414 Neuroanatomy: Draw It to Know It
Horizontal Saccade Circuitry
Here, we will draw the supra-ocular circuitry for hori-
zontal saccadic eye movements (aka horizontal sacca-
des). First, draw a brainstem in coronal view and divide
it into its midbrain, pons, and medulla, and then label its
anatomic left and right sides. Next, above the brainstem,
draw a right cerebral hemisphere and label the frontal
eye fi elds. Now, in the upper midbrain, draw the right
superior colliculus. For a long time, the importance of
the superior colliculus went unrecognized because iso-
lated lesions of the superior colliculus are rare, but exper-
imental inactivation of the superior colliculus has proven
its signifi cance. Th e superior colliculus is commonly
divided into a dorsal, “visuosensory” division and a ven-
tral, “motor” division. Th e dorsal division receives an
organized retinotopic map of the contralateral visual
hemifi eld from retinal ganglion cells and also receives
aff erent input from visual cortical regions, as well, includ-
ing the striate, extrastriate, and frontal cortices. Th e ven-
tral division projects to the contralateral abducens
nucleus (as drawn next).
Now, draw the bilateral abducens nuclei, which span
from the mid-pons to the low pons. For the remainder of
the diagram, we will draw the left brainstem ocular
nuclei, only. Next, in the mid-pons, draw the excitatory
burst neurons of the paramedian pontine reticular for-
mation; they lie anterior to the superior aspect of
the abducens nucleus. Th en, draw the inhibitory burst
neurons of the medullary reticular formation; they lie
within the rostral medulla anterior to the plane of the
abducens nucleus. Now, show that the neural integrator
lies along the dorsal tegmentum of the upper medulla;
it lies just anterior to the fourth ventricle. Finally, draw
the omnipause neurons in midline, which lie in between
the rootlets of the abducens nerves in the pontine
tegmentum.
Next, let’s add the functional circuitry for horizontal
saccades. First, show that the frontal eye fi elds directly
excite contralateral excitatory burst neurons, and then
show that the frontal eye fi elds also send an indirect
excitatory pathway that synapses within the superior col-
liculus, which, in turn, excites the contralateral excit-
atory burst neurons. Note that the indirect pathway is
actually more robust than the direct pathway. Now, draw
excitatory projection fi bers from the excitatory burst
neurons to the ipsilateral abducens nucleus. Th is excita-
tion stimulates the fi nal common pathway for horizontal
saccades (see Drawing 23-1 ).
Next, let’s interrupt our diagram to demonstrate
with our fi sts that the right hemisphere drives the eyes
to the left and that the left hemisphere drives the eyes to
the right. Hold your fi sts in front of you; they represent
the frontal eye fi elds. Point your index fi ngers inward in
a V shape to show that each cerebral hemisphere drives
the eyes in the opposite direction. Next, drop your right
fi st to show that a destructive lesion, such as a stroke,
causes a loss of innervation from the right frontal eye
fi elds, and as a result, the left frontal eye fi elds drive the
eyes to the right. Th en, shake your right hand to show
that an excitatory event, such as a seizure, causes over-
stimulation of the right frontal eye fi elds, and so the right
frontal eye fi elds overpower the left and drive the eyes to
the left.
Now, let’s continue with our diagram. Show that in
addition to exciting the abducens nucleus, the excitatory
burst neurons also excite the ipsilateral inhibitory burst
neurons, which inhibit the contralateral abducens
nucleus. Th us, when the left abducens nucleus is stimu-
lated, the right abducens nucleus is inactivated, which
prevents both nuclei from fi ring at the same time.
Next, show that the omnipause cells tonically inhibit
the excitatory burst neurons and also the inhibitory
burst neurons. Th en, indicate that the excitatory burst
neurons inhibit the omnipause neurons and inactivate
their tonic suppression of the burst neurons. Finally,
indicate that the neural integrator receives fi bers from
and projects fi bers to a wide array of nuclei, including
the excitatory and inhibitory burst neurons, to sustain
gaze, as detailed further in Drawing 23-3.^1 – 3