The Spinal Cord

Figure 9-1. Development of the spinal cord. Cross-sectional (A) and dorsal (B) views of the neural plate and the correlation of neural tube structures with the adult cord (C). The white area between the blue (posterior horn) and pink (anterior horn) of the adult spinal cord represents the approximate position of the intermediate gray. Malformations involving defects of the nerve tissue or surrounding bone include rachischisis (D), meningocele (E, solid cord) or meningomyelocele (E, dashed cord), and anencephaly with rachischisis (F). CSF, cerebrospinal fluid. (F Courtesy of Dr. Jonathan Fratkin.)

Beginning on day 21, the edges of the neural plate (neural folds) enlarge posteromedially to meet on the midline (Fig. 9-1B, C). The initial apposition of the neural folds to form the neural tube takes place at what will become, in the adult, cervical levels of the spinal cord. This closure simultaneously proceeds in rostral and caudal directions, ultimately creating small openings at either end between the cavity of the neural tube and the surrounding amniotic cavity. These openings are the anterior and posterior neuropores; they close at 24 and 26 days of gestation, respectively.

Neural Tube

The portion of the neural tube that will differentiate into the spinal cord initially consists of the ventricular zone and the marginal zone (see Chapter 5). The ventricular zone contains cells, neuroblasts, and glioblasts that are undergoing mitosis and the precursor cells that will form the ependymal cell layer lining the central canal. The marginal zone is initially narrow and contains the processes of cells of the ventricular zone but does not contain their cell bodies or nuclei.

After their final cell division, postmitotic neurons and glial cells migrate outward to form the intermediate zone. This establishes the basic three-layered structure—ventricular zone, intermediate zone, and marginal zone—of the neural tube. As development progresses, the intermediate zone enlarges, and the processes arising from the maturing neurons of this zone enter the marginal zone. The neural crests detach from the lateral edge of the neural plate and assume a location anterolateral to the neural tube.

Structures of the developing neural tube can be correlated with their adult counterparts (Fig. 9-1C). Neural crest cells differentiate into cells of the posterior root ganglia, among other structures. The intermediate zone (the term preferred instead of mantle zone) contains columns of maturing neurons, forming the paired alar plates posteriorly and the paired basal plates anteriorly. The alar and basal plates are separated from each other by the sulcus limitans. Maturing neurons of the alar plate differentiate into the tract neurons and interneurons of the posterior horn of the adult, and those of the basal plate become motor neurons and interneurons of the anterior horn. The intermediate gray (Fig. 9-1D, white area between blue and pink), an important region of the spinal cord insinuated between the posterior and anterior horns in the adult, originates from portions of both alar and basal plates.

The marginal zone is invaded by processes of maturing neurons located in the ventricular zone (Fig. 9-1C) and by the descending axons of neuroblasts found in the developing brainstem and cerebral cortex. These axons, most of which become myelinated, form the various tracts of the white matter of the adult spinal cord.

Neural Tube Defects

A variety of defects result from the failure of the neural tube to close properly (Fig. 9-1). Rachischisis occurs when the neural folds do not join at the midline and the undifferentiated neuroectoderm remains exposed. In its extreme form, rachischisis totalis (or holorachischisis), the entire spinal cord remains open. Rachischisis partialis (or merorachischisis) is the situation in which the spinal cord is partially closed and partially flayed open.

A failure of the anterior neuropore to close results in a failure of the skull and the underlying brain to properly develop. Although it is commonly called anencephaly (meaning without the brain), the term meroanencephaly (meaning without part of the brain) is actually more accurate because the brainstem may be fairly intact but the forebrain and cerebellum are largely absent. Rachischisis and meroanencephaly are catastrophic developmental defects that in most cases (particularly the latter) are not compatible with life.

In other cases, the neural tube may develop normally but the surrounding vertebrae may not form properly, resulting in spina bifida occulta or spina bifida cystica. Spina bifida occulta is characterized by partially missing vertebral arches; the area of the defect may be indicated by a patch of dark hairs. Spina bifida cystica is seen as enlargements that may contain only meninges and cerebrospinal fluid (meningocele) or meninges, cerebrospinal fluid, and portions of the spinal cord (meningomyelocele). These are described in greater detail in Chapter 5.

