416 Neuroanatomy: Draw It to Know It
Horizontal Saccade Details ( Advanced )
Here, we will review and elaborate on our discussion of
the anatomy and control of horizontal saccades. Across
the top of the diagram, write the following headings:
center, anatomy, action, and defi cit. First, list the supra-
ocular command centers for horizontal saccades. Begin
with the frontal eye fi elds, which comprise the frontal
eye fi e l d , a discrete region in the anterior precentral
sulcus near the superior frontal sulcus that is homolo-
gous to the location of the frontal eye fi eld of the macaque
monkey; and then show that the frontal eye fi elds also
comprise, most notably, the supplementary eye fi eld and
dorsolateral prefrontal cortex. Other important supra-
ocular command centers include the superior colliculus
and cerebellum, and also the anterior cingulate cortex
and basal ganglia. Now, in regards to their action, indi-
cate that the frontal eye fi elds and superior colliculus
drive the eyes to the contralateral side, and then show
that injury to these structures produces contralateral
volitional horizontal gaze palsy.^4 , 5 Note that horizontal
saccades via the vestibulo-ocular refl ex are spared in the
setting of frontal eye fi eld or superior colliculus injury.
Next, let’s address the excitatory burst neurons; indi-
cate that they lie within the nucleus reticularis pontis
caudalis of the paramedian pontine reticular formation
(PPRF) — the nucleus reticularis pontis caudalis lies
within the mid-pons, anterior to the superior aspect of
the abducens nucleus. Indicate that the excitatory burst
neurons activate the ipsilateral abducens nucleus, and
also show that they activate the ipsilateral inhibitory
burst neurons, which inhibit the contralateral abducens
nucleus. Th en, show that the excitatory burst neurons
also inhibit the omnipause neurons and innervate the
neural integrator, as well. Next, show that injury to the
PPRF produces ipsilateral horizontal gaze palsy. When
the PPRF is selectively injured, the fi nal common path-
way will be spared; however, due to its proximity to the
abducens nucleus, PPRF injury is most oft en associated
with abducens nucleus injury, which disrupts the fi nal
common pathway.
Now, let’s address the inhibitory burst neurons; show
that they lie within the nucleus paragigantocellularis
dorsalis of the medullary reticular formation — the
nucleus paragigantocellularis dorsalis lies within the
rostral medulla anterior to the plane of the abducens
nucleus. Indicate that the inhibitory burst neurons, most
notably, suppress the contralateral abducens nucleus
from fi ring , which prevents antagonist forces on the
intended horizontal saccade movement. Injury to the
inhibitory burst neurons is hypothesized to produce
ocular fl utter, high-frequency conjugate horizontal sac-
cades without an intersaccadic interval.^1
Next, let’s address the neural integrator; show that it
lies within the medial vestibular nucleus and the nucleus
prepositus hypoglossi of the perihypoglossal complex,
which lie along the dorsal tegmentum of the upper
medulla just anterior to the fourth ventricle. Indicate that
the neural integrator produces gaze holding. Show that
injury to the neural integrator causes a leaky integrator,
which means that the eyes do not remain in the intended
direction of gaze but instead drift back to center, prompt-
ing a corrective saccade.
Finally, let’s address the omnipause neurons; show that
they lie within the nucleus raphe interpositus, which sits
in midline in between the rootlets of the abducens nerves
in the pontine tegmentum. Indicate that omnipause cells
tonically suppress the excitatory and inhibitory burst
neurons except immediately before and during saccadic
eye movements. Th e predicted defi cit in omnipause cell
injury is opsoclonus: multidirectional saccadic oscilla-
tions without an intersaccadic interval, but what is actu-
ally observed, instead, is a slowing of saccades. Opsoclonus
is oft en ascribed to cerebellar injury; the cerebellum
receives projections from the frontal eye fi elds and supe-
rior colliculus, which fi rst relay, most notably, within the
nucleus reticularis tegmenti pontis, which lies within the
mid- to upper antero-central pontine tegmentum.^1 – 3