These
areas are connected to the oculomotor, trochlear, and abducens nuclei by the
medial longitudinal fasciculus (MLF) that runs through the majority of the
brain stem. The MLF allows for conjugate gaze, or the movement of the eyes in
the same direction, during horizontal movements that require the lateral and
medial rectus muscles. To elevate the eyes, the oculomotor nerve on either side
stimulates the contraction of both superior rectus muscles; to depress the
eyes, the oculomotor nerve on either side stimulates the contraction of both
inferior rectus muscles. Purely vertical movements of the eyes are not very
common. Movements are often at an angle, so some horizontal components are
necessary, adding the medial and lateral rectus muscles to the movement. The
rapid movement of the eyes used to locate and direct the fovea onto visual
stimuli is called a saccade.
Notice that the paths that are traced in are not strictly vertical. The movements between the nose and the mouth are
closest, but still have a slant to them. Also, the superior and inferior rectus
muscles are not perfectly oriented with the line of sight. The origin for both
muscles is medial to their insertions, so elevation and depression may require
the lateral rectus muscles to compensate for the slight adduction inherent in
the contraction of those muscles, requiring MLF activity as well. The left
panel of this figure shows a painting of a woman’s face, and the right panel
shows lines traced over the painting. These lines represent the shifts in the
gaze of a person looking at another face. Saccades are rapid, conjugate movements
of the eyes to survey a complicated visual stimulus, or to follow a moving
visual stimulus. This image represents the shifts in gaze typical of a person
studying a face. Notice the concentration of gaze on the major features of the
face and the large number of paths traced between the eyes or around the mouth.
Testing eye movement is simply a matter of having the patient track the tip of
a pen as it is passed through the visual field. This may appear similar to
testing visual field deficits related to the optic nerve, but the difference is
that the patient is asked to not move the eyes while the examiner moves a
stimulus into the peripheral visual field. Here, the extent of movement is the
point of the test. The examiner is watching for conjugate movements representing
proper function of the related nuclei and the MLF. Failure of one eye to abduct
while the other adducts in a horizontal movement is referred to as internuclear
ophthalmoplegia. When this occurs, the patient will experience diplopia, or
double vision, as the two eyes are temporarily pointed at different stimuli.
Diplopia is not restricted to failure of the lateral rectus, because any of the
extraocular muscles may fail to move one eye in perfect conjugation with the
other.
The final aspect of testing eye movements is to move the tip of the pen
in toward the patient’s face. As visual stimuli move closer to the face, the
two medial recti muscles cause the eyes to move in the one nonconjugate
movement that is part of gaze control. When the two eyes move to look at
something closer to the face, they both adduct, which is referred to as
convergence. To keep the stimulus in focus, the eye also needs to change the
shape of the lens, which is controlled through the parasympathetic fibers of
the oculomotor nerve. The change in focal power of the eye is referred to as
accommodation. Accommodation ability changes with age; focusing on nearer
objects, such as the written text of a book or on a computer screen, may
require corrective lenses later in life. Coordination of the skeletal muscles
for convergence and coordination of the smooth muscles of the ciliary body for
accommodation are referred to as the accommodation–convergence reflex. A
crucial function of the cranial nerves is to keep visual stimuli centered on
the fovea of the retina.
The vestibulo-ocular reflex (VOR) coordinates all of
the components ([link]), both sensory and motor, that make this possible. If
the head rotates in one direction—for example, to the right—the horizontal pair
of semicircular canals in the inner ear indicate the movement by increased
activity on the right and decreased activity on the left. The information is
sent to the abducens nuclei and oculomotor nuclei on either side to coordinate
the lateral and medial rectus muscles. The left lateral rectus and right medial
rectus muscles will contract, rotating the eyes in the opposite direction of
the head, while nuclei controlling the right lateral rectus and left medial
rectus muscles will be inhibited to reduce antagonism of the contracting
muscles. These actions stabilize the visual field by compensating for the head
rotation with opposite rotation of the eyes in the orbits. Deficits in the VOR
may be related to vestibular damage, such as in Ménière’s disease, or from
dorsal brain stem damage that would affect the eye movement nuclei or their
connections through the MLF. If the head is turned in one direction, the
coordination of that movement with the fixation of the eyes on a visual
stimulus involves a circuit that ties the vestibular sense with the eye
movement nuclei through the MLF. An iconic part of a doctor’s visit is the
inspection of the oral cavity and pharynx, suggested by the directive to “open
your mouth and say ‘ah.’” This is followed by inspection, with the aid of a tongue
depressor, of the back of the mouth, or the opening of the oral cavity into the
pharynx known as the fauces.
Whereas this portion of a medical exam inspects
for signs of infection, such as in tonsillitis, it is also the means to test
the functions of the cranial nerves that are associated with the oral cavity.
The facial and glossopharyngeal nerves convey gustatory stimulation to the
brain. Testing this is as simple as introducing salty, sour, bitter, or sweet
stimuli to either side of the tongue. The patient should respond to the taste
stimulus before retracting the tongue into the mouth. Stimuli applied to
specific locations on the tongue will dissolve into the saliva and may
stimulate taste buds connected to either the left or right of the nerves, masking
any lateral deficits. Along with taste, the glossopharyngeal nerve relays
general sensations from the pharyngeal walls. These sensations, along with Inteligen Further testing of vagus motor function has
the patient repeating consonant sounds that require movement of the muscles
around the fauces. The patient is asked to say “lah-kah-pah” or a similar set
of alternating sounds while the examiner observes the movements of the soft
palate and arches between the palate and tongue.