SPINAL CORD STRUCTURE

The adult spinal cord is composed of a butterfly-shaped central area of neuron cell bodies, the gray matter, and a surround of myelinated and unmyelinated fibers, the white matter. Although the cavity of the neural tube was prominent during development, this space is a small ependyma-lined central canal in the adult spinal cord (Fig. 9-1C).

Surface Features

The human spinal cord extends from the foramen magnum to the level of the first or second lumbar vertebra. It consists of 8 cervical, 12 thoracic, 5 lumbar, and 5 sacral levels plus 1 coccygeal level (Fig. 9-2). Although it is generally cylindric, the cord has cervical (C4 to T1) and lumbosacral (L1 to S2) enlargements, which serve, respectively, the upper and lower extremities.

Figure 9-2. Diagrammatic representation of the spinal cord, the meninges, and other adjacent structures. Note the location for a lumbar puncture (spinal tap).

There are eight cervical roots (and spinal cord levels) but only seven cervical vertebrae. So how do the roots relate to their corresponding vertebrae? The C1 root is located between the base of the skull and the C1 vertebra (Fig. 9-2). Therefore roots C1 through C7 are located above (rostral to) their respectively numbered vertebrae, and the C8 root is located between the C7 and T1 vertebrae. Beginning with the T1 vertebra and extending caudally, all roots are located caudal to their respectively numbered vertebrae (Fig. 9-2). It is also important to remember that each level or segment of the spinal cord is specified by the intervertebral foramen through which the posterior and anterior roots originating from that cord level exit (Fig. 9-2).

There are few superficial markings on the spinal cord (Fig. 9-3). The posterior median sulcus separates the posterior portion of the cord into halves and contains a delicate layer of pia, the posterior median septum. The posterolateral sulcus, which runs the full length of the cord, represents the entry point of posterior root (sensory) fibers. This area is frequently called the posterior (dorsal) root entry zone. In cervical and upper thoracic regions, a posterior intermediate sulcus and septum are found between the posterolateral and posterior median sulci. This sulcus and septum are insinuated between the medially located gracile fasciculus and the laterally located cuneate fasciculus (Figs. 9-3 and Fig. 9-12). Because of the organization of the gracile and cuneate fasciculi, the posterior intermediate septum is present only in upper thoracic and cervical cord levels.

Figure 9-3. The spinal cord at C7 showing the basic organization of the gray and white matter and the meninges (shown only on the right). The lamination pattern of the gray matter is shown only on the left.

On the anterolateral surface of the spinal cord, the anterolateral sulcus is the exit point for anterior root (motor) fibers (Fig. 9-3). Because the anterior roots exit in a somewhat irregular pattern, this sulcus is not as distinct as the posterolateral sulcus.

The anterior median fissure is a prominent space dividing the anterior part of the cord into halves (Fig. 9-3). This fissure contains delicate strands of pia and, more important, the sulcal branches of the anterior spinal artery.

Spinal Meninges

The tubular dural sac that encloses the spinal cord is attached cranially to the rim of the foramen magnum, and its closed caudal end is anchored to the coccyx by the filum terminale externum (Figs. 9-2 and 9-3). This dural sac is separated from the vertebrae by the epidural space. The spinal cord, in turn, is attached to the dural sac by the laterally placed denticulate ligaments and by the filum terminale internum. This latter structure extends caudally from the end of the spinal cord, the conus medullaris, and terminates in the attenuated (closed) portion of the dural sac, which is located adjacent to the S2 vertebra. The filum terminale externum extends caudally from the closed dural sac to its attachment on the inner aspect of the coccyx (Fig. 9-2).

The arachnoid mater adheres to the inner surface of the dura mater, and the pia mater is intimately attached to the surface of the cord. The subarachnoid space between these layers is continuous with the subarachnoid space around the brain and is likewise filled with cerebrospinal fluid. In adults the conus medullaris is located at the level of the L1 or L2 vertebral body. Extending caudally from this point to the end of the dural sac is an enlarged part of the spinal subarachnoid space, the lumbar cistern (Fig. 9-2). This cistern contains the posterior and anterior roots from spinal segments L2 to Coc1 as they sweep caudally. Collectively, these roots form the cauda equina. The method of choice to obtain a sample of cerebrospinal fluid for diagnostic purposes is the lumbar puncture (spinal tap), in which a large-bore needle is introduced between the L3 and L4 or L4 and L5 vertebral arches into the lumbar cistern (Fig. 9-2).

