligaments enclose the vertebral column to help protect the spinal cord and maintain the stability of the vertebral column. Two of the larger ligaments are the anterior and posterior longitudinal ligaments (Fig. 4.47). The anterior longitudinal ligament is a broad fibrous band that extends downward from Cl along the entire anterior surface of the vertebral bodies to the sacrum.
This ligament connects the anterior aspects of the vertebral bodies and intervertebral disks to maintain the stability of the joints and to help prevent hyperextension of the vertebral column. It is thicker in the thoracic region than in the cervical and lumbar regions, providing additional support to the thoracic spine. The posterior longitudinal ligament is narrower and slightly weaker than the anterior longitudinal ligament. It lies inside the vertebral canal and runs along the posterior aspect of the vertebral bodies (Figs. 4.47-4.53). Unlike the anterior longitudinal ligament, the posterior longitudinal ligament is attached only at the intervertebral disk and adjacent margins. It is separated from the middle of each vertebra by epidural fat, which provides passage of the basivertebral veins. The posterior longitudinal ligament runs the entire length of the vertebral column beginning at C2. This ligament helps prevent posterior protrusion of the nucleus pulposus and hyperflexion of the vertebral column.
The ligamenta flava are strong ligaments (consisting of yellow elastic tissue) present on either side of the spinous processes. They join the laminae of adjacent vertebral arches, helping to preserve the normal curvature of the spine (Figs. 4.47-4.53).
FIG. 4.53 Axial, Tl-weighted MRI of lumbar vertebra with spinal ligaments.
MUSCLES
The muscles of the back can be separated into three groupings or layers: the superficial layer (splenius muscles), the intermediate layer (erector spinae group), and the deep layer (transversospinal group). The muscle groups that run the length of the spine can be divided into regions according to their location: capitis, cervicis, thoracis, and lumborum (Table 4.1).
TABLE 4.1 Spinal Muscles
|
Muscle |
Origin |
Insertion |
|
Splenius Splenius capitis Splenius cervicis |
Nuchal ligament and spinous processes of C7-T4 Spinous processes of T3-T6 |
Mastoid process of temporal bone and lateral aspect of occipital bone Transverse processes of C1-C3 or C4 |
|
Erector Spinae Iliocostalis Iliocostalis cervicis Iliocostalis thoracis Iliocostalis lumborum Longissimus Longissimus capitis Longissimus cervicis Longissimus thoracis Spinalis Spinalis capitis Spinalis cervicis Spinalis thoracis |
Broad tendon arising from posterior iliac crest, sacrum, spinous processes of sacrum and inferior lumbar spine, and supraspinous ligament Broad tendon arising from posterior iliac crest, sacrum, spinous processes of sacrum and inferior lumbar spine, and supraspinous ligament Broad tendon arising from posterior iliac crest, sacrum, spinous processes of sacrum and inferior lumbar spine, and supraspinous ligament |
Fibers run superiorly to cervical transverse processes of C4 to C7 and angles of lower ribs, 7-12 Fibers run superiorly to mastoid process of temporal bone and to transverse processes of thoracic and cervical vertebrae and just medial to the angles of the lower ribs Fibers run superiorly to occipital bone and spinous processes of upper thoracic and cervical spine May blend with semispinalis capitis muscle |
|
Transversospinal Semispinalis |
||
|
Semispinalis capitis Semispinalis cervicis Semispinalis thoracis |
Transverse processes of cervical and thoracic spine |
Fibers span 4-6 vertebral segments, running superomedially to occipital bone and spinous processes in cervical and thoracic spine |
|
Multifidus |
||
|
Sacrum, ilium, transverse processes of T1-L5 and articular processes of C4-C7 |
Fibers span 4-6 vertebral segments, running superomedially to spinous processes |
|
|
Rotatores |
||
|
Transverse processes of vertebrae, well developed in thoracic spine |
Fibers run superomedially and attach to junction of lamina and transverse processes on same vertebra or spinous processes of vertebrae above their origin |
|
Superficial Layer
The splenius muscles are located on the lateral and posterior aspect of the cervical and upper thoracic spine. These bandage-like muscles originate on the spinous processes of C7-T6 and the inferior half of the ligamentum nuchae. They are divided into a cranial portion, the splenius capitis, which inserts on the mastoid process of the temporal bone and on the lateral aspect of the occipital bone, and a cervical portion, the splenius cervicis, which inserts on the transverse processes of C1-C3 (Figs. 4.54-4.58). Together they act to extend the head and neck.
