116 Neuroanatomy: Draw It to Know It
Major Ascending & Descending Tracts
Here, we will draw the posterior column pathway, the
anterolateral system (which includes, most notably, the
spinothalamic tract), and the lateral corticospinal tract.
First, draw an axial cross-section through the spinal cord.
Next, label the origin and termination points of our
pathway; label the right peripheral nerve and the left
cerebral hemisphere’s primary motor cortex and primary
sensory cortex.
Now, label the posterior column of the right side of
the posterior white matter. Posterior column fi bers ascend
the spinal cord ipsilateral to their side of origin. Next,
label the anterolateral system bundle on the left side of
the spinal cord — the anterolateral system fi bers ascend
the spinal cord contralateral to their side of origin. Finally,
draw the lateral corticospinal tract on the right side of the
spinal cord — the lateral corticospinal tract fi bers descend
the spinal cord contralateral to their side of origin.
Next, let’s label cell bodies for each pathway and then
draw each pathway’s course. We will abbreviate the cell
bodies for each pathway as follows: posterior column
pathway cell body as PCP, anterolateral system cell body
as ALS, and lateral corticospinal tract cell body as CST.
Show that the fi rst cell body (the fi rst-order sensory
neuron) for the posterior column pathway lies in the
dorsal root ganglion. Th is cell body is pseudo-unipolar:
indicate that it projects a single axon bundle over a very
short distance, which divides into a peripheral process
(the peripheral nerve) and a central process (the poste-
rior nerve root). Next, show that the second-order sen-
sory neurons lie in the gracile and cuneate nuclei in the
medulla. Th en, draw the third-order sensory neuron in
the contralateral thalamus. Now, draw the posterior
column pathway, itself. Indicate that the central process
enters and ascends the posterior column without form-
ing a synapse in the spinal cord and that it instead fi rst
synapses in the gracile and cuneate nuclei in the medulla.
Next, show that the gracile and cuneate nuclei send
decussating fi bers across the medulla via the internal
arcuate fasciculus; these fi bers ascend the brainstem via
the medial lemniscus and synapse in the third-order
neuron in the ventroposterior lateral nucleus of the thal-
amus. Finally, indicate that the thalamus projects to the
sensory cortex.
Now, let’s draw the cell bodies and fi ber pathway
for the anterolateral system. Indicate that the fi rst-
order neuron lies within the dorsal root ganglion; then,
show that the second-order neuron lies within the dorsal
horn of the spinal cord; and fi nally, show that the third-
order neuron lies within the contralateral thalamus (also
within the ventroposterior lateral nucleus). Now, draw
the anterolateral system pathway, itself. Show that ante-
rolateral system central processes project from the dorsal
root ganglion to the dorsal horn. Th ese inputs ascend
and descend a variable number of spinal cord levels
before synapsing in the spinal cord. Th en, show that at or
near their level of entry into the spinal cord, the antero-
lateral system fi bers decussate via the ventral commissure
and bundle in the anterolateral spinal cord, where they
ascend the spinal cord and brainstem to synapse in the
thalamus. Finally, show that the thalamus projects to the
sensory cortex. Note that whereas the posterior column
pathway ascends the spinal cord ipsilateral to its side of
origin, the anterolateral system ascends the spinal cord
contralateral to its side of origin.
Lastly, let’s draw the cell bodies and pathway for the
lateral corticospinal tract. Indicate that the fi rst-order
neuron lies in the motor cortex, most notably, but also in
the premotor and sensory cortices, and then that the
second- order neuron lies within the contralateral ante-
rior gray matter horn of the spinal cord. Now, show
that lateral corticospinal fi bers descend from the motor
cortex through the ipsilateral brainstem, decussate
within the medullary pyramids at the cervicomedullary
junction, and then descend through the spinal cord in
the lateral corticospinal tract to synapse in spinal motor
neurons. Th en, show that motor neurons project nerve
fi bers via the anterior nerve root, which joins the poste-
rior nerve root to form a mixed spinal nerve.^2 , 4 – 12