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The cranial nerves
innervate the muscles of the jaw, face, tongue, neck,
pharynx, and larynx. Some of them are motor, some are
sensory and some are mixed nerves, containing both sensory
and motor fibers. Six of them are involved in speech and
swallowing, and are therefore very important to the speech,
language pathologist.
The motor nuclei
and processes of the cranial nerves are lower motor neurons
as they form a final common pathway for information
descending from the cerebrum to the periphery. Because the
motor roots of the cranial nerves are located in the brain
stem, messages from the precentral and postcentral gyri
reach the cranial nerves on the corticobulbar portion of the pyramidal
tract. With the exception of part of CN VII (the facial
nerve) and parts of CN XII the hypoglossal nerve, this
innervation is bilateral.
Along
with the spinal nerves, the cranial nerves are part of the
peripheral nervous
system.
There
are twelve pairs of cranial nerves.
Cranial
Nerves Involved in Smell and Vision
CN I, the olfactory nerve, is a purely sensory
nerve. It has receptors within the mucous membrane of the
nose. Information runs posteriorly along the olfactory tract
and through the olfactory bulb to the temporal lobe where it
is processed. Remember that this is the only sense not
mediated by the thalamus.
CN
II,
the optic nerve, is also a
sensory nerve. Visual information
from the retina is carried back to the superior colliculus
of the midbrain on the optic tract which is immediately
below the olfactory tract in the more anterior part of the
brain. From the superior colliculus, messages are first
passed on the lateral geniculate body in the thalamus and
then to the cortex of the occipital lobe.
CN
III
is the oculomotor nerve. The nucleus of this motor
nerve is located in the midbrain. It mediates movements of
the eyeball and constriction and dilation of the pupil.
CN
IV
or the
trochlear nerve, is a motor nerve. Its
nucleus also lies in the mesencephalon. This nerve also
innervates eye movements and damage to it will cause double
vision.
CN VI
is
the abducens nerve. The nucleus of this motor
nerve is found in the pons. This nerve provides innervation
for movements of the eyeball.
The Cranial Nerves Involved in
Speech, Hearing and Swallowing
CN V is the trigeminal
nerve.
The
motor nucleus of this large mixed nerve originates in the pons. It
provides motor innervation to the muscles that control the
mandible (jaw), the tensor veli palatini muscle of the
velum, and the tensor tympani muscle of the middle ear.
It mediates
sensation from the head, jaw, face, some of the sinuses and
tactile sensation from the anterior two thirds of the
tongue.
CN VII
or
the facial nerve, is most often classified
as a motor nerve, but can also be considered a
mixed nerve.
Its
motor nucleus which is located in the junction of the pons and medulla innervates all of
the muscles of facial expression including those in the
forehead, cheeks, and lips, as well as the stapedius muscle
of the middle ear. It also sends motor impulses to the rest
of the ear; if you can wiggle your ears, this action is
mediated by CN VII.
The part of the
nucleus that sends commands to the upper part of the face
receives bilateral (ipsalateral and contralateral)
innervation from upper motor neuronal tracts. However, the
portion that controls the lower part of the face receives
only contralateral (unilateral) innervation. This means that
unilateral lesions of the pyramidal tract may have
noticeable effects on voluntary movements of the cheeks and
lips. Note that involuntary facial expressions of emotion
will not be impaired in the case of a pyramidal tract lesion
as they are controlled by the extrapyramidal tract.so that if a patient smiles both sides of the mouth will lift normally. With lower motor neuron lesions the entire side of the face will be paralyzed and won't respond to either voluntary or involuntary upper motor neuron input.
The facial nerve mediates taste in the anterior two
thirds of the tongue.
CN VIII
is
the vestibular/acoustic
nerve. The two branches of this sensory
nerve carry information from the cochlea and from the
vestibular end organs in the inner ear. This nerve
originates in the medulla. The vestibule which connects to the pons and then to the cerebellum regulates balance and equilibrium including the coordination of head and body movements.The auditory fibers go to the inferior colliculi of the midbrain and then to the medial geniculate bodies of the thalamus and finally project bilaterally to the temporal cortex. Lesions on the cell bodies in the medulla will not result in total hearing loss because of the bilateral auditory projections.
CN
IX,
or the glossopharyngeal
nerve, is a mixed nerve.
Its
motor aspect contributes to the action of the middle
pharyngeal constrictor muscle and innervates the
stylopharyngeus muscle.
Its sensory aspect
carries input from the posterior one third of the tongue,
the velum, and the pharynx including the tonsils.
CN X
is
the vagus nerve. This mixed nerve originates in the
medulla.
One of
the motor nuclei of the vagus innervates the majority of the
viscera, including the heart, respiratory system, and
digestive system.
Another motor
nucleus sends motor commands to the pharyngeal constrictor
muscles and completely controls the intrinsic musculature of
the larynx. The superior branch of the vagus innervates
the cricothyroid muscle and so is involved in pitch changes.
