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Basic anatomical
structures for swallowing and speech
The Oral
Cavity
The Oral Cavity is
defined as the space between the lips and pharynx.
The muscles
involved in chewing are all innervated by the trigeminal
nerve. They include: the temporalis which elevates,
retracts, and assists in closing the mandible, the masseter
which elevates and closes the mandible, the medial which
also elevates the mandible and aids in its closure, the
lateral pterygoid which depresses, opens, and protrudes the
mandible, as well as moving it laterally.
Other muscles
involved in chewing are the obicularis oris and the
zygomaticus. Both are lip muscles. The buccinator holds food
in contact with the teeth. All three are innervated by the
facial nerve (CN. VII).
Five muscles
control the movements of the velum. The palatoglossal and
the levator veli palatini both raise the velum. They are
innervated by the vagus nerve (CN. X). The tensor veli
palatini tenses the velum. It receives innervation from the
trigeminal (CN. V).
The
palatopharyngus depresses the velum and constricts the
pharynx. It is innervated by the spinal accessory (CN. XI).
The muscularis uvula shortens the velum. It is also
innervated by the spinal accessory.
Poor
velopharyngeal closure will affect speech but is not a
matter of great concern in regard to swallowing. Patients
may be concerned about it and believe that it is very
important. While the entrance of food into the nasopharynx
may be unpleasant, it is certainly not
life-threatening.
The
Pharynx
The hypopharynx is
the lower portion of the pharynx. It is also known as the
laryngopharynx.
There are three
pharyngeal recesses: food boluses can lodge in these
recesses.
The vallecula is the space or depression
between the base of the tongue and the epiglottis.
The
two
pyriform sinuses are located in the pharynx, beside
the larynx. They are formed by the shape of muscle
attachments to the pharyngeal walls.
The
superior, middle, and inferior pharyngeal constrictor muscles make up the
external circular layer of the pharynx.
The
stylopharyngus m. and the
salpingopharyngus m. make up the internal
longitudinal layer of the pharynx.
The pharyngeal
constrictor muscles help move food down toward the esophagus
via a stripping action. (This process should not
be confused with peristalsis which is the wave-like
motions of muscles that occur in the esophagus. In some of
the literature the action of the pharyngeal constrictor
muscles is mistakenly called peristalsis.)
The latest
research on swallowing suggests that the action of the
pharyngeal constrictor muscles is not the most critical
factor in the movement of food down the pharynx. It seems
that the plunger action of the tongue, or the
tongue
driving force, plays a major role in this
process.
The
cricopharyngus m. or pharyngeal-esophageal
(P.E)
segment separates the pharynx from the esophagus. At the end
of the pharyngeal stage of the swallow, it must relax to
allow the bolus to enter the esophagus. (It is normally
closed to prevent the reflux of food and to keep air out of
the digestive system.) If the P.E. segment does not relax,
food will build up in the pharynx and eventually spill over
the top of the larynx into the airway. The cricopharyngus is
innervated by the vagus (CN. X).
Problems with the
P.E. segment are rare. According to Logemann (1983, 1989,1997),
only 5% of dysphagias are caused by
malfunction of the cricopharyngus muscle. In the past,
physicians frequently treated all types of swallowing
problems by cutting the P.E. segment. This procedure is
called myotomy.
The Neuroanatomy of
Swallowing
(Zemlin, 1997; Logemann, 1997, 1989; Morrell, 1984; Dobie,
1978)
Definition
In the past, the swallow was
classified as a reflex. Now most sources agree that
swallowing is a pattern-elicited response.
The gag reflex, in
contrast, is a good example of a true reflex. It is
"triggered" whenever a noxious substance touches the back of
the tongue, back of the pharynx, or soft palate. The swallow
response, on the other hand, cannot be initiated by touching
any particular area in the oral cavity. The gag reflex and
the swallow response also differ in terms of neurological
control. The gag reflex is completely controlled by the
brain stem. The swallow, on the other hand, is only
partially controlled by the brain stem. It also receives
cortical input, and input from muscle spindles, including
feedback from tongue movements.
(It is important
to note that the gag reflex and the swallow response are not
related. In the past, many physicians would determine
feeding status based on the presence or absence of a
patient's gag. Actually, the presence or absence of a gag
reflex does not predict the status of the swallow
response.)
Neurological
Control
Both
sensory and motor information are necessary for the
initiation of the swallow response; swallowing is dependent
on both sensory and motor control or on information from
both afferent and efferent systems. Sensory feedback plays a
more important role in swallowing than it does in speech.
