Three
of the nerves are strictly responsible for special senses whereas four others
contain fibers for special and general senses. Three nerves are connected to
the extraocular muscles resulting in the control of gaze. Four nerves connect
to muscles of the face, oral cavity, and pharynx, controlling facial
expressions, mastication, swallowing, and speech. Four nerves make up the
cranial component of the parasympathetic nervous system responsible for pupillary
constriction, salivation, and the regulation of the organs of the thoracic and
upper abdominal cavities. Finally, one nerve controls the muscles of the neck,
assisting with spinal control of the movement of the head and neck. The cranial
nerve exam allows directed tests of forebrain and brain stem structures. The
twelve cranial nerves serve the head and neck. The vagus nerve (cranial nerve
X) has autonomic functions in the thoracic and superior abdominal cavities. The
special senses are served through the cranial nerves, as well as the general
senses of the head and neck. The movement of the eyes, face, tongue, throat,
and neck are all under the control of cranial nerves. Preganglionic
parasympathetic nerve fibers that control pupillary size, salivary glands, and
the thoracic and upper abdominal viscera are found in four of the nerves. Tests
of these functions can provide insight into damage to specific regions of the
brain stem and may uncover deficits in adjacent regions.
Sensory Nerves The
Nooflex olfactory, optic, and vestibulocochlear nerves (cranial nerves I, II, and VIII)
are dedicated to four of the special senses: smell, vision, equilibrium, and
hearing, respectively. Taste sensation is relayed to the brain stem through
fibers of the facial and glossopharyngeal nerves. The trigeminal nerve is a
mixed nerve that carries the general somatic senses from the head, similar to
those coming through spinal nerves from the rest of the body. Testing smell is
straightforward, as common smells are presented to one nostril at a time. The
patient should be able to recognize the smell of coffee or mint, indicating the
proper functioning of the olfactory system. Loss of the sense of smell is
called anosmia and can be lost following blunt trauma to the head or through aging.
The short axons of the first cranial nerve regenerate on a regular basis. The
neurons in the olfactory epithelium have a limited life span, and new cells
grow to replace the ones that die off. The axons from these neurons grow back
into the CNS by following the existing axons—representing one of the few
examples of such growth in the mature nervous system. If all of the fibers are
sheared when the brain moves within the cranium, such as in a motor vehicle
accident, then no axons can find their way back to the olfactory bulb to
re-establish connections. If the nerve is not completely severed, the anosmia
may be temporary as new neurons can eventually reconnect. Olfaction is not the
pre-eminent sense, but its loss can be quite detrimental.
The enjoyment of food
is largely based on our sense of smell. Anosmia means that food will not seem
to have the same taste, though the gustatory sense is intact, and food will
often be described as being bland. However, the taste of food can be improved
by adding ingredients (e.g., salt) that stimulate the gustatory sense. Testing
vision relies on the tests that are common in an optometry office. The Snellen
chart ([link]) demonstrates visual acuity by presenting standard Roman letters
in a variety of sizes. The result of this test is a rough generalization of the
acuity of a person based on the normal accepted acuity, such that a letter that
subtends a visual angle of 5 minutes of an arc at 20 feet can be seen. To have
20/60 vision, for example, means that the smallest letters that a person can
see at a 20-foot distance could be seen by a person with normal acuity from 60
feet away. Testing the extent of the visual field means that the examiner can
establish the boundaries of peripheral vision as simply as holding their hands
out to either side and asking the patient when the fingers are no longer
visible without moving the eyes to track them. If it is necessary, further
tests can establish the perceptions in the visual fields. Physical inspection
of the optic disk, or where the optic nerve emerges from the eye, can be
accomplished by looking through the pupil with an ophthalmoscope. The Snellen
chart for visual acuity presents a limited number of Roman letters in lines of
decreasing size. The line with letters that subtend 5 minutes of an arc from 20
feet represents the smallest letters that a person with normal acuity should be
able to read at that distance.
The different sizes of letters in the other
lines represent rough approximations of what a person of normal acuity can read
at different distances. For example, the line that represents 20/200 vision
would have larger letters so that they are legible to the person with normal
acuity at 200 feet. The optic nerves from both sides enter the cranium through
the respective optic canals and meet at the optic chiasm at which fibers sort
such that the two halves of the visual field are processed by the opposite
sides of the brain. Deficits in visual field perception often suggest damage
along the length of the optic pathway between the orbit and the diencephalon.
