Human Anatomy Part 2 - K. A. Diubenko, A. K. Kolomiitsev, Yu. B. Chaikovskyi 2008

Special Section
Nervous system, systema nervosum - Special Section
Spinal cord - External structure

The Spinal Cord, medulla spinalis (Greek myelos), lies within the vertebral canal. It has the appearance of a cylindrical cord (Figs. 123, 125) consisting of two symmetrical halves connected by a narrow commissure, in the center of which is the central canal, canalis centralis (a remnant of the neural tube cavity). The upper boundary of the spinal cord corresponds to the decussation of the pyramids, or the exit of the first pair of spinal nerve roots, and projects onto the margin of the foramen magnum; the lower boundary lies at the level of the I—II lumbar vertebrae. In its upper portion, the spinal cord merges without a distinct boundary into the Medulla Oblongata of the Brain. In the lower regions, at the level of LІ-ІІ, the spinal cord tapers into the conus medullaris, which continues as the filum terminale. The filum terminale, along with the dura mater of the spinal cord, dura mater spinalis, extends into the sacral canal. The portion of the filum terminale that lies within the dural sac but does not fuse with it is called the internal filum terminale, filum terminale іnternum, while its other part fuses with the sheath as the external filum terminale, terminale externum. The filum terminale is accompanied by the anterior spinal Arteries and Veins, as well as one or two roots of the sacral nerves.

Knowledge of the lower boundary of the spinal cord is of great practical importance. For therapeutic and diagnostic purposes, a lumbar puncture is performed in this region to collect CEREBROSPINAL FLUID or to inject therapeutic substances into the subarachnoid space. In adults, the puncture is performed between the ІІІ-IV or IV-V lumbar vertebrae along the projection line (Jacoby's line) drawn between the crests of both iliac bones. It corresponds to the IV lumbar vertebra. The length of the spinal cord depends on the person's height. In newborns, the spinal cord is 15-17 cm long and weighs about 3 g; by two years of age, its length reaches 20 cm, and by 10 years, the length doubles. In an adult, the length of the spinal cord is 41-45 cm, and its weight is 26-28 g.

The spinal cord performs two main Functions: reflex (segmental) and conductive, which is carried out by the White matter. Complex relationships exist between these two functions, which can be characterized as the subordination of the segmental reflex activity of the spinal cord to suprasegmental centers and The formation of various functional levels in the brain.

The thickness and shape of the spinal cord are not uniform throughout its length. In the cervical and lumbar regions, There are two enlargements: the cervical enlargement, intumescentia cervicalis, and the lumbosacral enlargement, intumescentia lumbosacralis, which correspond to the exit sites of the nerve roots supplying the upper and lower limbs.

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Fig. 123. The spinal cord, medulla spinalis, its segments, segmenta; Meninges, meninges; roots, radices, and Spinal Nerves, nn. spinales.

A - anterior view, B - posterior view, C - cross-sections of the human spinal cord at different levels: cervical (C I; V), thoracic (Th I; VIII), lumbar (L I; III), sacral (S III; IV)

The cervical enlargement begins at the level of the III—IV cervical vertebrae and extends to the II thoracic vertebra, reaching its greatest width at the level of the V-VI cervical vertebrae. The lumbar enlargement extends from the IX-X thoracic vertebrae to the I lumbar vertebra, reaching its greatest width at the level of the XII thoracic vertebra.

Along the anterior surface of the spinal cord, throughout its entire length, runs the deep anterior median fissure, fissura mediana anterior, into which a fold of the pia mater—the anterior median septum—invaginates. Along the posterior surface of the cord runs a very narrow posterior median sulcus, sulcus medianus posterior, into which a sheet of glial tissue—the posterior median septum, septum medianum posterius—penetrates.

The fissure and the sulcus divide the spinal cord into right and left halves, which are connected by a narrow bridge of Nervous Tissue containing the central canal, canalis centralis, of the spinal cord. On the lateral surface of each half, there are two shallow sulci: the anterolateral sulcus, sulcus lateralis anterior, located laterally to the anterior median fissure, and the posterolateral sulcus, sulcus lateralis posterior, lying laterally to the posterior median sulcus. From or near the anterolateral sulcus, anterior rootlets, filia radicularia anteriora, emerge from each segment, representing the processes of motor Cells of the anterior horns. The anterior rootlets form the motor anterior ROOT, radix ventralis (anterior).

The posterior rootlets, filia radicularia posteriora, enter the posterolateral sulcus; they represent the processes of cells located in the spinal ganglion, ganglion spinale. The posterior rootlets form the sensory dorsal root, radix dorsalis (posterior).

The spinal ganglion, ganglion spinale, has the appearance of a spindle-shaped enlargement located on the posterior root. It is a collection of primarily pseudounipolar Nerve Cells. The peripheral processes (dendrites) of these cells join the axons of motor Neurons from the anterior horns outside the spinal ganglion to form the spinal nerve, nervus spinalis, which is thus a mixed nerve. Each pair of spinal nerves (right and left) corresponds to a specific segment of the spinal cord. A segment, segmentum, is defined as a horizontal section of the gray and WHITE MATTER OF the spinal cord that corresponds to two pairs of roots (Fig. 124). Spinal cord segments represent partially functional centers that provide segmental or radicular innervation to specific areas of The Human Body. Short reflex arcs close at the level of the spinal cord segments. For more details on the segmental innervation of body regions, see the section: "Peripheral Nervous system".

The spinal cord has 31 pairs of segments arranged in sequence (Fig. 125):

Cervical segments - segmenta cervicalia (1-8);

Thoracic segments - segmenta thoracica (1-12);

Lumbar segments - segmenta lumbalia (1-5);

Sacral segments - segmenta sacralia (1-5);

Coccygeal segment - segmentum coccygeum (1);

To designate the segments, initial Latin letters are used to indicate a particular region of the spinal cord, followed by numbers corresponding to the sequential number of the segment (C-cervicalia; T-thoracica; L-lumbalia; S-sacralia; Co-cocygeum).

Fig. 124. Diagram of spinal cord segments (after F. Kiss and J. Szentágothai)

Determining the exit site of the spinal nerve roots from the spinal cord in relation to the spinous processes of the vertebrae (skeletotopy) is of great practical importance. In neurology, Chipault's rule is used, which is based on the orientation of the spinal roots (the angle between the roots and the spinal cord becomes smaller the further down the root emerges): in the cervical spine, one is added to the number of the palpated spinous process; for example, the VI cervical nerve emerges below the processus spinosus of the V cervical vertebra; in the upper thoracic spine, the number 3 is added; finally, the processus spinosus of the XI thoracic vertebra and the space between it and the processus spinosus of the XII thoracic vertebra correspond to the exit site of the last three lumbar roots, while the exit site of the sacral nerve roots lies directly below the processus spinosus of the XII thoracic vertebra.

Fig. 125. Sequential arrangement of the spinal cord segments.