264 Neuroanatomy: Draw It to Know It
Corticopontocerebellar Pathway
Here, we will draw the corticopontocerebellar pathway.
For this diagram, we need to draw one side of the brain,
the brainstem, and then the opposite side of the cerebel-
lum, and we need to establish the midline of the diagram.
Indicate that the bulk of the pathway originates in
the primary motor and sensory cortices; we exclude the
lesser contributions from more wide-reaching brain
regions. To get a sense of the corticopontocerebellar
pathway’s importance in movement, consider that nearly
20 million fi bers are dedicated to the corticopontocere-
bellar pathway, whereas only 1 million fi bers are dedi-
cated to the corticospinal tract.^24
Now, show that the corticopontocerebellar fi bers fi rst
descend to the pontine nuclei, where they make their
primary synapse. Next, show that the pontine nuclei
project across midline through the middle cerebellar
peduncle into the contralateral cerebellar cortex. As a
reminder, the inferior and middle cerebellar peduncles
are the main infl ow pathways into the cerebellum, and
the superior cerebellar peduncle is the main outfl ow
pathway from the cerebellum.
Th en, show that the cerebellar cortex projects to
the dentate nucleus, which lies deep within the cerebel-
lum. Now, draw the superior cerebellar peduncle, red
nucleus, and ventrolateral nucleus of the thalamus.
Indicate that the dentate projects fi bers out of the cere-
bellum through the superior cerebellar peduncle, which
cross midline within the midbrain, inferior to the red
nucleus, to synapse in the ventrolateral nucleus of the
thalamus. Th en, show that select fi bers synapse directly
in the red nucleus, instead. Note that the red nucleus
projections typically originate from the globose and
emboliform nuclei, which lie medial to the dentate
nucleus, whereas the thalamic projections typically orig-
inate from the dentate nucleus. Now, show that the thal-
amus projects back to the primary motor strip to
complete the corticopontocerebellar pathway.
Th e clinical application of this pathway comes in the
analysis of cerebellar defi cits. Cerebellar injuries cause
ipsilateral defi cits. If a person has a cerebellar defi cit (for
instance, incoordination), then either the ipsilateral cer-
ebellum or a portion of this pathway (somewhere along
its course) is aff ected. If the area of injury localizes con-
tralateral to the side of the body that is aff ected, think of
a corticopontocerebellar pathway lesion. Consider a
stroke in which there is a left third nerve palsy and right-
side hemiataxia. Where could the injury lie? On a sepa-
rate sheet of paper, draw the midbrain. Defi ne right
and left. Draw the left third nerve and its exiting fascicles
and then draw the right cerebellum. How can we con-
nect these disparate regions? Show that fi bers exit the
right cerebellum through the superior cerebellar pedun-
cle and pass adjacent to the third nerve fi bers on the left.
Indicate that injury here produces the aforementioned
defi cits (see Drawing 10-3, Claude's syndrome, for
details).^1 , 3 , 7 – 11