Somatosensation
of the face is conveyed along the nerve to enter the brain stem at the level of
the pons. Synapses of those axons, however, are distributed across nuclei found
throughout the brain stem. The mesencephalic nucleus processes proprioceptive
information of the face, which is the movement and position of facial muscles.
It is the sensory component of the jaw-jerk reflex, a stretch reflex of the
masseter muscle. The chief nucleus, located in the pons, receives information
about light touch as well as proprioceptive information about the mandible,
which are both relayed to the thalamus and, ultimately, to the postcentral
gyrus of the parietal lobe. The spinal trigeminal nucleus, located in the medulla,
receives information about crude touch, pain, and temperature to be relayed to
the thalamus and cortex. Essentially, the projection through the chief nucleus
is analogous to the dorsal column pathway for the body, and the projection
through the spinal trigeminal nucleus is analogous to the spinothalamic
pathway. Subtests for the sensory component of the trigeminal system are the
same as those for the sensory exam targeting the spinal nerves. The primary
sensory subtest for the trigeminal system is sensory discrimination. A
cotton-tipped applicator, which is cotton attached to the end of a thin wooden
stick, can be used easily for this. The wood of the applicator can be snapped
so that a pointed end is opposite the soft cotton-tipped end.
The cotton end
provides a touch stimulus, while the Nooflex pointed end provides a painful, or sharp,
stimulus. While the patient’s eyes are closed, the examiner touches the two
ends of the applicator to the patient’s face, alternating randomly between
them. The patient must identify whether the stimulus is sharp or dull. These
stimuli are processed by the trigeminal system separately. Contact with the
cotton tip of the applicator is a light touch, relayed by the chief nucleus,
but contact with the pointed end of the applicator is a painful stimulus
relayed by the spinal trigeminal nucleus. Failure to discriminate these stimuli
can localize problems within the brain stem. If a patient cannot recognize a
painful stimulus, that might indicate damage to the spinal trigeminal nucleus
in the medulla. The medulla also contains important regions that regulate the
cardiovascular, respiratory, and digestive systems, as well as being the
pathway for ascending and descending tracts between the brain and spinal cord.
Damage, such as a stroke, that results in changes in sensory discrimination may
indicate these unrelated regions are affected as well. Gaze Control The three
nerves that control the extraocular muscles are the oculomotor, trochlear, and
abducens nerves, which are the third, fourth, and sixth cranial nerves. As the
name suggests, the abducens nerve is responsible for abducting the eye, which
it controls through contraction of the lateral rectus muscle. The trochlear
nerve controls the superior oblique muscle to rotate the eye along its axis in
the orbit medially, which is called intorsion, and is a component of focusing
the eyes on an object close to the face. The oculomotor nerve controls all the
other extraocular muscles, as well as a muscle of the upper eyelid.
Movements
of the two eyes need to be coordinated to locate and track visual stimuli
accurately. When moving the eyes to locate an object in the horizontal plane,
or to track movement horizontally in the visual field, the lateral rectus
muscle of one eye and medial rectus muscle of the other eye are both active.
The lateral rectus is controlled by neurons of the abducens nucleus in the
superior medulla, whereas the medial rectus is controlled by neurons in the
oculomotor nucleus of the midbrain. Coordinated movement of both eyes through
different nuclei requires integrated processing through the brain stem. In the
midbrain, the superior colliculus integrates visual stimuli with motor
responses to initiate eye movements. The paramedian pontine reticular formation
(PPRF) will initiate a rapid eye movement, or saccade, to bring the eyes to
bear on a visual stimulus quickly. 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. Control of conjugate gaze
strictly in the vertical direction is contained within the oculomotor complex.
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 [link] 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 , 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 certain taste stimuli, can stimulate the gag reflex.
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