Spinal Cord
Surface features
At the level of the upper border of
the atlas, the spinal cord is directly continuous with the medulla oblongata.
Inferiorly, the cord usually extends as far as the first lumbar intervertebral
disc, where it terminates as the conus medullaris. In transverse section, the
spinal cord is oval, with its smaller diameter anteroposteriorly. The cord is
especially wide at the cervical and lumbar enlargements due to increased
numbers of nerve cell bodies within the spinal cord, which innervate the upper
and lower limbs.
On the anterior aspect of the cord
lies the anterior median fissure, whereas posteriorly, there is a relatively
shallow posterior median sulcus. On each side, a posterolateral sulcus marks
the site of attachment of the posterior nerve roots.
Spinal nerve roots
The spinal nerves are attached to the
spinal cord by anterior and posterior nerve roots (Fig. 8.36). The region of the spinal cord to which one pair
of anterior and posterior nerve roots attaches is called a spinal segment
(Table 8.3).
Each anterior spinal root emerges from
the cord as a series of small rootlets, while similar branches from each
posterior spinal root sink into the posterolateral sulcus. The anterior and
posterior roots cross the subarachnoid space and unite in the appropriate
intervertebral foramen to form a spinal nerve. Each posterior root is
characterized by a ganglion just proximal to the union of anterior and
posterior roots.
Because the spinal cord is shorter
than the vertebral column, the lower spinal nerve roots descend within the
vertebral canal before leaving through their intervertebral foramina (Table
8.3). These lumbar, sacral and coccygeal nerve roots are clustered around the
conus medullaris and filum terminale to form the cauda equina (Figs 8.35 &
8.37). Compression of the cauda equina, for example by a large central prolapse
of an intervertebral disc, may disrupt bladder function and sphincter control
and also produce bilateral sensory and motor abnormalities in the lower limbs.
The spinal cord receives its arterial
supply from anterior and posterior spinal arteries that arise from vertebral
arteries and are reinforced by branches of the deep cervical, intercostal and
lumbar arteries. These vessels form a longitudinal anastomosis that runs the
length of the spinal cord. One main branch from an upper lumbar or lower
thoracic artery often provides an important supply to the spinal cord. Damage
to this vessel, the artery of Adamkiewicz, may threaten the viability of part
of the spinal cord. For example, lumbar arteries may become blocked in aortic
aneurysm disease, resulting in spinal cord ischaemia leading to paraplegia with
paralysis and loss of sensation in both lower limbs. The venous return from the
spinal cord drains into a series of longitudinal anastomosing channels, which
empty into veins corresponding to the arteries.
