Point 13
Intro
Eye
movements induced by the vestibular apparatus are
compensatory. That is, they oppose head movements or changes
in head position and act to keep the fovea of the retina on
an object of interest. For example, a quick turn (or push)
of your head to the RIGHT will result in a
compensatory reflex turning of the two eyes to the
LEFT. You already know some of the receptors and
pathways underlying this reflex. Thus a quick rotation of
the head to the RIGHT will turn on the hair cells in
the RIGHT horizontal semicircular canal, increase the
firing of the right vestibular nerve and increase the firing
of neurons in the RIGHT vestibular nuclei. Cells in
the RIGHT vestibular nuclei send their axons
across the midline to the contralateral PARAMEDIAN
PONTINE RETICULAR FORMATION (PPRF). The PPRF, which lies
within the medial portion of the pontine tegmentum, ventral
to the abducens nucleus, is an integrative region involved
in the generation of horizontal eye movements. Neurons in
the LEFT PPRF project to the LEFT ABDUCENS
nucleus. The abducens nucleus lies just off the midline
within the dorsal part of the pons, just under the fourth
ventricle. It contains two types of neurons. The larger
motor neurons in this nucleus possess axons that pass
ventrally through the pons to exit on the ventral surface of
the brain stem (at the pontomedullary junction). Axons of
C.N. VI then innervate the ipsilateral LATERAL RECTUS
(LR6). There are also other smaller neurons in the
abducens nucleus whose axons do not leave the brain stem,
but rather CROSS and ASCEND in the medial
longitudinal fasciculus (MLF; this is referred to as the
ascending part of the MLF) to terminate in the oculomotor
nucleus (C.N. III). In particular, these crossed axons from
the abducens nucleus end only upon those neurons within the
oculomotor nucleus that innervate the MEDIAL RECTUS
muscle. Remember, neurons in the oculomotor nucleus that
innervate other eye muscles, as well as the preganglionic
parasympathetic neurons that innervate the ciliary ganglion,
do not receive this crossed input from the abducens
nucleus. Only those neurons that innervate the medial rectus
muscle receive this ascending, crossed input.
Now you can see how a rotatory movement of the head to the RIGHT results in a increase in discharge of the RIGHT vestibular nerve, an increase in firing of the RIGHT vestibular nuclei, an increase in firing of neurons in the LEFT PPRF, an increase in firing of both small and large neurons in the LEFT abducens nucleus and reflex turning of the left eye to the LEFT (via LEFT lateral rectus; C.N. VI) and the right eye to the LEFT (via ascending MLF input to the RIGHT medial rectus; C.N. III). This is called the VESTIBULO-OCULAR REFLEX (VOR), which is a critically important reflex for stabilizing visual images in the presence of a continuously moving head.
Now
we can examine the results of a lesion in the LEFT
vestibular nerve on eye movements. Such a lesion puts the
RIGHT vestibular nerve in "control". This imbalance
results in the eyes being pushed slowly to the LEFT
(right vestibular nerve turns on right
vestibular nuclei, which turns on the left PPRF,
which turns on the left abducens which turns both
eyes to the left). When the eyes are pushed as far
LEFT as possible, they snap back very quickly to the
RIGHT by mechanisms not fully understood. The eyes
then slowly move to the LEFT again, and this vicious
cycle continues. This nodding back and forth is called
NYSTAGMUS (to nod). It is named (i.e., right or left)
by the FAST direction. For instance, a lesion of the
LEFT vestibular nerve will result in a RIGHT
nystagmus. Thus, the RIGHT (intact) vestibular nerve
is "driving" the LEFT PPRF and LEFT ABDUCENS
to move the eyes slowly to the LEFT, after which they
reflexively snap back to the RIGHT (i.e., the
direction of the nystagmus).
Now
we need to consider pathways involved in VOLUNTARILY turning
both of our eyes horizontally to the LEFT in order to
see a new object of interest. This is called a left
horizontal saccade (jerk). We already know that to do this
we need to have the LEFT lateral rectus and the
RIGHT medial rectus contract synchronously. The two
eyes will then move together (conjugately) to the
LEFT. To VOLUNTARILY do this, we use a pathway that
begins in the frontal eye fields of the cerebral cortex
(area 8). This is a cortical area that lies rostral to
the primary motor area (area 4). To voluntarily move
your eyes to the LEFT, information from your
RIGHT frontal eye fields is conveyed to the
LEFT (contralateral) PPRF. You should know the rest
from here, but I'll help! The RIGHT frontal eye field
will tell your LEFT PPRF to turn on both large and
small neurons in the LEFT abducens nucleus. Two
things will then happen. The LEFT eye will turn
LEFT (laterally) and the RIGHT eye will turn
LEFT (medially). This is a voluntary LEFT
horizontal saccade.