Intermediate Layer
The intermediate muscle group, the erector spinae muscle group, consists of massive muscles that form a prominent bulge on each side of the vertebral column. The erector spinae muscle group is the chief extensor of the vertebral column and is arranged in three vertical columns, the iliocostalis layer (lateral column), longissimus layer (intermediate column), and the spinalis layer (medial column) (Figs. 4.58 and 4.59). This muscle group arises from a common broad tendon from the posterior part of the iliac crest, sacrum, and inferior lumbar spinous processes. The iliocostalis muscles run superiorly to attach to the angles of the lower ribs and transverse processes of C7-C4. The longissimus muscles run superiorly to insert into the tips of the transverse processes of the thoracic and cervical regions, medial to the angles of the lower ribs, and the mastoid process. The narrow spinalis muscle group extends from the spinous processes of the upper lumbar and lower thoracic regions to the spinous processes of the superior thoracic spine, cervical spine, and occipital bone (Figs. 4.58-4.63). The spinalis muscles are most prominent in the thoracic region and may be absent or blend with the semispinalis muscles in the cervical region.
Deep Layer
The transversospinal muscles consist of several short muscles that are located in the groove between the transverse and spinous processes of the vertebrae. They can be separated into the semispinalis, multifidus, and rotatores and have a primary function to flex and rotate the vertebral column (Figs. 4.59-4.64). The semispinalis muscles arise from the thoracic and cervical transverse processes and insert on the occipital bone and spinous processes in the thoracic and cervical regions. The semispinalis muscles form the largest muscle mass in the posterior portion of the neck. The multifidus muscles consist of many fibrous bundles that extend the full length of the spine and are the most prominent in the lumbar region. The deepest of the transversospinal muscles are the rotatores, which connect the lamina of one vertebra to the transverse process of the vertebra below. They are best developed in the thoracic region.
Two additional muscles that are commonly visualized in the lumbar region of the spine are the quadratus lumborum and the psoas muscles, which are considered abdominal muscles (Figs. 4.65-4.68). Further information on these muscles can be found in Chapter 7.
SPINAL CORD
Spinal Meninges
Throughout its length, the delicate spinal cord is surrounded and protected by cerebrospinal fluid, which is contained in the thecal sac formed by the spinal meninges (Fig. 4.69). The spinal meninges are continuous with the cranial meninges and can be broken into the same three layers: dura, arachnoid, and pia. The dura mater is the tough outer layer that extends to approximately the level of S2, creating the thecal sac (Figs. 4.29, 4.49, and 4.694.72). The anterior thecal sac adheres to the posterior longitudinal ligament and is separated from the vertebral column by an epidural space that contains fat and vessels. Each spinal nerve is surrounded by dura mater that extends through the intervertebral foramen called the dural nerve root sleeve. The arachnoid mater is the thin transparent membrane that is attached to the inner surface of the dura mater. A potential space called the subdural space runs between the arachnoid and dura mater. The arachnoid mater is connected to the pia mater by numerous delicate strands, creating the spiderlike appearance associated with the arachnoid mater. The space between the arachnoid mater and pia mater is the subarachnoid space, which is filled with cerebrospinal fluid and the blood vessels that supply the spinal cord (Figs. 4.69-4.74). The pia mater is a highly vascular layer that closely adheres to the spinal cord. At the distal end of the spinal cord, approximately LI, the pia mater continues as a long, slender strand called the filum terminale. The filum terminale descends through the subarachnoid space to the inferior border of the thecal sac, where it is reinforced by the dura mater. After leaving the thecal sac, it eventually exits the sacral canal through the sacral hiatus and attaches to the coccyx, providing an anchor between the spinal cord and the coccyx (Figs. 4.49, 4.69, and 4.70). In addition, lateral extensions of the pia mater leave the spinal cord to form pairs of denticulate ligaments, which attach to the dura, preventing lateral movement of the spinal cord within the thecal sac. The denticulate ligaments run between the ventral and dorsal nerve roots within the spinal column (Fig. 4.75).
