114 Neuroanatomy: Draw It to Know It
Descending Pathways (Advanced)
Here, we will draw the descending spinal cord pathways
in axial cross-section. First, draw one half of an outline of
an axial cross-section of the spinal cord and include a
gray matter horn. Th en, label the following terms across
the top of the page: tract, origin, termination, and func-
tion. Next, label the lateral corticospinal tract in the lat-
eral funiculus and then the anterior corticospinal tract
along the anterior median fi ssure. Indicate that the ante-
rior corticospinal tract fi bers originate in the motor cor-
tices (most notably), terminate in contralateral spinal
motor neurons, and innervate proximal musculature for
gross motor movements. Next, outside of our table, make
a notation that the lateral corticospinal tract fi bers inner-
vate distal musculature for fi ne motor movements (we
draw this pathway in Drawing 7-4 ). Th en, label the
somatotopic organization of the lateral corticospinal
tract: indicate that the medial aspect of the lateral corti-
cospinal tract carries the arm fi bers and the lateral aspect
carries the leg fi bers.
Next, label the hypothalamospinal tract alongside the
lateral corticospinal tract in the lateral funiculus. Indicate
that this tract originates in the hypothalamus, descends
ipsilaterally, and carries hypothalamic control of auto-
nomic function. Now, label the rubrospinal tract near to
the lateral corticospinal tract. Indicate that it originates
from the red nucleus in the midbrain, crosses in the ven-
tral tegmental area of the brainstem, and innervates the
upper cervical spinal cord to produce arm fl exion. Next,
label the tectospinal tract in the anterior-medial spinal
cord. Indicate that it originates in the superior colliculus
of the midbrain, decussates in the dorsal midbrain teg-
mentum, and innervates the upper cervical spinal seg-
ments to produce contralateral head turn.
Now, let’s address the reticulospinal tracts and ves-
tibulospinal tracts, both of which maintain posture
through the activation of antigravity muscles. In our
axial cross-section, label the medial reticulospinal tract
and medial vestibulospinal tract in the anterior-medial
spinal cord and label the lateral reticulospinal tract and
lateral vestibulospinal tract in the lateral funiculus (the
lateral vestibulospinal tract borders the anterior funicu-
lus). Next, indicate that the medial reticulospinal tract
originates in the medial zone of the pontine reticular
formation and passes predominantly ipsilaterally to acti-
vate the axial and proximal limb extensors. Th en, indi-
cate that the lateral reticulospinal tract originates in the
medial zone of the medullary reticular formation and
passes predominantly ipsilaterally to inhibit the axial and
proximal limb extensors (and to a lesser degree it also
excites axial and proximal limb fl exors). Now, make a
notation that the vestibulospinal tracts excite the axial
and proximal limb extensors. Specifi cally, the medial
vestibulospinal tract acts on the cervical spinal cord to
excite neck extensor musculature and the lateral ves-
tibulospinal tract projects along the height of the spinal
cord to excite paravertebral and proximal limb extensor
muscles.^7 , 9 , 13 , 14
Next, consider that normally the cerebral cortex
inhibits the rubrospinal and reticulo- and vestibulospi-
nal tracts, but when cortical inhibition is lost, these three
tracts are left unchecked, which results in upper extrem-
ity fl exion and lower extremity extension, referred to as
decorticate posturing. Th en, consider that a lesion that
cuts off both the cortical and rubrospinal tracts (ie, a
lesion below the level of the midbrain) leaves the retic-
ulo- and vestibulospinal tracts unchecked, which results
in neck and limb extension, referred to as decerebrate
posturing.
Finally, in regards to the somatotopy of the anterior
gray matter horns, show that the posterior nuclei inner-
vate the fl exor muscles and the anterior nuclei innervate
the extensor muscles, and then show that the medial nuclei
innervate the proximal muscles and the lateral nuclei
innervate the distal muscles. Notice how the somatotopic
organization of the gray matter parallels the positions of
the functionally-related white matter pathways.^2 , 4 – 12