The spinal cord is the part of the CNS located within the vertebral canal. It is involved in controlling movements of the trunk and limbs, regulates visceral functions, and transmits information to and from the brain.


 It extends from the foramen magnum (in the occipital bone) to the sixth or seventh lumbar vertebra in dogs of medium size and to the L7-S1 junction in small dogs.


It has a cylindrical shape with two enlargements located at the level of C5-T1 vertebrae (involving the spinal segments from C6 to T2) and L3-L6 vertebrae (involving the spinal segments from L4 to S3). They are the cervical intumescence and the lumbar intumescence respectively. The cervical intumescence forms the nerves of the brachial plexus and the lumbar intumescence gives rise to the lumbosacral plexus. The caudal elongation of the spinal cord has a cone shape named conus medullaris. It extends caudally by a thin cord of ependymal and glial cells, the terminal filament.


Externally, the dorsal median sulcus and the ventral median fissure divide the spinal cord in two halves. The dorsolateral sulcus marks the site of entry of sensory fibers and the ventrolateral sulcus, almost imperceptible, marks the point where motor fibers leave the spinal cord.


The spinal cord has a segmental organization. In dogs and cats it consists of between 35 and 38 segments divided into: 8 cervical, 13 thoracic, 7 lumbar, 3 sacral and between 4 and 7 caudal. As the growth of the spinal canal is greater than that of the spinal cord, some of the spinal segments do not correspond with the vertebrae. This is the reason why the spinal roots of the last lumbar spinal segments, and the sacral and caudal spinal roots, extend caudally to reach the corresponding vertebral foramina. Inside the vertebral canal, they arrange around the terminal filament, and form the cauda equine[1].


Each spinal segment has, on each side, a dorsal root (formed by afferent[2] fibers) and a ventral root (form by efferent[3] fibers)  Closer to the spinal cord, each root divides into rootlets. The dorsal and ventral roots of each side unite to form a spinal nerve that passes through the intervertebral foramen or the lateral vertebral foramen in the case of the atlas. Then, each nerve branches off in a dorsal and a ventral branch, each one made of afferent and efferent fibers. For the sacral nerves, the branching in dorsal and ventral branches takes place inside the vertebral canal. The dorsal branches exits through the dorsal sacral foramina, and the ventral ones through the ventral sacral foramina. As the sensory neurons are derived from the neural crest, their bodies are grouped outside the spinal cord, forming a thickening in the dorsal root near its junction with the ventral root, called the spinal ganglion[4]. Motor neurons originate from the neural tube wall and their bodies are located at the level of the gray matter of the ventral horn.


The cervical spinal nerves leave the vertebral canal in front of the vertebrae with the same number. However, as they are seven cervical vertebral and eight cervical spinal segments, the C8 spinal nerve leaves the vertebral canal between the C7 and the T1 vertebrae. For the rest of the spinal nerves (thoracic, lumbar and sacral), they leave the vertebral canal behind the corresponding vertebrae.


Functionally, the spinal cord can be divided into four sections: from C1 to C5, from C6 to T2 (cervical enlargement or intumescence), from T3 to L3, and from L4 to S3 (lumbar enlargement or intumescence). This division is very helpful to localize lesions.


The spinal cord segment C1 is located between the foramen magnum and the atlas, the C2 spinal segment is located at the level of the atlas, the C3 one (the longest spinal segment) is in the axis, and so on. The spinal cord segments from C6 to T2 are located between the fifth cervical and first/second thoracic vertebrae. The spinal cord segments from T3 to L3 are located between the second thoracic vertebra and the third lumbar vertebra, and the spinal cord segments from L4 to S3 are located between the third and sixth lumbar vertebrae.

[1] From the Latin cauda, "tail"; equina, "horse".

[2] From the Latin afferens, “to take out”.

[3] From the Latin efferens, “to take in”.

[4] From the Greek ganglion, “knot”.