For example, loss of peripheral vision may be the result of a pituitary tumor
pressing on the optic chiasm ([link]). The pituitary, seated in the sella
turcica of the sphenoid bone, is directly inferior to the optic chiasm. The
axons that decussate in the chiasm are from the medial retinae of either eye,
and therefore carry information from the peripheral visual field. Pituitary
Tumor The left panel of this figure shows the top view of the brain. The center
panel shows the magnified view of a normal pituitary, and the right panel shows
a pituitary tumor. The pituitary gland is located in the sella turcica of the
sphenoid bone within the cranial floor, placing it immediately inferior to the
optic chiasm. If the pituitary gland develops a tumor, it can press against the
fibers crossing in the chiasm. Those fibers are conveying peripheral visual
information to the opposite side of the brain, so the patient will experience
“tunnel vision”—meaning that only the central visual field will be perceived.
The vestibulocochlear nerve (CN VIII) carries both equilibrium and auditory
sensations from the inner ear to the medulla. Though the two senses are not
directly related, anatomy is mirrored in the two systems. Problems with balance,
such as vertigo, and deficits in hearing may both point to problems with the
inner ear. Within the petrous region of the temporal bone is the bony labyrinth
of the inner ear. The vestibule is the portion for equilibrium, composed of the
utricle, saccule, and the three semicircular canals. The cochlea is responsible
for transducing sound waves into a neural signal. The sensory nerves from these
two structures travel side-by-side as the vestibulocochlear nerve, though they
are really separate divisions. They both emerge from the inner ear, pass
through the internal auditory meatus, and synapse in nuclei of the superior
medulla. Though they are part of distinct sensory systems, the vestibular
nuclei and the cochlear nuclei are close neighbors with adjacent inputs.
Deficits in one or both systems could occur from damage that encompasses
structures close to both. Damage to structures near the two nuclei can result
in deficits to one or both systems. Balance or hearing deficits may be the
result of damage to the middle or inner ear structures. Ménière's disease is a
disorder that can affect both equilibrium and audition in a variety of ways.
The patient can suffer from vertigo, a low-frequency ringing in the ears, or a
loss of hearing. From patient to patient, the exact presentation of the disease
can be different. Additionally, within a single patient, the symptoms and signs
may change as the disease progresses. Use of the neurological exam subtests for
the vestibulocochlear nerve illuminates the changes a patient may go through.
The disease appears to be the result of accumulation, or over-production, of
fluid in the inner ear, in either the vestibule or cochlea. Tests of
equilibrium are important for coordination and gait and are related to other
aspects of the neurological exam. The vestibulo-ocular reflex involves the
cranial nerves for gaze control. Balance and equilibrium, as tested by the
Romberg test, are part of spinal and cerebellar processes and involved in those
components of the neurological exam, as discussed later. Hearing is tested by
using a tuning fork in a couple of different ways. The Rinne test involves
using a tuning fork to distinguish between conductive hearing and sensorineural
hearing. Conductive hearing relies on vibrations being conducted through the
ossicles of the middle ear. Sensorineural hearing is the transmission of sound
stimuli through the neural components of the inner ear and cranial nerve. A
vibrating tuning fork is placed on the mastoid process and the patient
indicates when the sound produced from this is no longer present. Then the fork
is immediately moved to just next to the ear canal so the sound travels through
the air. If the sound is not heard through the ear, meaning the sound is
conducted better through the temporal bone than through the ossicles, a
conductive hearing deficit is present.
The Weber test also uses a tuning fork
to differentiate between conductive versus sensorineural hearing loss. In this
test, the tuning fork is placed at the top of the skull, and the sound of the
tuning fork reaches both inner ears by travelling through bone. In a healthy
patient, the sound would appear equally loud in both ears. With unilateral
conductive hearing loss, however, the tuning fork sounds louder in the ear with
hearing loss. This is because the sound of the tuning fork has to compete with
background noise coming from the outer ear, but in conductive hearing loss, the
background noise is blocked in the damaged ear, allowing the tuning fork to
sound relatively louder in that ear. With unilateral sensorineural hearing
loss, however, damage to the cochlea or associated nervous tissue means that
the tuning fork sounds quieter in that ear.
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