Sensory input involved in the initiation in the swallow
comes from the trigeminal, facial, and glossopharyngeal
nerves. Information about motor movement is received from
the muscle spindles in the tongue via the hypoglossal
nerve.
Sensory and motor
information from these sources is carried to the swallowing
center, which is believed to be located in the medulla,
within the nuclei of the reticular formation; specifically
the nucleus ambiguous. When the swallow response is
initiated, this center causes messages to be sent to the
glossopharyngeal, the vagus, and the hypoglossal nerves. The
glossopharyngeal is considered the major nerve for the
swallowing center.
Six of the cranial
nerves provide the innervation for both swallowing and
speech. The link is to CMSD 620, Unit 9 cranial nerves:
- CN. V The
Trigeminal Nerve
- CN. VII The
Facial Nerve
- CN. IX The
Glossopharygeal Nerve
- CN. X The
Vagus Nerve
- CN. XI The
Spinal Accessory Nerve
- CN. XII The
Hypoglossal Nerve
The Trigeminal Nerve
(CN. V):
Motor
Component
The efferent
portion of the trigeminal nerve innervates the muscles
involved in chewing. These include the temporalis, the
masseter, the medial, and the lateral pterygoid.
The trigeminal
nerve also innervates the tensor veli
palatine muscle, which tenses the
velum.
In addition, the
trigeminal assists the glossopharyngeal nerve in raising the
larynx and pulling it forward during the laryngeal substage
of the pharyngeal swallow.
Sensory
Components
The trigeminal
nerve carries feedback about all kinds of sensation except
taste from the anterior 2/3 of the tongue.
CN. V also carries
sensory information from the face, mouth and
mandible.
The Facial Nerve (CN.
VII)
Motor
Components
The facial
innervates the lip muscles including the orbicularis oris
and the zygomaticus. The muscles must contract during the
oral preparatory and oral transport stages of the swallow to
prevent food from dribbling out of the mouth.
The facial also
innervates the buccinator muscles of the cheeks.
These must remain tense during the oral component of the
swallowing process to prevent the pocketing of food between
the teeth and the cheeks.
Sensory
Component
The facial carries
information about taste from the anterior 2/3 of the
tongue.
The Glossopharyngeal
Nerve (CN. IX)
Motor
Components
It innervates the
3 salivary glands in the mouth. The saliva from these glands
mixes with the chewed up food to form a bolus.
CN. IX has motor,
sensory, and autonomic nervous system nerve fibers. It,
along with the vagus (CN. X), provides some innervation to
the upper pharyngeal constrictor muscles (Zemlin,
1997).
It innervates the
stylopharyngeus muscle which elevates the
larynx and pulls it forward during the pharyngeal stage of
the swallow. This action also aids in the relaxation and
opening of the cricopharyngeus muscle.
Sensory
components
The
glossopharyngeal nerve mediates all sensation, including
taste, from the posterior 1/3 of the tongue (The facial carries
information about taste from the anterior 2/3 of the
tongue).
CN. IX also
carries sensation from the velum and the superior portion of
the pharynx. A lesion may have impaired the gag reflex
unilaterally (Zemlin, 1997).
The Vagus Nerve (CN.
X)
Motor
Components
The vagus is
responsible for raising the velum as it innervates the
glossopalatine and the levator veli palatine muscles.
The vagus along
with CN. IX innervates the pharyngeal constrictor
muscles.
The vagus along
with CN. XI innervates the intrinsic musculature of the
larynx. It is responsible for vocal fold adduction during
the swallow.
The vagus also
innervates the cricopharyngeus muscle.
The vagus controls
the muscles involved in the esophageal stage of the swallow
as well as those that control respiration. (This is the only
cranial nerve that influences structures inferior to the
neck.)
Sensory
Component
The vagus carries
sensory information from the velum and posterior and
inferior portions of the pharynx.
The vagus also
mediates sensation in the larynx.
The Spinal Accessory
Nerve (CN. XI)
Motor
Components
CN. XI innervates
the palatopharyngeus muscle which depresses the velum and
constricts the pharynx.
It also innervates
the muscularis uvula which tenses the velum. It, along with
CN.X, innervates the levator veli palatini.
(CN. XI is strictly a motor nerve.)
The Hypoglossal Nerve
(CN. XII)
Motor
Components
The hypoglossal
innervates all extrinsic and intrinsic tongue muscles
(It is strictly a motor nerve.)
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