240 Neuroanatomy: Draw It to Know It
The Vestibular System
Here, we will draw the vestibulo-ocular refl ex, vestibu-
lospinal pathways, and a few additional central vestibu-
lar projections. We will begin with the vestibulo-ocular
refl ex. Each of the semicircular canals has its own projec-
tion pattern (see Figure 14-4); here, we will draw the
horizontal canal’s excitation of the ocular nuclei because
it is the most clinically helpful pathway to learn. Draw a
coronal view of the brainstem and include an axial sec-
tion through a pair of eyes. Next, draw the left vestibular
nuclear complex, which spans much of the height of the
pons and the medulla. Now, label the left oculomotor
nucleus and also the right abducens nucleus. Show that
the vestibular nuclear complex excites the contralateral
abducens nucleus, which causes the right lateral rectus
muscle to produce eye a b duction: it drives the right eye
to the right. Th en, indicate that the abducens nucleus
also projects fi bers across midline that ascend the medial
longitudinal fasciculus and excite the contralateral ocul-
omotor nucleus, which causes the left medial rectus
muscle to produce eye a d duction: it drives the left eye to
the right. Lastly, show that the vestibular nuclear com-
plex also sends direct projections to the ipsilateral oculo-
motor nucleus via the ascending tract of Deiters. Note
that the vestibular nuclear complex simultaneously
inhibits the ipsilateral abducens nucleus and contralat-
eral oculomotor nucleus, and also note that of the four
vestibular nuclei (drawn later), the medial and lateral
nuclei have the most robust ocular projections.^2
For a simple way to remember this circuitry at the
bedside, hold your fi sts in front of you and show that the
left vestibular nuclear complex drives the eyes to the right
and the right vestibular nuclear complex drive the eyes to
the left. Next, drop one fi st to demonstrate that when
one vestibular nuclear complex is damaged, the eyes devi-
ate toward the damaged side, away from the intact side.
Next, we will draw the vestibulospinal pathways.
Redraw a brainstem. Include an outline of the vestibular
nuclear complex, which spans from the upper pons to
the inferior fl oor of the fourth ventricle (in the medulla).
To draw the individual nuclei of the vestibular nuclear
complex, divide the complex into medial and lateral
divisions. Label the top of the medial division as the
superior nucleus and the rest as the medial nucleus; then,
label the top half of the lateral division as the lateral
nucleus and the bottom half as the inferior nucleus. Th e
lateral nucleus is also called Deiters’ nucleus and the
inferior nucleus is also called the descending nucleus.
Note that although not shown as such in our diagram,
the lateral and inferior nuclei actually overlap in the low
pons and the medial and superior nuclei overlap in the
mid-pons.
Now, to continue our vestibulospinal pathway dia-
gram, include upper and lower levels of the spinal cord.
Indicate that the lateral vestibular nucleus projects
uncrossed descending fi bers, which descend the ventral
spinal cord and innervate extensor motor neurons
throughout the spinal cord, and label this projection as
the lateral vestibulospinal tract. Th en, indicate that the
medial vestibular nucleus projects crossed and uncrossed
fi bers that descend the medial spinal cord to innervate
extensor motor neurons in the upper spinal cord, only,
and label this as the medial vestibulospinal tract. Note
that the medial vestibulospinal tract also receives minor
inputs from the lateral and inferior vestibular nuclei, as
well. Th ese vestibulospinal pathways provide antigravity
movements for the maintenance of posture, as do the
reticulospinal tracts (see Drawing 7-3).
Next, let’s list a few additional central vestibular func-
tions. Indicate that the vestibular nuclei project to the
thalamus for somatosensory detection of vestibular
movement and to the reticular formation, which induces
nausea and vomiting. Lastly, indicate that the vestibulo-
colic refl ex maintains the head at a level position during
movement.^2 – 4