Spinal Cord Segments

The spinal cord lies within the vertebral canal and it has at least 33 pairs of spinal nerves attached through which the spinal cord receives axons from (afferent) and sends (efferent) axons to the peripheral nervous system.

The spinal cord arises at the level of the foramen magnum and extends to the level of the sixth lumbar vertebra in most dogs and the seventh lumbar vertebra in cats, where it tapers to form the conus medullaris, the filum terminale and the cauda equina. It comprises a central core of grey matter and an outer layer of white matter. The diameter of the spinal cord is not constant throughout its length. In the caudal part of the cervical region and the lumbar region, it widens to form the cervicothoracic and lumbosacral intumescences, respectively, from which the lower motor neurons (LMNs) to the thoracic and pelvic limbs arise.

The spinal cord is surrounded by the three layers of meninges. The most external layer is the dura mater, which lies on the surface of the arachnoid membrane (this is the external surface of the subarachnoid space, which contains the CSF) with thin collagenous extensions that attach to the inner layer of the meninges, the pia mater. It is important to note that the spinal cord parenchyma (as for all the parts of the central nervous system) does not have nociceptors (receptors activated in the noxious stimulus); however, these receptors are widely present in the meninges.

A spinal cord segment is defined as a portion of the spinal cord that gives rise to one pair of spinal nerves. There are 8 cervical, 13 thoracic, 7 lumbar, 3 sacral and at least 2 caudal/coccygeal spinal cord segments in the dog and cat. There are 8 cervical spinal nerves with only 7 cervical vertebrae because the cervical spinal nerve C1 leaves the vertebral canal through the lateral vertebral foramen in the atlas, and the cervical spinal nerve C8 exits caudal to the seventh cervical vertebra. Some spinal cord segments lie at the level of the vertebra of the same annotation (mainly in the caudal thoracic area), whilst others lie cranial to the corresponding vertebra, and the spinal nerves course caudally in the vertebral canal to exit at the correct intervertebral foramen.

Spinal lesion localization refers to the spinal cord segments rather than the vertebrae.