The Normal Spinal Cord and Meninges

Internal Organization

Grossly, the spinal cord is organized as a series of hollow cylinders arrayed concentrically along the length of the cord. From deep to superficial these are the central canal of the cord, the deep gray matter of the cord, and the surrounding white matter. The ratio of gray matter to white matter within each spinal segment varies systematically along the length of the cord ( Fig. 5-4 ). Upper spinal segments show larger proportions of white matter, because they carry the white matter tracts to and from the entire distal cord. Lower spinal segments carry only the white matter for the few segments distal to them. The spinal gray matter is usually sharply delimited from the white matter, except in the cervical cord where strands of gray matter extend into the lateral white matter as the reticular formation.

The central canal extends downward from the caudal end of the fourth ventricle into the proximal filum terminale (see Fig. 5-4 ). It is lined by ciliated ependyma and surrounded by neuroglia and a network of fine nerve fibers constituting the substantia gelatinosa centralis. The central canal typically measures less than 1 mm in diameter in youth, partially closes in adulthood, and is mostly obliterated by age 40 years. Within the distal cord and/or proximal filum terminale, the caudal end of the central canal widens into a fusiform “terminal ventricle,” 8 to 10 mm long, that contains less than 1 mL of cerebrospinal fluid. There may be ependymal rests and lumina of abortive accessory canals within the conus medullaris and proximal filum.

Just external to the central canal, and surrounding it, the longitudinal column of spinal gray matter forms a “fluted gray column” that appears H-shaped in cross section ( Figs. 5-4 and 5-5 ). The ventral arms of the H are the symmetric ventral horns containing motor neurons. The dorsal arms of the H are the symmetric dorsal horns receiving the central fibers of sensory neurons that reside in the dorsal root ganglia. The crossbar of the H is the central gray matter. In the thoracic region, paired additional lateral horns project into the lateral white matter and contain the preganglionic sympathetic neurons. Considered vertically over the length of the spinal cord, the ventral, dorsal, and lateral horns form ventral, dorsal, and lateral columns of gray matter.

External to the gray matter, the white matter of the spinal cord is subdivided into paired dorsal, lateral, and ventral columns (funiculi) by fissures, septa, sulci, and nerve roots (see Figs. 5-4 and 5-5 ). In the ventral midline, the deep ventral median fissure extends nearly to the central canal, dividing the white matter of the ventral cord into paired ventral columns (anterior columns, anterior funiculi). Anterolaterally, the paired ventrolateral sulci mark the line of exit of the ventral motor nerve roots. The ventral columns are the paired white matter tracts situated to each side of the ventral median sulcus as far laterally as the ventrolateral sulci (including the emerging ventral roots). In the dorsal midline, the shallow dorsal median sulcus and subjacent posterior median glial septum extend nearly to the central canal, dividing the dorsal white matter into paired dorsal columns (posterior columns, dorsal funiculi). Posterolaterally, the paired dorsolateral sulci mark the line of entry of the dorsal nerve roots. The dorsal columns are the paired white matter tracts situated to each side of the dorsal median sulcus and septum as far laterally as the dorsolateral sulci (but not including the entering dorsal roots). At cervical and thoracic levels above T6, a posterointermediate sulcus and septum further divide each dorsal column into a medial fasciculus gracilis and a lateral fasciculus cuneatus. The lateral columns are the white matter tracts situated between the ventrolateral and dorsolateral sulci.

The fibers coursing within the longitudinal columns may cross the midline close to the central canal of the cord via the gray and white commissures. The dorsal and ventral gray commissures are tracts of crossing unmyelinated (hence “gray”) fibers situated just dorsal and ventral to the central canal. The ventral white commissure is a tract of crossing myelinated (hence “white”) fibers situated ventral to the ventral gray commissure and dorsal to the depth of the ventral median fissure ( Fig. 5-6A ).

The ventral motor roots arise from cell bodies situated within the ventral horns of the spinal gray matter. The cell bodies are organized longitudinally into medial, central, and lateral longitudinal cell columns, which supply different groups of muscles. The motor roots emerge into the subarachnoid space through the ventrolateral sulci and course ventrolaterally toward the exiting segmental root sleeves.

