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General Considerations

Anatomically, the spinal cord can be divided into four regions: cervical, containing 7 vertebrae and 8 spinal nerves; thoracic, containing 12 vertebrae and spinal nerves; lumbar, containing 5 vertebrae and spinal nerves; and sacral, containing 5 fused vertebrae and spinal nerves. In cross-section, the spinal cord reveals butterfly-shaped gray matter surrounded by white matter. The gray matter contains the neuronal cell bodies, and the white matter consists of nerve tracts (Figure 18–1).

Figure 18–1.

Transverse section through the spinal cord, composite representation, illustrating the principal ascending (left) and descending (right) pathways. The lateral and ventral spinothalamic tracts ascend contralateral to the side of the body that is innervated. C = cervical; D = distal; E = extensors; F = flexors; L = lumbar; P = proximal; S = sacral; T = thoracic. (Reproduced with permission from Braunwald E, Fauci AS, Kasper DL, et al: Harrison’s Principles of Internal Medicine, 15th ed. New York, NY: McGraw-Hill Education; 2001.)

The major clinically relevant tracts of the cord are the dorsal columns (ascending), which convey tactile discrimination, vibration, and joint position sense; the spinothalamic tracts (ascending), which convey pain, temperature, and crude touch; and the corticospinal tracts (descending), which convey fibers used for motor control. The spinal cord ends between the first and second lumbar vertebrae in adults. This distal area is called the conus medullaris, and its continuation as the filum terminale is composed of connective tissue that attaches to the coccyx. The cauda equina is a collection of nerve roots that begins at the end of the spinal cord and exits from the third lumbar vertebra to the fifth sacral vertebra. The spinal cord is insulated from the bony canal by a layering of fatty connective tissue and by the meninges. The three meninges from inner to outer are pia, arachnoid, and dura. The subarachnoid space contains cerebrospinal fluid and separates the pia from the arachnoid.

Other structures that protect the spinal cord can be divided based on their location relative to the cord (Figure 18–3). The intervertebral foramen is the opening between the pedicles of adjacent vertebrae for the spinal nerve to pass through. Spinal nerves are composed of dorsal and ventral roots. The first seven pairs of cervical spinal nerves exit above the same-numbered vertebral bodies, whereas all the subsequent nerves exit below the same-numbered vertebral bodies because of the presence of eight cervical spinal cord nerves but only seven cervical vertebrae. Intervertebral disks separate the vertebral bodies and respond dynamically to applied loads to reduce the forces the vertebrae are exposed to; in other words, they act as shock absorbers. The avascular disk consists of an eccentrically located nucleus pulposus and the surrounding annulus fibrosus. The nucleus pulposus is a semigelatinous ...

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