White Matter

The white matter of the spinal cord is divided into three large regions, each of which is composed of individual tracts or fasciculi. The posterior funiculus is located between the posterior median septum and the medial edge of the horn (Fig. 9-3). At cervical levels, this area consists of the gracile and cuneate fasciculi; collectively, these are commonly referred to as the posterior columns.

The lateral funiculus is the area of white matter located between the posterolateral and anterolateral sulci (Fig. 9-3). This region of the cord contains clinically important ascending and descending tracts, the locations of which are shown in Figure 9-12. Those most important in diagnosis of the neurologically impaired patient are the lateral corticospinal tract and the anterolateral system (ALS).

Located between the anterolateral sulcus and the ventral median fissure is a comparatively small region, the anterior funiculus (Fig. 9-3). This area contains reticulospinal and vestibulospinal fibers, portions of the ALS, the anterior corticospinal tract, and a composite bundle called the medial longitudinal fasciculus (MLF).

Two small but important components of the white matter are the anterior white commissure and the posterolateral (dorsolateral) tract (Fig. 9-3). The anterior white commissure is located on the anterior midline and is separated from the central canal by a narrow band of small cells. The posterolateral tract, the tract of Lissauer, is a small bundle of lightly myelinated and unmyelinated fibers capping the posterior horn.

Gray Matter

The gray matter of the spinal cord is composed of neuron cell bodies, their dendrites and the initial part of the axon, the axon terminals of fibers synapsing in this area, and glial cells. Because this area has few myelinated fibers, it appears distinctly light and has a characteristic shape in myelin-stained sections (Figs. 9-3 and Fig. 9-5).

The spinal gray matter is divided into a posterior (dorsal) horn, an anterior (ventral) horn, and the region where these meet, commonly called the intermediate zone (or intermediate gray). On the basis of the shape, size, and distribution of neurons located in these areas, the gray matter is divided into laminae (Rexed laminae) I to IX and an area X around the central canal (Fig. 9-3). These laminae are also characterized by the input they receive and the trajectory of axons originating from each lamina.

The posterior horn is composed of laminae I to VI (Fig. 9-3). The most distinct structure in the posterior horn, the substantia gelatinosa (lamina II), is capped by cells of the posteromarginal nucleus (lamina I). Laminae III to VI are located sequentially below the substantia gelatinosa. Laminae III and IV may also be called the nucleus proprius (posterior or dorsal proper sensory nucleus); their cells have elaborate dendrites that extend into lamina II. Laminae V and VI, which form the base of the posterior horn, are usually divided into medial and lateral portions.

The intermediate zone, lamina VII, extends from the area of the central canal to the lateral edge of the spinal gray and varies in shape at different levels of the cord. Particularly characteristic of lamina VII at thoracic levels are the posterior thoracic nucleus (dorsal nucleus of Clarke) and the lateral horn, which contains the intermediolateral nucleus, frequently called the intermediolateral cell column.

The anterior horn is made up of laminae VIII and IX (Fig. 9-3). Lamina VIII contains a population of smaller cells that are interneurons and tract cells. Lamina IX consists of several distinct clusters of large motor neurons whose axons directly innervate skeletal muscle.

Blood Supply

The blood supply to the spinal cord is derived from the anterior and posterior spinal arteries and from branches of segmental arteries (Fig. 9-4). The segmental branches that serve the posterior and anterior roots and the posterior root ganglia are the radicular arteries, and branches that largely bypass the roots to supplement the blood supply to the cord are the spinal medullary arteries. One especially large spinal medullary artery, the artery of Adamkiewicz, is most often seen at L2 on the left. This vessel is an important source of blood supply to the cord and must be preserved during surgery in this area. At each level, terminal branches of the spinal medullary arteries join to form an arterial network, the arterial vasocorona, on the surface of the spinal cord (Fig. 9-4).

Figure 9-4. Blood supply to the spinal cord. Note that the lateral corticospinal tract and the anterolateral system receive a dual blood supply; also note the topographic arrangement of corticospinal fibers. L, leg–lower extremity; T, trunk; A, arm–upper extremity.