At each level the dorsal (sensory) roots arise from cell bodies situated within the dorsal root ganglia in the neural foramina. Each dorsal root ganglion cell gives rise to one short fiber that quickly divides into a central and a peripheral process. The central process (central axon) of the dorsal root ganglion cell passes centrally through the subarachnoid space toward the spinal cord. Multiple central processes arise from each segmental ganglion. These fan out to form six to eight dorsal rootlets that enter the corresponding segment of the spinal cord at the dorsolateral sulcus. As the central processes approach the cord, they become sorted by size and degree of myelination. Larger, more heavily myelinated fibers pass medially to enter the dorsal root entry zone as a medial bundle of central fibers (see Fig. 5-6B,C ). Lightly myelinated and unmyelinated fibers pass laterally to enter the dorsal root entry zone as a lateral bundle of central fibers. Each entering central process bifurcates into ascending and descending branches. Long ascending branches of the medial fibers enter the dorsal columns directly or pass through the medial portion of the dorsal horn to deeper layers of gray matter. Thin lightly myelinated or unmyelinated branches of the lateral fibers enter the dorsolateral tract (fasciculus) of Lissauer (see later).

The peripheral process of the dorsal root ganglion cell courses laterally through the root sleeve toward the sensory receptor. Within each segmental neural foramen, the ventral motor rootlets that arise from the spinal cord unite with the peripheral sensory processes that arise from the dorsal root ganglion distal to (on the far side of) the dorsal root ganglion to form the spinal nerve. Thus, the term nerve signifies the united motor and sensory roots of a segment starting distal to the dorsal root ganglion. In humans, there are 8 cervical nerve roots, 12 thoracic roots, 5 lumbar roots, 5 sacral roots, and 1 coccygeal root. The C1 nerve exits the spinal canal between the foramen magnum and C1, superior to the bony C1 ring. The C8 nerve exits the spinal canal between C7 and T1, superior to the T1 vertebra. All of the subsequent thoracic, lumbar, and sacral roots exit the spinal canal immediately inferior to the pedicles of the same-named vertebrae. That is, the T7 nerve exits the canal immediately under the T7 pedicle via the T7-T8 neural foramen and the L4 nerve exits the spinal canal immediately under the L4 pedicle via the L4-L5 neural foramen. In the upper cervical spine, some nerve roots also emerge directly through the lateral surface of the lateral columns of white matter to form the spinal accessory nerves (cranial nerve XI) that course superiorly through the subarachnoid space and foramen magnum to join the medullary portion of the spinal accessory nerve within the posterior fossa.

Vascularization of the Spinal Cord

Arterial Supply

The spinal cord receives its arterial supply from a single anterior spinal axis and paired dorsal and/or dorsolateral arteries ( Figs. 5-1 to 5-3 and 5-7 to 5-9 ). Ventrally, the anterior spinal axis is a long, usually continuous midline trunk that is 0.2 to 0.8 mm in diameter. It arises near the foramen magnum by union of the paired anterior spinal branches of the vertebral arteries and courses caudad within the anterior median fissure. It varies in caliber along its length, being relatively narrow in the thoracic region and widest inferior to the anastomosis with the artery of Adamkiewicz (0.5-1.0 mm). The anterior spinal axis receives supplemental feeders at multiple, irregularly spaced intervals. These typically join the main arterial axis with a hairpin turn. One large feeder entering between C4 and C8 may be designated the artery of the cervical enlargement. The largest supplementary feeder (arteria radicularis magna) typically arises between T9 and T12 and passes to the thoracolumbar enlargement. It may be designated the artery of the lumbar enlargement (i.e., the artery of Adamkiewicz). Within the anterior median fissure, the anterior spinal axis gives rise to numerous sulcomarginal branches that supply the anterior two thirds of the spinal cord, including most of the gray matter and much of the white matter.

At each feeder, the blood entering the anterior spinal axis is directed both craniad and caudad. This creates multiple less well-vascularized “watersheds” at the junctions of the feeder territories. At these watersheds, therefore, flow within the anterior axis may be markedly reduced or nearly absent.

Dorsally, the paired arterial trunks form a variably complete plexus of longitudinal vessels (<0.5 mm in diameter) that course along the dorsal surface of the cord, either medial to (dorsal spinal arteries) and/or lateral to (dorsolateral spinal arteries) the entry of the dorsal nerve roots at the dorsolateral sulci. The paired dorsal/dorsolateral trunks do not supply the spinal cord directly. Instead they feed into a circumferential pial vascular plexus called the vasocorona. The vasocorona then gives rise to multiple small arterial branches that enter each segment of the cord radially, perpendicular to the circumference of the cord, to supply the posterior one third of the spinal cord.

