Download Spinal Cord Structure

Spinal Cord Structure
Recall that the central nervous system tissues can generally be divided
into white matter and gray matter. White matter is the myelincontaining region composed of axons, which make up the tracts of the
CNS. These carry information between different regions and structures
in the CNS. Gray matter contains the cell bodies and dendrites and
therefore is the site of synaptic transmission.
In the cortex of the brain, gray matter makes up the cortical (outer)
regions, while the white matter tracts tend to make up the majority of
the deep tissues of the brain, although there are exceptions to the latter,
such as the deep basal and thalamic nuclei that are composed of gray
matter. In contrast to this general arrangement of the brain, the spinal
cord is arranged with the white matter surrounding the central gray
matter, indicating that the spinal tracts carry information up and down
the cord along the outer aspects, while synaptic transmission tends to
occur more centrally.
Cross-
In the image above, you can see how the central gray matter is
somewhat butterfly shaped, with each side of the “butterfly” containing
a posterior (dorsal) horn and an anterior (ventral) horn. Each
of the horns is contiguous with the posterior and anterior spinal
nerve roots, respectively. The posterior root of the nerve carries
sensory information into the posterior horn, often synapsing there. The
anterior horn contains the cell bodies of somatic motor neurons,
and it sends its axons out the anterior root of the spinal nerve to the
muscle cells it innervates. The lateral horn is not found at all levels of
the spinal cord, but is limited to thoracic and lumber segments of the
cord. This is because the lateral horns contain the neurons of
the sympathetic nervous system, which leave the cord only in these
segments. Even though the cell bodies are found in the lateral horns,
their axons leave via the anterior nerve roots, just like those that control
skeletal muscle. The matched horns on each side of the “butterfly” are
connected via the gray commissure, which also surrounds the
cerebrospinal fluid filled central canal.
The white matter of the spinal cord is divided into columns. Each
segment
of
the
cord
contains
matched
posterior, lateral and anterior columns. The anterior columns
and posterior columns are partially separated by the anterior median
fissure and posterior median sulcus, respectively. Each pair is also
connected by a commissure of white matter that runs adjacent to the
gray commissure, termed the anterior and posterior commissures.
The columns are further divided into tracts that carry sensory
information up the spinal cord (ascending tracts) and motor
information down the spinal cord (descending tracts).
Although each segment of the spinal cord has similar features, there are
some differences along its length, as you may be able to determine from
the image above. The main difference is that the ratio of gray matter to
white matter varies among segments of the spinal cord. At the lower
levels of the spinal cord there is a greater ratio of grey matter to white
matter. This should make sense, as there are less ascending and
descending tracts of whiter matter as you move lower. As previously
mentioned, the lateral horns are only found in the thoracic and lumber
regions of the spinal cord, where they contain the motor nuclei of the
sympathetic nervous system. Finally, the size of the anterior and
posterior horns varies, depending on the amount of tissue they are
innervating. For example, the thoracic segments have relatively small
anterior horns, as there is little skeletal muscle to innervate in the
thorax and abdomen, while the cervical and thoracolumbar regions have
large anterior horns, used to innervate the skeletal muscles of the arms
and legs, respectively.