The pharyngeal reflex or gag reflex is a reflex contraction of the back of the throat, evoked
by touching the roof of the mouth, the back of the tongue, the area around the tonsils
and the back of the throat.
It, along with other aero digestive reflexes such as reflexive pharyngeal swallowing, prevents
something from entering the throat except as part of normal swallowing and helps prevent
choking.
Reflex arc In a reflex arc, a series of physiological
steps occur very rapidly to produce a reflex.
Generally a sensory receptor receives an environmental stimulus, in this case from objects reaching
nerves in the back of the throat, and sends a message via an afferent nerve to the central
nervous system.
The CNS receives this message and sends an appropriate response via an efferent nerve
to effector cells located in the same initial area that can then carry out the appropriate
response.
In the case of the pharyngeal reflex: the sensory limb is mediated predominantly
by CN IX the motor limb by CN X.
The gag reflex involves a brisk and brief elevation of the soft palate and bilateral
contraction of pharyngeal muscles evoked by touching the posterior pharyngeal wall.
Touching the soft palate can lead to a similar reflex response.
However, in that case, the sensory limb of the reflex is the CN V. In very sensitive
individuals, much more of the brain stem may be involved; a simple gag may enlarge to retching
and vomiting in some.
Suppression and activation Swallowing unusually large objects or placing
objects in the back of the mouth may cause the pharyngeal reflex.
Some people, for instance sword swallowers have learned how to suppress it.
In contrast, triggering the reflex is sometimes done intentionally to induce vomiting, for
example by those who suffer from bulimia nervosa.
According to one study, one in three people lacks a gag reflex.
However, on the other end of the spectrum are people with a hypersensitive gag reflex.
This hypersensitivity can lead to a variety of issues, from swallowing a pill or large
bites of food to visiting the dentist.
Hypersensitivity is generally a conditioned response, usually occurring following a previous
experience.
There are a variety of ways to desensitize one’s hypersensitivity, from relaxation
to numbing the mouth and throat to training one's soft palate to get used to being touched.
Of course, the effectiveness of these techniques varies with the person.
Its absence About a third of healthy adults do not have
a gag reflex.
However, in certain cases, absence of the gag reflex and pharyngeal sensation can be
a symptom of a number of severe medical conditions, such as damage to the glossopharyngeal nerve,
the vagus nerve, or brain death.
In unilateral glossopharyngeal nerve damage, there will be no gag response when touching
the pharyngeal wall on the same side of the damaged nerve.
With one-sided vagal nerve damage, the soft palate will elevate and pull toward the intact
side regardless of the side of the pharynx that is touched.
This is because the sensory component is intact on both sides, but only the motor nerves supplying
one side of the soft palatine and pharyngeal muscles is working, therefore the contraction
of the muscles in the reflex is asymmetrical.
If both CN IX and X are damaged on one side, stimulation of the normal side elicits only
a unilateral response, with deviation of the soft palate to that side; no consensual response
is seen.
Touching the damaged side produces no response at all.
At one point, it was thought that a lack of the gag reflex in stroke patients was a good
predictor for dysphagia or laryngeal aspiration, and was therefore commonly checked for.
However, in one study, 37% of healthy people did not have a gag reflex, yet all subjects
except for one still retained an intact pharyngeal sensation.
These results suggest that the muscles that control the gag reflex remain independent
of those that control normal swallowing.
Since this reflex is commonly not found in healthy people, its predictive value in determining
the risk for swallowing disorders is severely limited.
Pharyngeal sensation, on the other hand, as seen by this study, is rarely absent, and
could prove better at predicting future problems with swallowing.
Reflexive pharyngeal swallow Closely related to the gag reflex, in which
food or other foreign substances are forced back out of the pharynx, swallowing generally
pushes food through the digestive system into the stomach.
This reflex in particular functions as a protective system for the upper respiratory tract as
it not only forces the glottis to close, thereby preventing any substances getting into the
airways, but also clears the pharynx of any residual substances by a swallow.
This particular reflex is simply one of several aero digestive reflexes such as the reflexive
pharyngeal swallow, the pharyngoglottal closure reflex, in which no swallowing occurs yet
the glottis still closes, and the pharyngo-upper esophageal sphincter contractile reflex, occurring
mainly during gastroesophageal reflux episodes.
All either forcibly close the glottis or allow the pharynx to remove particles into the digestive
tract that may have been forced back up by both this tract and the upper respiratory
tract.
These reflexes can also protect the airways from any food or liquids that may spill over
from the hypopharynx.
The hypopharynx is the bottom part of pharynx, and can be considered the first area where
the digestive tract splits from the airways.
However, if the maximum capacity of fluids that the hypopharynx can safely hold is exceeded,
then this excess fluid spills into the larynx and from there into the lungs.
Therefore these reflexes prevent levels reaching this maximum volume.
Since both the digestive system and the respiratory system are connected by the pharynx, there
are many problems and diseases that occur when the body is unable to regulate passage
of food and air into the appropriate tracts.
Perhaps the most preventable cause of damage to these reflexes originates from smoking.
One study has shown that, when compared to non-smokers, the threshold volumes for both
the pharyngo-upper esophageal sphincter contractile reflex and reflexive pharyngeal swallowing
is increased.
See also Retching, also known as dry heaving
References