418 Neuroanatomy: Draw It to Know It
Vertical Saccades ( Advanced )
Here, we will draw the anatomic substrate for vertical
and torsional saccades. First, draw a coronal section
through the midbrain and pons. Label the left and right
sides of the brainstem. At the top of the diagram, at the
midbrain–diencephalic junction, in the plane of the
mammillary bodies, near the midline of the diagram,
draw the bilateral rostral interstitial nuclei of the medial
longitudinal fasciculus (riMLF). Next, just beneath the
bilateral riMLF, still in the midbrain–diencephalic junc-
tion, near midline in the dorsal tegmentum, draw the
bilateral interstitial nuclei of Cajal (INC). Now, beneath
the bilateral INCs, show that the oculomotor complex
straddles the midline of the mid- to upper midbrain; it
lies within the dorsal tegmentum just anterior to the
periaqueductal gray area. Note that although we oft en
represent the oculomotor nuclei as comprising two dis-
crete nuclei, it actually forms a single oculomotor com-
plex consisting of many subnuclei (see Drawing 12-5).
Next, beneath the oculomotor complex, in the lower
midbrain (in the plane of the inferior colliculi), draw the
bilateral trochlear nuclei, which also lie within the dorsal
tegmentum, near midline. Lastly, draw the nucleus raphe
interpositus in midline in the pontine tegmentum; it lies
in between the rootlets of the abducens nerves.
Now that we have completed an anatomic view of the
relevant ocular nuclei for vertical and torsional saccades,
let’s describe their supranuclear control. Write out the
headings: nucleus and function. Th en, indicate that the
riMLF comprises the excitatory burst neurons for verti-
cal and torsional saccades; it acts in similar fashion to the
nucleus reticularis pontis caudalis of the PPRF for hori-
zontal saccades. Now, show that the INC is the neural
integrator for vertical and torsional saccades, similar to
the medial vestibular nucleus and nucleus prepositus
hypoglossi for horizontal saccades. Next, show that the
nucleus raphe interpositus contains the omnipause neu-
rons for vertical and torsional saccades just as it does for
horizontal saccades. Lastly, note that certain texts
describe the INC as containing the inhibitory burst neu-
rons for vertical and torsional saccades whereas others
describe the riMLF as containing them; as a reminder,
for comparison, the nucleus paragigantocellularis dorsa-
lis of the medullary reticular formation contains the
inhibitory burst neurons for horizontal saccades.
Next, we will list the high points of vertical and tor-
sional saccade circuitry. First, indicate that the riMLF
produces bilateral projections for upward gaze but only
ipsilateral projections for downward gaze. Th us, a unilat-
eral riMLF lesion will cause isolated downward gaze
palsy (observed as slowing of downward gaze), but
redundant innervation for upward gaze preserves the
function and speed of upward gaze. Next, indicate the
laterality of riMLF innervation for torsional saccades.
Show that from the subject’s point of view, the right
riMLF produces clockwise torsional rotation of the eyes,
whereas the left riMLF produces counterclockwise tor-
sional rotation of the eyes. Stated diff erently, from the
subject’s perspective, the top poles of the eyes rotate
toward the riMLF that is activated. To demonstrate this,
hold your fi sts in front of you and point your index fi n-
gers straight up. Now, activate the right riMLF and rotate
your fi ngers to the right; then, activate the left riMLF
and rotate your fi ngers to the left. Note that the torsional
defi cits observed in unilateral riMLF lesions are more
substantial than the vertical defi cits.
Finally, consider that whereas the frontal eye fi elds
and superior colliculus project to the contralateral PPRF
excitatory burst neurons for horizontal saccades, they
project to the ipsilateral riMLF for vertical and torsional
saccades.^1 – 3 , 6