The
Spinal Cord
The spinal cord is part of the central nervous system. It is a
continuation of the medulla and passes out of the cranial vault through the
foramen magnum and continues to run caudally in the vertebral canal, formed from
the adjacent vertebral foramina.
Realize that the spinal cord does not extend the entire length of
the vertebral column in the adult. In the developing embryo the spinal cord
extends all the way down to the sacral region, however, during growth and development
the vertebral column grows faster than the spinal cord thus spinal levels and
vertebral levels do not coincide.
1) realize that
the spinal cord ends at approx. L3 in infants, and approx. L1-L2 in adults.
2) The spinal
cord segments do not correspond to the vertebral levels.
Notice that the spinal cord has two areas of enlargement
1) Cervical
enlargement
2) Lumbosacral
enlargement
These enlargements are caused by the
corresponding nerves making up the brachial plexus and the lumbar and sacral
plexuses.
Spinal Nerves
There are 31 pair of spinal nerves
8 cervical
12 thoracic
5 lumbar
5 sacral
1 coccygeal
Note that there are only 7 cervical
vertebra, thus one pair of cervical spinal nerves exits the spinal cord above
C1.
Spinal nerves are made of dorsal and ventral roots.
Ventral roots
leave the anterior (ventral) horn of the spinal cord and carry motor (efferent)
fibers.
Dorsal roots
enter the posterior (dorsal) horn of the spinal cord and carry sensory
(afferent) fibers.
This means that the cell bodies whose
axons make up the ventral root are found in the ventral horn of the spinal
cord. Note that these are unipolar neurons.
The dorsal root, however, is comprised of axons
from pseudounipolar neurons whose cell bodies are located in the dorsal root
ganglion (spinal ganglion). The spinal
ganglion is found in the intervertebral foramen, here it can lay on the pedicle
of the lower vertebra.
Distal to the spinal ganglion, and
immediately outside the vertebral foramen, the dorsal and ventral roots join to
form a spinal nerve. This spinal nerve subsequently divides into a dorsal and
ventral rami.
DO NOT
confuse dorsal and ventral rami with dorsal and ventral roots.
Due to the differential growth of the spinal cord and the
vertebra, we said the spinal nerves do not correspond with their vertebral
levels.
Rationalize this phenomenon
Notice the spinal cord ending at approx. L1-L2 yet the dorsal
and ventral roots continue down the vertebral canal to exit at their
appropriate vertebral levels. This
collection of dorsal and ventral roots is termed the cauda equina
(horses tail).
The tip (caudal end) of the spinal cord is cone shaped and is
termed the conus medullaris.
Extending from the tip of the conus medullaris is a delicate
filament which helps anchor the spinal cord, the filum terminale. It ends at S2 where it attaches to the
inferior portion of the dural sac. The filum terminale has
no functional significance.
The Meninges
The meninges are thin tissue layers which surround and support the
spinal cord.
There are three
layers of meninges:
1) dura mater
2) arachnoid
mater
3) pia mater
the arachnoid and
pia mater together are termed the leptomeninges.
Dura Mater
(tough - mother)
is the outermost layer of the
meninges and is composed of fibrous and elastic tissue. It surrounds the brain and the spinal cord.
The spinal portion is a long tube which hangs from the foramen magnum and is
usually anchored at the inferior border of S2 in adults
Arachnoid
Mater (weblike)
is the middle layer of the meninges. It
very closely follows the dura mater but is not adhered to it. The dura and
arachnoid are separated by a potential space termed the subdural space.
Note that the arachnoid also surrounds the cauda equina.
Pia Mater
(tender - mother)
is the deepest layer of the meninges. it is separated from the arachnoid by an actual
space, the subarachnoid space. These two layers are connected by delicate
strands of connective tissue called arachnoid trabeculae. The subarachnoid
space is filled with CSF (cerebrospinal fluid) which baths the brain and spinal
cord with shock absorbing fluid.
is made up of two fused layers of loose
connective tissue. These enclose a fine meshwork of blood vessels and adhere
very closely to the spinal cord.
forms the denticulate ligaments. These are ligaments (21 of them) which
anchor the spinal cord to the dura mater.
Blood supply of the spinal cord
The spinal cord receives its blood supply principally from 3
longitudinal vessels
1 anterior spinal artery and
2 posterior spinal arteries
Anterior Spinal Artery
runs down the entire length of the spinal
cord in the ventral median fissure
supplies the anterior 2/3 of the spinal
cord
Notice that it begins to taper until it is
resupplied by a major radicular artery. Thus it is smallest in the area of T4
to T8.
Posterior Spinal Artery
arises off of either the vertebral arteries
or the posterior inferior cerebellar arteries and pass down the posterior
aspect of the spinal cord.
These arteries have extensive anastomosies
between themselves and the anterior spinal artery.
In general the posterior spinal arteries
supply the posterior 1/3 of the spinal cord.
Notice that these vessels also taper until
resupplied by radicular arteries.
The anterior and posterior arteries alone can only supply enough
blood to maintain the upper cervical segments of the spinal cord hence they must
be supplied with blood at increasingly lower levels. This is accomplished via
radicular arteries.
Anterior radicular arteries supply the
anterior spinal artery.
Posterior radicular arteries supply the
posterior spinal arteries.
The radicular arterial supply arises from
spinal branches of the following arteries:
1) vertebral
2) ascending cervical
3) posterior intercostal
4) lumbar
5) lateral sacral
The radicular arteries also supply the vertebra and the meninges
These arteries enter through the intervertebral foramen and pass
along the anterior and posterior (ventral and dorsal) roots of the spinal
nerves to reach the cord.
The Great Radicular Artery (artery of Adamkiewicz)
is a large
artery, usually one of the anterior radiculars, which supplies the lumbosacral
enlargement of the cord.
is
also known as the arteria radicularis magnus. It is important because it
is the major blood supply to the inferior 2/3 of the spinal cord.
Notice the watershed area of the spinal cord in the midthoracic
region.
Venous Drainage of the Spinal Cord
The veins of the spinal cord are distributed similar to the
arteries. There are usually 3 anterior veins and 3 posterior veins. These veins
communicate extensively with each other and are drained via radicular veins
which lead to
intervertebral veins and then to vertebral veins, ascending lumbar
veins, and to the azygous system.