The Vagus Nerve

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The Vagus Nerve
Sensory fibers
There are two types of vagal sensory
fibers, each responsible for transmitting
a different type of sensory information.
Visceral sensory fibers carry
information from stretch receptors and
chemoreceptors (regarding oxygen
levels) in the abdomen and thorax, in
addition to other sensations from the
abdomen, thorax, tongue, pharynx,
larynx, bronchi and esophagus—they
tell us when we “feel bad.” These fibers
terminate in the solitary tract, which
mediates the gag and cough reflexes,
as well as keeping mucous membranes
hydrated, which is important for
swallowing. Visceral pain is carried
separately in fibers of the sympathetic
system.
A Most
Multi-talented
Nerve
Cranial Nerve X­ The Vagus
General sensory fibers transmit pain,
touch and temperature information
from the skin of the external ear, external
auditory canal, external tympanic
membrane, larynx and pharynx to the
spinal trigeminal nucleus, which is the
pain and temperature center for all the
cranial nerves.
Motor fibers
begin in the dorsal vagal nucleus,
the parasympathetic center in the
brainstem. The parasympathetic system
is responsible for initiating “rest and
digest” activities in the body. Vagus
nerve fibers originating in this nucleus
activate smooth muscles and mucosal
glands in the pharynx, larynx and
esophagus—as well as the thoracic and
abdominal viscera—to aid in swallowing
and digestion. The nerve fibers synapse
on nerve cell bodies in the ganglion and
then travel in the pharyngeal branch
and internal laryngeal branch of the
superior laryngeal.
Pharyngeal branch (not shown)
The pharyngeal branch is the principal
motor nerve of the pharynx and soft
palate, supplying all the striated muscles
except the sylopharyngus and the
tensor veli palati. It branches from the
inferior ganglion and innervates all
the pharyngeal constrictors, which aid
in swallowing, and the levator palate,
which closes off the nasal cavity from
the oral cavity. It also innervates the
salpingopharyngeus, palatopharyngeus
and palatoglossus, all of which change
the pharynx’s shape for speech and
swallowing. Oral examination reveals
deviation of the uvula to the unaffected
side because of unopposed muscular
action and drooping of the soft palate
on the affected side. Unilateral damage
to this branch of the nerve results in
dysphagia.
Auricular branch (not shown)
The auricular branch, a general sensory
fiber, is one of the few branches
containing no motor fibers. Stimulating
the auricular branch by means of
something as benign as placing an otoblock to make earmold impressions can
result in coughing, vomiting or fainting
in some people—what’s known as a
vasovagal response.
Internal laryngeal nerve
The internal laryngeal nerve transmits
visceral sensory information from most of
the larynx, the aryepiglottic folds, mucous
membrane of the epiglottis and the base
of the tongue. In addition, general sensory
fibers convey touch, pain and temperature
information from areas above the vocal
folds. The internal laryngeal nerve exits
the larynx through the thyrohyoid
membrane and joins with the external
laryngeal branch to create the superior
laryngeal nerve. The superior laryngeal
nerve joins the other branches to create
the vagus.
External portion of the superior
laryngeal branch
People with damaged visceral sensory
fibers of the vagus present a significant
risk of aspiration. Their ability to detect
foreign matter in the larynx is impaired,
so they cannot initiate a cough response
to clear the airway. Speech-language
pathologists play a key role in determining
the patient’s ability to eat and drink
safely, as well as teaching compensatory
strategies to the family and patient.
Inferior pharyngeal
constrictor muscle
The superior laryngeal branch arises from
the inferior ganglion. At the larynx it splits
into the internal and external laryngeal
nerves. Although primarily a sensory
nerve, the motor segment of the external
laryngeal branch supplies the cricothyroid
muscle, a primary mediator of vocal pitch.
Unilateral damage to this portion of
the nerve results in voice disorders. The
internal branch does not contain motor
fibers.
Cricothyroid muscle
Cricopharyngeus muscle
(part of the inferior
pharyngeal constriction)
Recurrent laryngeal nerve
At and below the vocal folds, visceral and
general sensory fibers alike travel in the
recurrent laryngeal nerve. Thus, the vocal
folds create a division in the pathways
conveying all sensory information.
Recurrent laryngeal nerve
The superior and recurrent nerves merge
into the trunk of the vagus. The general
sensory fibers’ destination is the spinal
trigeminal nucleus, while visceral sensory
fibers travel to the solitary nucleus. The
solitary tract’s nerve fibers, and the
mucosa they serve, are often damaged
in radiation therapy for head and neck
cancers, which presents additional
challenges for patients and SLPs who work
with them.
Continuing journeys ...
The vagus nerve continues its travels
through the esophageal, cardiac,
pulmonary and gastrointestinal branches
in the thorax and abdomen.
Netter medical
illustration used with
permission of Elsevier.
All rights reser ved.
»
Sources for this article are
available at www.asha.org/leader
This branch of the vagus nerve literally
bypasses the larynx and then loops back
or “recurs” superiorly to innervate all the
intrinsic muscles of the larynx except the
cricothyroid. Thus, virtually all the muscles
that open, close, tense or relax the vocal
folds are supplied by this nerve. It has
different pathways on the left and the
right sides of the body. The right recurrent
laryngeal loops under the subclavian
artery and then travels upward between
the trachea and the esophagus to enter
the larynx. The left recurrent laryngeal
loops under the aorta and ascends in
the groove between the trachea and
esophagus on the left. As it enters the
larynx, the recurrent becomes the inferior
laryngeal nerve and innervates the
lateral cricoarytenoids, thyroarytenoids,
posterior cricoarytenoid and the
transverse arytenoids. Unilateral damage
results in weakness or paralysis on the
affected side, resulting in breathiness, a
weak voice or hoarseness. Coughing or
choking can occur while eating.

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