Anatomy of the spine and spinal cord

  Vertebral column

This provides a mobile yet protective midline dorsal scaffold housing the spinal cord and cauda equina. Comprising a series of interlocking vertebrae the spinal nerves emerge via intervertebral foramina, the cord joining the brain stem at the foramen magnum (Fig. 33.1a and b).

Developed from the union of adjacent mesenchymal blocks, the basic pattern of anterior body and posterior arch exists throughout. These surround the central canal through which passes the spinal cord, a derivation of the neural plate, and lower down the nerves of the cauda equina. Between each vertebra  which are linked by anterior intervertebral disc and lateral facet joints — runs a series of connecting liga­ments to provide significant stability to the spine. These include particularly the anterior and posterior longitudinal ligaments running the full length of the vertebral column from the foramen magnum, attached to the periosteum of each vertebral body by an annulus of each disc. The interspinous ligaments link each spinous process and the supraspinous ligaments skip between the tips of each spinous process along the full length of the spine (Fig. 33.2).

Cervical spine

This comprises seven vertebrae with areas of specialisation. The Cl and C2 vertebrae join to form the atlantoaxial complex, the body of Cl being incorporated into the body of C2 to form the odontoid peg or dens. The Cl or atlas vertebra, therefore, becomes a ring between the occipital condyles of the skull and the C2 or axis vertebra. The posterior arch of Cl has no spinous process and may be incomplete.

           The axis or C2 vertebra forms an easily radiologically visible structure with the dens anteriorly and a large bifid spine posteriorly.

The remaining cervical vertebrae (C3 to C?) also have identifying features. Running in the lateral mass is the foramen transversarium through which passes the vertebral artery from its point of origin at the subclavian artery to where it loops over the lamina of C2 to gain access to the cranial cavity via the foramen magnum. The spinous process of these vertebrae (except C7) are usually bifid. The vertebral artery does not traverse the C? vertebra and this also has a prominent spinous process which can be palpated, giving rise to its name, the vertebra prominens.

Thoracic spine

Uniform in shape, the heads of the ribs articulate with adjacent bodies of these 12 vertebrae. Gradually the size of the vertebral body and the articular processes increases down towards the more solid lumbar vertebrae.

Lumbar vertebrae

These represent a more mobile part of the spine and yet represent an area of great strength. Formed of five vertebrae the lateral mass, pedicles and laminae are thicker and the spinous processes shorter and more vertical. The last mobile joint between L5 and S1 (lumbosacral junction) joins the spine to the rigid pelvis via the sacrum.

Sacrum

Situated between the iliac bones and joined by the sacroiliac joint, the bones of the sacrum are fused into one triangular shape through which traverse the terminal sacral nerve roots - S2, S3 and S4.

The basic vertebral structure of each component of the sacrum is maintained, but the bones blend into one another to form a smooth anterior and posterior surface.

Coccyx

These very small vertebrae do not transmit any neural structure and are attached to the sacrum.

The spinal canal

This contains the spinal cord and cauda equina enclosed in layers of meninges.

The spinal cord extends from the foramen magnum at the cervicomedullary junction to the conus medullaris — an area of expansion of the cord at the Ll/L2 junction.

From that point there is no spinal cord, the nerves continuing as the cauda equina. Surrounding the cord and the cauda equina is arachnoid mater, defining the cerebrospinal fluid (CSF) containing subarachnoid space, and the spinal dura or theca which is continuous with the cranial dura at the foramen magnum and forms a sleeve around each emerging nerve root. It is via this root sleeve that the sequential blood supply gains access to the spinal cord. The epidural space between the spinal theca and the bone of the spinal canal is filled with epidural fat and the epidural venous plexus.

The spinal cord

The cord itself is part of the central nervous system but arising from it are pairs of spinal nerves, each one composing a dorsal sensory root and a ventral motor root to form a numbered mixed spinal nerve (Fig. 33.3).

These emerge via the intervertebral foramenae and are numbered Cl to C8, Ti to T12, L1 to L5 and SI to S4.

The spinal cord is uniform in appearance with two areas of expansion — in the cervical region corresponding to the origin of the nerve roots to the arm and in the lumbar region at the conus medullaris.

The blood supply to the spinal cord is formed from the descending anterior and spinal arteries, from the vertebral arteries and from the segmental spinal arteries which form an anastomosis up and down the spine. Some of these segmental arteries are significantly enlarged, for example the artery of Adamkewitz at D10.