After producing chickenpox, the herpes zoster virus can lie dormant within the ventral horns of the spinal cord for years. When reactivated, the virus attacks the dorsal roots of peripheral nerves, producing a painful rash with a distribution that corresponds to the affected sensory nerve. This condition is termed shingles.
Spinal Cord and Nerve Roots
The spinal cord functions as a large nerve cable that connects the brain with the body. It begins as a continuation of the medulla oblongata at the inferior margin of the brainstem and extends to approximately the first lumbar vertebra. The spinal cord tapers into a cone-shaped segment called the conus medullaris (Figs. 4.69-4.72 and 4.76-4.79). The conus medullaris is the most inferior portion of the spinal cord and is located at approximately the level of T12-L1. At the termination of the spinal cord, nerves continue inferiorly in bundles. This grouping of nerves has the appearance of a horse’s tail and is termed the cauda equina, which exits through the lumbosacral foramina (Figs. 4.69, 4.76, 4.77, 4.80, and 4.81).
The spinal cord retains the anterior and posterior fissures of the medulla oblongata, which extend along the length of the spinal cord. The anterior median fissure extends into the spinal cord at an average depth of 3 mm. The posterior median sulcus is shallower but together with the anterior median fissure separates the spinal cord into symmetric right and left halves. There is a shallow depression on each side of the posterior median sulcus called the posterolateral sulcus. These depressions mark the location where the ventral nerve rootlets enter the spinal cord (Figs. 4.73 and 4.91).
The spinal cord is composed of white and gray matter. The white matter (myelinated axons) comprises the external borders of the cord and is more abundant. The gray matter is composed of nerve cells and runs the entire length of the cord. It is centrally located and surrounds the central canal, which contains cerebrospinal fluid and is continuous with the ventricles of the brain (Figs. 4.73, 4.82, and 4.91). In cross-section, the gray matter has the appearance of a butterfly. The two posterior projections are the dorsal horns, and the two anterior projections are the ventral horns (Fig. 4.73). The dorsal horns contain neurons and sensory fibers that enter the cord from the body periphery via the dorsal roots at the posterolateral sulcus. These are called the afferent (sensory) nerve roots (Figs. 4.74, 4.78, 4.79, 4.88, and 4.91). The dorsal root ganglion, an oval enlargement of the dorsal root that contains the nerve cell bodies of the sensory neurons, is located in the intervertebral foramen (Figs. 4.73 and 4.84-4.91). The ventral horns contain the nerve cell bodies of the efferent (motor) neurons. The efferent (motor) nerve roots exit the spinal cord via the ventral root to be distributed throughout the body (Figs. 4.78, 4.79, 4.83, 4.84, and 4.88). Just outside the intervertebral foramina, the ventral and dorsal roots unite to form the 31 pairs of spinal nerves. Of these nerve pairs, 8 correspond to the cervical region, 12 belong to the thoracic section, 5 correspond to the lumbar region, 5 correspond to the sacrum, and 1 belongs to the coccyx (Figs. 4.76, 4.88, and 4.91-4.93). Each spinal nerve provides a specific cutaneous distribution that can be demonstrated on a dermatome map (Fig. 4.94).
PLEXUSES
The spinal cord is enlarged in two regions by the cell bodies of nerves that extend to the extremities. The cervical enlargement extends from the vertebral bodies of approximately C3-C7, and the lumbosacral enlargement occurs within the lower thoracic region. Cross-sectional images of the spinal cord at various levels show considerable differences in size and shape because of the changing proportion of gray and white matter (Figs. 4.91 and 4.95-4.100).
