Anatomy
of the spine and spinal cord
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
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
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.