The posterior columns and peripheral parts of the lateral and anterior funiculi are served by the posterior spinal arteries and arterial vasocorona. Most of the gray matter and the adjacent parts of the white matter are served by the central branches of the anterior spinal artery (Fig. 9-4). These central branches tend to alternate: one serves the left side of the cord, the next serves the right side.

Trauma, as in hyperextension of the cervical spine or mechanical injury to the cord, may cause occlusion or spasm of the anterior spinal artery. The result is bilateral damage to the cervical cord (central cervical cord syndrome). The characteristic features of this syndrome are bilateral weakness of the extremities, primarily evident in the arms, forearms, and hands; a patchy loss of sensation below the lesion; and urinary retention.

REGIONAL CHARACTERISTICS

Although all spinal levels have posterior, lateral, and anterior funiculi and posterior and anterior horns, their shapes and proportions vary between major spinal regions (Fig. 9-5). For example, cervical (C4 to T1) and lumbosacral (L1 to S2) cord levels have prominent posterior and anterior horns because of the extensive sensory input from and motor outflow to the upper and lower extremities. In contrast, the posterior and anterior horns at thoracic levels are small, sensory input is less dense, and there is no appendicular musculature at these levels.

Figure 9-5. Representative levels of the spinal cord in anatomic (left two columns) and clinical (right two columns) orientations. The overall shape of the spinal cord at each level is clear in the myelogram and essentially identical to the corresponding anatomic section. Note that in CT, as shown here, or in MRI, the anterior and posterior horns (AH and PH) in the clinical orientation are the reverse of those seen in the anatomic orientation (compare far right and left columns). (CT myelogram From Haines DE: Neuroanatomy: An Atlas of Structures, Sections, and Systems, 8th ed. Baltimore, Lippincott Williams & Wilkins, 2012.)

When viewing the spinal cord in the clinical setting, as in magnetic resonance imaging (MRI) or computed tomography (CT), it is important to note that anterior and posterior are reversed compared with the orientation commonly used in the anatomic setting (Fig. 9-5). It is absolutely essential to remember the orientation and consequent location of tracts in the spinal cord in CT and MRI because these images are used in the evaluation of the neurologically compromised patient. In short, anterior is up in the image and posterior is down (Fig. 9-5). In addition, the right and left sides of the patient are standardized; the observer’s right is the patient’s left and the observer’s left is the patient’s right.

Cervical Levels

The cervical cord is round to oval and proportionately larger than at other spinal levels (Fig. 9-5). There is a large amount of white matter because a full complement of ascending and descending fiber tracts is present. The gracile and cuneate fasciculi are especially obvious structures at cervical levels.

At cervical levels C1 to C3, the posterior and anterior horns are comparatively small, which results in a more round than oval shape of the upper cervical spinal cord. The horns at C3 to C4 are becoming larger as part of the cervical enlargement; thus the shape of the spinal cord is transitioning from more round to more oval. At cervical levels C4 to C8, the posterior and anterior horns are large, reflecting the sensory input from and motor innervation to the upper extremity. At these levels, the spinal cord is distinctly oval (Fig. 9-5). The oval shape of the spinal cord is also obvious in a myelogram at lower cervical levels (Fig. 9-5).

Thoracic Levels

In general, the thoracic cord is round and the posterior and anterior horns are small (Fig. 9-5). From upper to lower thoracic levels, there is a progressive decrease in the amount of white matter. Although both the gracile and cuneate fasciculi are present at upper thoracic levels (above T6), only the gracile fasciculus is present at lower thoracic levels (below T6). However, the small size of the posterior and anterior horns makes the white matter in thoracic levels appear proportionately large. As seen in a myelogram at about midthoracic levels, there is more space around the spinal cord than at lower cervical levels (Fig. 9-5).

Two structures especially obvious in the gray matter of lamina VII at thoracic levels are the posterior thoracic nucleus (dorsal nucleus of Clarke) and the lateral horn (Fig. 9-5). The posterior thoracic nucleus, a prominent medial cell group, contains neurons whose axons project to the cerebellum. The lateral horn is a protrusion into the lateral funiculus formed by the underlying intermediolateral cell column. These cells are preganglionic sympathetic neurons whose axons will terminate in either paravertebral or prevertebral ganglia.