Shortly after emerging from the intervertebral foramen, each nerve divides into dorsal and ventral rami, which contain both motor and sensory fibers (Fig. 4.101). The dorsal rami of all spinal nerves extend posteriorly to innervate the skin and muscles of the posterior trunk. The ventral rami of T2-T12 pass anteriorly as the intercostal nerves to supply the skin and muscles of the anterior and lateral trunk. The ventral rami of all other spinal nerves form complex networks of nerves called plexuses. These plexuses serve the motor and sensory needs of the muscles and skin of the extremities. The four major nerve plexuses are the cervical, brachial, lumbar, and sacral (Fig. 4.102).
Cervical Plexus
The cervical plexus arises from the upper four ventral rami of C1-C4 to innervate the neck, the lower part of the face and ear, the side of the scalp, and the upper thoracic area. The major motor branch of this plexus is the phrenic nerve, which is formed by the branches of C3, C4, and the upper division of C5. This nerve descends vertically down the neck and passes into the superior thoracic aperture, where it continues inferiorly to the diaphragm (Figs. 4.102-4.106).
The phrenic nerve, which innervates the diaphragm, is formed by motor fibers from C3-C5. A primary danger of a broken neck is that an injury at or above the level of C4 may result in paralysis of respiratory muscles, resulting in breathing difficulties and impaired speech production.
Brachial Plexus
The brachial plexus is a large, complex network of nerves arising from the five ventral rami of C5-C8 and T1. The brachial plexus is located posterior to the subclavian artery as it courses toward the axillary region of the shoulder (Figs. 4.107-4.109). The roots of the brachial plexus emerge between the anterior and middle scalene muscles to form three trunks: superior, middle, and inferior. The trunks continue laterally and inferiorly to form three cords just posterior to the clavicle. The cords extend through the axilla to form five terminal branches: the musculocutaneous, axillary, median, radial, and ulnar nerves. These nerves provide innervation for the muscles of the upper extremity and shoulder, with the exception of the trapezius and levator scapula muscles (Figs. 4.107-4.115).
Lumbar Plexus
The lumbar plexus consists of six nerves arising from the ventral rami of T12 and L1-L4. The lumbar plexus is situated on the posterior abdominal wall, between the psoas major muscle and the transverse processes of the lumbar vertebrae. In general, it serves the lower abdominopelvic region and anterior and medial muscles of the thigh. The femoral nerve is the largest branch of the lumbar plexus descending beneath the inguinal ligament (Fig. 4.117). At the level of the lesser trochanter, the femoral nerve divides into several branches, the largest being the saphenous nerve, which descends along the medial aspect of the leg to the ankle, accompanied by the great saphenous vein. The saphenous nerve innervates the anterior lower leg, some of the ankle, and part of the foot (Figs. 4.116-4.119).
Paraplegia will result from transection of the spinal cord between the cervical and lumbosacral enlargements. Quadriplegia will result if the transection occurs above the level of C3.
Sacral Plexus
Arising from L4-L5 and S1-S4, the nerves of the sacral plexus innervate the buttocks, posterior thigh, and feet. These nerves converge toward the inferior sacral foramina to unite into a large, flattened band. Most of this nerve network continues into the thigh as the sciatic nerve, which is the largest nerve in the body. The sciatic nerve exits the pelvis through the greater sciatic foramen and continues to descend vertically along the posterior thigh. In its course, it divides into the tibial and peroneal nerves, which innervate the posterior aspect of the lower extremity. The sacral plexus lies against the posterolateral wall of the pelvis between the piriformis muscle and internal iliac vessels, just anterior to the sacroiliac joint (Figs. 4.116, 4.117, and 4.120-4.124).
FIG. 4.116 Anterior view of lumbar and sacral plexuses.
FIG. 4.117 Anterior and posterior views of lumbar and sacral plexuses.