Its recurrent branch innervates all of the other intrinsic
laryngeal musculature.
The vagus also
innervates the glossopalatine and levator veli palatine
muscles, making it primarily responsible for palatal
functioning.
CN XI
is
the spinal accessory
nerve, a motor nerve that originates in the
medulla. It innervates the trapezius and sternocleidomastoid
muscles of the neck. It also sends some motor messages to
the uvula and the levator veli palatine (raises the velum).
CN
XII,
is the hypoglossal
nerve, another motor nerve that originates in
the medulla. It controls tongue movement, innervating both
the intrinsic and extrinsic tongue muscles. The part of the
nucleus that innervates the genioglossus, the muscle
involved in tongue protrusion, is connected only to
contralateral fibers from the pyramidal tract. Thus,
unilateral upper motor lesions can affect this type of
movement. All other tongue muscles receive bilateral
innervation for voluntary movements.
If
there is damage to the hypoglossal nerve itself, the tongue
may reveal the presence of the lower motor neuron lesion by
fasciculating (twitching).
The Six Cranial
Nerves Involved in Speech and Swallowing
CN V -
- the trigeminal nerve
CN VII - - the
facial nerve
CN IX - - the
glossopharyngeal nerve
CN X - - the vagus
nerve
CN XI - - the
spinal accessory nerve
CN XII - - the
hypoglossal nerve
Stages of Deglutition
(Logemann,1989, 1994, 1998. Morrell,1984.
In Groher,1984.)
In the
Oral-preparatory
stage, food is moved around the mouth,
chewed and tasted. Time needed for this stage is variable.
During
the
oral stage, food is moved to the back of the
mouth by the tongue. This stage lasts for about
1
second, ending when the bolus touches the
back of the oropharynx.
In the first part
of the pharyngeal stage, or laryngeal
substage, a number of things happen
simultaneously. The larynx moves up and forward, the vocal
folds approximate and the epiglottis falls over the top of
the larynx.
During
the second part of this stage, the bolus of food is moved
down the pharynx by the stripping action of the pharyngeal
constrictor muscles. This phase of the pharyngeal stage ends
when the cricopharyngus muscle, also known as the
p.e.
segment, opens, allowing the food to enter
the esophagus. It is not known what leads the p.e. segment
to open at the right time during the swallowing sequence.
Some think that it opens due to stretching caused by the
elevation of the larynx. The total time required for the
entire pharyngeal phase is 1 second.
During the
esophageal stage, food travels down the
esophagus via a wave-like motion called peristalsis. This
phase lasts between 8 and 20 seconds.
Note
that peristalsis occurs only in the esophagus and colon, not in
the pharynx. In some older literature on swallowing, there
is confusion about this, and the stripping action of the
pharyngeal constrictor muscles is mislabeled as peristalsis.
Swallowing is not
just a motor reflex; it requires a combination of sensory
and motor control. The swallowing center, which is located
in the nucleus ambiguous of the medulla, recognizes a
pattern that consists of both sensory and motor elements and
then triggers the swallowing response. This is why the
swallow is considered to be a type of patterned
response or pattern recognition
system.
Sensory input that
initiates the swallow response comes from the trigeminal,
facial and glossopharyngeal nerves.
The trigeminal
nerve is involved in the oral preparatory and oral stages.
It provides the innervation that controls jaw movement for
chewing and it also mediates tactile sensation in the
anterior two thirds of the tongue.
During the oral
preparatory and oral stages, taste is carried from the
anterior two thirds of the tongue by the facial nerve.
The
glossopharyngeal nerve is responsible for taste in the
posterior one third of the tongue and for tactile sensation
to the posterior part of the oral cavity, including the
velum, tonsils, and walls of the oropharynx. It provides the
feedback that is most important in the elicitation of the
swallow.
Feedback from
motor movements, especially tongue movements which are
mediated by the hypoglossal nerve, also help to trigger the
swallow.
Input from both
the cerebral cortex and the cerebellum is responsible for
the coordination and timing of the motor movements involved
in swallowing.
Mnemonic
for the Cranial Nerves
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On
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(olfactory)
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Some
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(sensory)
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Old
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(optic)
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Say
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(sensory)
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Olympus'
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(oculomotor)
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Marry
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(motor)
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Towering
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(trochlear)
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Money
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(motor)
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Top
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(trigeminal)
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But
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(both)
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A
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(abducens)
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My
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(motor)
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Finn
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(facial)
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Mother*
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(motor)
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And
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(auditory)
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Says
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(sensory)
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German
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(glossopharyngeal)
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Bad
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(both)
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Vended
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(vagus)
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Business
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(both)
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At
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(accessory)
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Marry
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(motor)
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Hopps
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(hypoglossal)
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Money
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(motor)
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The facial nerve
could also be classified as both sensory (taste for anterior
two thirds of tongue) and motor, in which case the word in
this part of the rhyme would change to "brother." It is
usually classified as a motor nerve.
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