Spinal Cord and Spinal Nerves
The spinal cord, or spinal medulla, is located in the channel of the spinal column, the vertebral canal, and is surrounded by the cerebrospinal fluid. It has two spindle-shaped swellings: one in the neck region, the cervical enlargement (C1), and one in the lumbar region, the lumbar enlargement (C2). At the lower end, the spinal cord tapers into the medullary cone (BC3) and ends as a thin thread, the terminal filament (C4). The ante-rior median fissure at the ventral side and the posterior median sulcus (BC5) at the dorsalside mark the boundaries between the two symmetrical halves of the spinal cord. Nerve fibers enter dorsolaterally and emerge ven-trolaterally at both sides of the spinal cord and unite to form the dorsal roots, posteriorroots, and theventral roots, anterior roots. Theroots join to form short nerve trunks of 1 cm in length, the spinal nerves. Intercalated into the posterior roots are spinal ganglia (B6) containing sensory nerve cells. Only the posterior roots of the first cervical spinal nerves do not have a spinal ganglion, or only a rudimentary one.
In humans, there are 31 pairs of spinal nerves which emerge through the intervertebralforamina from the vertebral canal. Each spi-nal nerve pair supplies one body segment. The spinal cord itself is unsegmented. The impression of segmentation is created by the bundling of nerve fibers emerging from the foramina.
The spinal nerves are subdivided into cervi-cal nerves, thoracic nerves, lumbar nerves, sacral nerves, and coccygeal nerves (A). There are
8 pairs of cervical nerves (C1 – C8) (the first pair emerges between occipital bone and atlas)
12airs of thoracic nerves (T1 – T12) (thefirst pair emerges between the first and second thoracic vertebrae
5 pairs of lumbar nerves (L1 – L5) (the firstpair emerges between the first and sec-ond lumbar vertebrae)
5 pairs of sacral nerves (S1 – S5) (the firstpair emerges through the upper sacral foramina)
one pair of coccygeal nerves (emerging be-tween the first and second coccygeal vertebrae)
Spinal cord and vertebral canal are initially of the same length so that each spinal nerve emerges from the foramen lying at its own level. During development, however, the vertebral column increases much more in length than does the spinal cord. As a result, the lower end of the spinal cord moves further up in relation to the surrounding vertebrae. In the newborn, the lower end of the spinal cord lies at the level of the third lumbar vertebra, and in the adult, at the level of the first lumbar or twelfth thoracic vertebra. Thus, the spinal nerves no longer emerge at their levels of origin; instead, their roots run down a certain distance within the vertebral canal to their foramen where they emerge. The more caudally the roots originate from the spinal cord, the longer their run within the vertebral canal. The levels where the spinal nerves emerge are therefore no longer identical with the corresponding levels of the spinal cord.
From the medullary cone (BC3) onward, the vertebral canal contains only a dense mass of descending spinal roots, known as the cauda equina (tail of a horse) (B7).