VASCULATURE
Spinal Arteries
The spinal cord is supplied by the anterior spinal artery, paired posterior spinal arteries, and a series of spinal branches. The anterior spinal artery is formed, just caudal to the basilar artery, by the union of two small branches of the vertebral arteries (Fig. 4.125). It runs the entire length of the spinal cord in the anterior median fissure and supplies the anterior two-thirds of the spinal cord (Figs. 4.125-4.127). The posterior spinal arteries arise as small branches of either the vertebral or the posterior inferior cerebellar arteries. They descend along the dorsal surface of the spinal cord and supply the posterior one-third of the spinal cord (Fig. 4.128). There exist frequent anastomoses joining the two posterior spinal arteries with each other and with the anterior spinal artery.
Arising from the posterior aspect of the descending aorta are segmental arteries that supply the vertebral column and spinal cord. They are termed the intercostal arteries in the thoracic region and lumbar arteries in the lumbar region. These vessels extend toward the intervertebral foramen, where they divide into spinal branches (Figs. 4.128-4.131). After giving off an anterior and posterior branch to the walls of the vertebral column, the spinal branches divide into anterior and posterior radicular arteries that pass along the ventral and dorsal roots into the spinal cord (Fig. 4.128). The anterior radicular arteries contribute blood to the anterior spinal artery, and the posterior radicular arteries contribute blood to the posterior spinal arteries. The largest of the radicular arteries is the great anterior radicular artery (artery of Adamkiewicz), which arises in the lower thoracic and upper lumbar region, typically between T12 and L3 (Figs. 4.125 and 4.127). This vessel makes a major contribution to the anterior spinal artery and provides the main blood supply to the inferior two-thirds of the spinal cord.
An injury of the great radicular artery (artery of Adamkiewicz) may result in paralysis of the lower limbs because the artery provides the main blood supply to the inferior two-thirds of the spinal cord.
Spinal Veins
Veins of the Spinal Cord. The veins that drain the spinal cord follow the same segmental organization as their arterial counterparts. The central gray matter of the cord is drained by the anterior and posterior central veins located in the anterior median fissure and posterior sulcus, respectively (Fig. 4.132). The outer white matter is drained by small radial veins that encircle the spinal cord within the pia mater. The venous blood collected by these tiny veins drains into the anterior and posterior median (spinal) veins created by the longitudinal venous channels within the pia mater on the anterior and posterior surfaces of the spinal cord (Fig. 4.132). The anterior and posterior median veins drain into the anterior and posterior radicular veins that parallel the ventral and dorsal nerve roots. They eventually empty into the intervertebral veins that accompany the spinal nerves through the intervertebral foramina.
Veins of the Vertebral Column. The veins of the vertebral column form an extensive network of internal and external venous plexuses, named according to their corresponding location in the vertebral column (Figs. 4.132-4.135). The internal venous plexuses lie within the vertebral canal in the epidural space and are divided into anterior and posterior internal plexuses The valveless external venous plexuses communicate freely with the vertebral veins and intracranial venous sinuses and are located at the outer surfaces of the vertebral column. They can be divided into the anterior and posterior external plexuses. The anterior external venous plexuses run directly in front of the vertebral bodies, and the posterior external venous plexuses run along the posterior aspect of the vertebral arches (Fig. 4.133). The anterior sections of the internal and external plexuses communicate via a network of veins called the basivertebral veins, which drain the vertebral bodies. The large basivertebral veins emerge from the posterior surfaces of the vertebral bodies (Figs. 4.132, 4.133, 4.136, and 4.137). The internal and external venous plexuses, along with the radicular veins, drain into the intervertebral veins, ending in the vertebral, intercostal, lumbar, and sacral veins.
Because the vertebral venous plexuses are valveless, an increase in intra-abdominal pressure (e.g., coughing, straining) may cause backflow of blood into the basivertebral veins of the spine or dural sinuses of the brain. This creates a potential pathway for metastatic disease or other pathology to spread to the central nervous system.
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