Venous Drainage

The venous drainage of the spinal cord is more variable than the arterial supply and shows no real parallelism to the arterial tree ( Figs. 5-2 , 5-3 , and 5-7 to 5-11 ). On the ventral surface, a continuous anterior median spinal vein (0.4-1.0 mm) is located within the ventral median fissure, deep to the anterior spinal axis. This vein is largest in the lumbosacral region and, in 60% to 70% of cases, continues along the filum terminale as a very large terminal vein. The anterior median spinal vein is typically larger than the corresponding anterior spinal axis. The largest vein draining the anterior thoracolumbar cord is designated the great anterior radiculomedullary vein (see Fig. 5-2 ). This is easily mistaken for the artery of Adamkieciwz.

On the dorsal surface, the most nearly constant vein is a large posterior median spinal vein. This is largest rostral to the thoracolumbar enlargement and continues into the cervical region. It is easily recognized because it has a tortuous course at the dorsal aspect of the lumbar enlargement and because there is no posterior midline artery.

A secondary system of short, smaller-caliber veins also courses along the surface of the spinal cord. The most nearly continuous of these are the anterolateral veins, which may replace the median veins wherever the median veins are interrupted. An additional plexus of superficial transverse veins interconnects the longitudinal channels. The intrinsic veins of each segment drain outward, radially, to the superficial venous plexus. Within the spinal cord, true transmedullary anteroposterior venous anastomoses may arise from the ventral median vein, course through the ventral median fissure into the spinal cord, zigzag craniocaudad as they circle around the central canal, and then pass directly outward along the dorsal median septum to enter the dorsal median vein (see Fig. 5-10 ). Such transmedullary venous anastomoses typically ascend rostrally as they pass from ventral to dorsal. They receive few, if any, veins draining from the spinal cord.

Gray Matter of the Spinal Cord

The gray matter of the spinal cord is composed of neurons, their terminal axons and dendrites (collectively designated neurites), and the neuroglial support cells. The neurons may be designated intrasegmental if their processes remain within the same spinal segment, intersegmental if their processes extend over adjacent segments, and suprasegmental if their processes extend beyond the spinal cord into the brain. The neurons and neurites of the spinal cord interconnect widely across the midline commissures, so the paired halves of the spinal gray matter form one functional system (see Figs. 5-4 and 5-6A ).

In cross section, the spinal gray matter resembles an H or a “butterfly” with dorsal horns, ventral horns, and lateral horns (at T2-L1 levels only) (see Figs. 5-4 to 5-6 ). The volume of gray matter varies with the spinal level (see Fig. 5-5 ). The gray matter is most extensive at lumbosacral levels, where it supplies the lower extremities, perineum, and pelvis. It is thinnest in the thoracic region, where it supplies only the axial musculature, and second most prominent in the cervical region, where it supplies the shoulder girdle. The broad ventral horns are considered to have a base oriented toward the central canal and a large head that projects ventrolaterally. The narrow dorsal horns have a similar base near to the central canal, a constricted neck, an oval to fusiform head, and an apex that projects dorsolaterally (see Figs. 5-4 to 5-6 ). The junction of the dorsal and ventral horns centrally is designated the intermediate gray matter.

The spinal gray matter contains multiple specific nuclei. The most important of these are discussed below.

White Matter of the Spinal Cord

The spinal white matter contains nerve fibers, neuroglia, and blood vessels. Those nerve fibers that are well myelinated appear white. Nonmyelinated and lightly myelinated nerve fibers appear gray, hence the terms white commissure and gray commissure to indicate different groups of fibers crossing the midline near to the central canal.

The white matter tracts are typically considered in three groups: propriospinal tracts, long ascending tracts, and long descending tracts ( Fig. 5-14 ). The major propriospinal tracts lie immediately external to the gray matter of the spinal cord within each of the ventral, lateral, and dorsal columns. The major ascending tracts include the fasciculi gracilis and cuneatus in the dorsal columns, the spinocerebellar tracts in the lateral columns, and the spinothalamic tracts in adjoining portions of the ventral and lateral columns (“anterolateral” columns). The major descending tracts include the corticospinal and vestibulospinal tracts in the ventral and lateral funiculi, and possibly a minor contribution from the rubrospinal tract in the dorsal portion of the lateral funiculus.