Now, I want you to understand the fact that the gray matter is divided into finer divisions. We looked at
dorsal, ventral, and intermediate horns. But the gray matter is divided into divisions that are referred to as
Rexed laminae. These are going to number I through X. I will be located most dorsally. IX is located out
here ventrally. Then X is going to be in an area of decussation of axons. Now, I want you to understand
some of the basic functions that are associated with these Rexed laminae. First, we’re going to bundle
Rexed laminae I through IV together. You can see where they’re located within the dorsal gray horn
or dorsal horn. These are responsible for exteroceptive sensations. Here you’re receiving information
from the periphery of the body as it might relate to fine touch, temperature, vibration, or pressure.
Rexed laminae V and VI shown here and here are responsible for proprioceptive sensation. These also
serve as a relay between the midbrain and the cerebellum. Rexed lamina number VI also will have some
neurons that will give rise to the spinocerebellar pathways. This will be ascending in nature. Number VII
shown in through here would only be found where you have visceral motor output, sympathetic motor
output. So you’re looking at this particular lamina being found in spinal cord segments T1 through T12,
L1, L2 and perhaps L3. VIII and IX are shown here and here. These Rexed laminae are responsible for
final motor output to the periphery. Then lastly, between the right and left gray horns, you have this
more centrally located area where you have a decussation of axons between the right and left sides.
If we highlight specifically Rexed lamina number IX, again this is coordinating final motor output along
with VIII but in Rexed lamina number IX, there is a topographic organization by motor function.
Here you’re going to find alpha and gamma motor neurons. There are two rules for you to remember
about this topographic organization by function. One is the proximal to distal rule. What we have here
in the upper limb within Rexed lamina number IX, the more proximal musculature of the upper limb is
found medially. The distal musculature of the upper limb is found more laterally. The proximal to distal
rule is topographic organization is then medial to lateral. There’s also a flexor extensor rule regarding
this topographic organization. That is the flexors, in this case the upper limb are oriented more
posteriorly within the Rexed lamina number IX, whereas extensor muscles become arranged so that
they are more anteriorly located within this Rexed lamina. Now, I want you to understand that the spinal
cord tracts are going to be involved in transmitting or conveying functional information. This functional
information can be ascending, going from the spinal cord upwards to higher brain centers such as the
cerebral cortex. You can have functional information being conveyed in a descending manner coming
from the cerebral cortex, for example, down to the level of the spinal cord. Highlighted in blue are
various ascending pathways or tracts. An example here would be the lateral spinothalamic tract shown
in through here. You’d also have, yet as another example, the anterior spinothalamic tract. These are
conveying sensory information from the cord level which is receiving information from the periphery
and then sending that up to the cerebral cortex so that it may be perceived. The other type of functional
transmission is going to be descending in nature and this is conveying motor information. Some of the
examples here are shown in red. This would be your lateral corticospinal tract, for example, here.
Then this one arranged here would be your anterior corticospinal tract. So you want to be able to use
these tracts to innervate your peripheral skeletal musculature.