Anatomic Correlations

External Structure

The spinal cord has major functional and teleologic importance, yet it is almost paradoxical that its total mass represents just 2% of the entire central nervous system volume. The spinal canal dimensions are slightly larger, allowing the cord to move freely within the canal during neck and back flexion/extension as its perspective changes with movement. The spinal cord per se is a cylindrical elongated structure flattened dorsoventrally, having a length of 42–45 cm (Fig. 44-1). It lies within the vertebral canal extending from the atlas, continuous with the medulla through the foramen magnum, to the level of the 1st and 2nd lumbar vertebra. Here it tapers into the conus medullaris and terminates as the cauda equina (Fig. 44-1). In conditions such as meningitis, one must not place the spinal needle above the L3 vertebra in order not to specifically damage the underlying spinal cord. It is safe to enter the spinal canal distal to this level because the spinal nerves originating from the distal cord and forming the cauda equina, as nerve roots L2-S5, are anatomically arranged so that they are gently moved aside by the passage of the needle in contrast to the fixed spinal cord that may be easily pierced by the needle insertion and thus cause significant damage.

Figure 44-1 Spinal Cord and Vental Rami in Situ.

The cervical and lumbar enlargements of the spinal cord provide the nerve roots innervating, respectively, the upper and lower limbs. There are 31 pairs of spinal nerves, each having dorsal sensory and ventral motor roots that exit the cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal). Although there are 7 cervical vertebrae, there are 8 cervical nerve roots (Fig. 44-2). The C1-7 roots exit above their respective vertebrae whereas C8 exits below the 7th cervical vertebra and all thoracic, lumbar, and sacral roots exit below their specific vertebrae.

Figure 44-2 Relation of Spinal Nerve Roots to Vertebrae.

Three protective membranes, the meninges including the dura mater, being the outer layer, then the arachnoid, and the most inner one, the pia, surround the cord (Fig. 44-2). Cerebrospinal fluid flows between the arachnoid and pia. Epidural fat is present in the epidural space between the spinal canal and dura mater. When clinical myelopathies develop, these various disorders are classically categorized as intramedullary, that is, intrinsic to the cord, or extramedullary, occurring secondary to disorders extrinsic to the cord. Extramedullary disorders are further subdivided into those with either an intradural extramedullary locus or a purely extradural site of pathology.

Internal Structure

White matter, consisting of myelinated fibers, surrounds the butterfly or H-shaped gray matter that contains cell bodies and their processes within the cord’s center. These include both primary ascending sensory fibers and descending motor fibers. The sensory pathways are most superficial, while the motor fibers such as the corticospinal fibers are more deeply situated but still superficial and superior to the ventral horn containing the anterior horn cells that are the primary receptors for the corticospinal tract fibers. Longitudinal furrows on the cord’s surface divide the white matter into columns or funiculi that are large bundles of nerve fibers having diverse functions. The anterior median fissure, posterior median, posterior intermediate, and the anterolateral sulci divide the dorsal or posterior, lateral, and anterior columns. The posterior columns are further divided into two fasciculi, gracilis medially (present at all spinal levels) and cuneatus laterally (T6 and above).

Specific Spinal Tracts

Ascending and descending tracts within the cord are interrupted at sites of cord damage (Figs. 44-3 to 44-5). The subsequent clinical sequelae develop based on the specific tracts that are affected.

Figure 44-3 Principal Fiber Tracts of Spinal Cord.

Figure 44-4 Spinal Cerebral Afferent Systems.

Figure 44-5 Cerebral Cortex: Efferent Pathways.

Ascending Sensory Tracts (see Fig. 44-4)

Dorsal columns subserving touch, pressure, position, and movement sensations arise from dorsal root fibers and ascend posteriorly. Their fibers do not decussate (i.e., cross) until within the medulla. They then travel to the ventral posterolateral thalamic nuclei.

Lateral spinothalamic tracts arise from secondary pain and temperature neurons within the spinal cord, cross into the anterior commissure, and ascend in the lateral funiculus to the reticular formation and ventral posterolateral thalamic nuclei.

Anterior spinothalamic tracts arise from dorsal horn neurons within the spinal cord, cross in the anterior commissure, and ascend anterolaterally to the posterior thalamic and ventral posterolateral thalamic nuclei. The anterior spinothalamic tract provides light touch sensation.

Dorsal spinocerebellar tracts are uncrossed, ascending in the lateral funiculus to the cerebellar vermis. At subconscious levels, they provide fine coordination of posture and limb muscle movement.

Anterior (ventral) spinocerebellar tracts are dedicated to lower extremity movement and posture, initially cross and then ascend the lateral funiculus to the cerebellum.

Cuneocerebellar tracts (not illustrated) contribute to upper extremity coordination and movement, are uncrossed, and ascend to the cerebellum.

Descending Motor Tracts

Corticospinal tracts are responsible for voluntary, skilled movement. They originate in the motor cortex (precentral and premotor areas 4 and 6), postcentral gyrus, and adjacent parietal cortex (see Fig. 44-5). These primary motor fibers descend via the corona radiata, posterior limb of internal capsule, pons, and into the medulla where most distally they then divide into three separate motor tracts (see Fig. 44-3). Up to 90% of these fibers descend in the lateral funiculus as the lateral corticospinal tract. Most decussate within the distal medulla; the uncrossed lateral corticospinal tract is much smaller. The anterior corticospinal tract travels in the anterior funiculus, crossing within the cord.

Tectospinal, rubrospinal, and vestibulospinal tracts (see Fig. 44-3) originate in the superior colliculus, red nucleus, and the lateral vestibular nucleus, respectively. These tracts variously affect reflex postural movements, tone in flexor muscles, and facilitate antigravity and extensor muscles.