NEURAL CONTROL OF SWALLOWING
Neural Control of Swallowing
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Deglutition is best understood as a specialized
example of motor control.
It involves a dynamic interplay of descending motor
tracts and ascending sensory tracts.
Cortical and subcortical motor systems, such as the
cerebellum and the basal ganglia, play an
important role in maintaining postural stability and
head position.
Corticospinal and corticobulbar tracts carry inputs
from cortical motor centers in the frontal lobe and
converge on central pattern generators in the lower
brainstem.
Motor Tracts
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The motor cranial nerves participating in deglutition
include:
o Trigeminal Nerve (V) for muscles of mastication;
o Facial Nerve (VII) for lip sphincter and buccal
muscles;
o Glossopharyngeal (IX) and Vagus (X) Nerves for
muscles of the palate, pharynx, esophagus, larynx,
and respiratory control centers; and
o Hypoglossal Nerve (XII) for the extrinsic muscles of
the tongue.
CN V Trigeminal Nerve
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The trigeminal nerve is the
largest cranial nerve and
originates in the pons.
Its motor root supplies the
muscles of mastication,
some of the muscles of the
soft palate, and the
muscles inserting into the
floor of the mouth.
CN V Trigeminal Nerve
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Damage to the trigeminal nerve can affect the
eating process.
Specific signs of trigeminal nerve damage or lack
of innervation by the trigeminal nerve includes:
o Absence/loss of bite reflex in children;
o Partial/total paralysis of the muscles of
mastication, affecting mandible movement;
o Lock jaw, or trismus, resulting from tonic spasm or
rigid contraction of the masseter, temporalis, or
either pterygoid muscles.
CN VII Facial Nerve
o The
special visceral efferent fibers
of the facial nerve supply muscles
of facial expression, including the
buccinator, as well as the posterior
belly of the digastric, stylohyoid
and stapedius muscles.
o Signals for voluntary movement of
the facial muscles originate in the
motor cortex and pass via the
corticobulbar tract in the posterior
limb of the internal capsule to the
motor nuclei of CN VII.
CN VII Facial Nerve
o Fibers
pass to both the ipsilateral
and contralateral motor nuclei of
CN VII in the caudal pons.
o The portion of the nucleus that
innervates the muscles of the
forehead receives corticobulbar
fibers from both the contralateral
and ipsilateral motor cortex.
CN VII Facial Nerve
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The portion of the nucleus that
innervates the lower muscles of
facial expression receives
corticobulbar fibers from only
the contralateral motor cortex.
This is very important clinically
as central (upper motor
neuron) and peripheral (lower
motor neuron) lesions will
present differently.
CN VII Facial Nerve
o The
general visceral efferent
fibers innervate the lacrimal,
submandibular, and sublingual
glands, as well as mucous
membranes of nasopharynx, hard
and soft palate.
o They originate from a diffuse
collection of cell bodies in the
caudal pons just below the facial
nucleus known as the superior
salivatory nucleus.
CN VII Facial Nerve
Damage to neuronal cell bodies in the cortex or their
axons that project via the corticobulbar tract through the
posterior limb of the internal capsule to the motor nucleus
of CN VII are upper motor neuron lesions.
o Upper motor neuron lesions are usually the result of
stroke.
o Voluntary control of only the lower muscles of facial
expression on the side contralateral to the lesion will be
lost.
o Voluntary control of muscles of the forehead will be
spared due to the bilateral innervation of that portion of
the CN VII motor nucleus.
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CN VII Facial Nerve
o Damage
to the motor nucleus of CN VII or its axons
results in a lower motor neuron lesion.
o Paralysis of all muscles of facial expression (including
those of the forehead) will be expressed ipsilateral to
the lesion.
oDamage to the facial nerve affecting the eating
process can include:
o Decreased salivary production and mucosal
dryness.
CN VII Facial Nerve
oLoss
of symmetry to mouth,
which may droop to one side;
and
oInability of buccinator muscles
to monitor and control food
during chewing, causing food to
slip outside molar surfaces and
collect between gums and
cheeks.
CN IX Glossopharyngeal Nerve
oThe
special visceral efferent fibers of the
glossopharyngeal nerve work in close conjunction with
the vagus nerve to supply motor fibers of the
stylopharyngeus muscle and glands of the pharynx
and larynx.
oSignals for voluntary elevation and/or dilation of the
pharynx by the stylopharygeus muscle originate in the
pre-motor and motor cortex.
CN IX Glossopharyngeal Nerve
oThey
pass via the corticobulbar tract in the posterior
limb of the internal capsule to synapse bilaterally on
the ambiguus nuclei in the reticular formation of the
medulla.
oDamage to the motor fibers of the glossopharyngeal
nerve may contribute to difficulty or loss of the ability
to move food through the pharynx because of
decreased functioning of the pharyngeal constrictor
muscles.
CN IX Glossopharyngeal Nerve
o The
general visceral
efferent fibers innervate
the ipsilateral parotid
gland.
o Salivation is produced in
response to smelling food
(mediated by the olfactory
system).
CN X Vagus Nerve
oThe
motor branches of
the vagus nerve supply
the voluntary muscles of
the pharynx, soft palate,
most of the larynx, and
one muscle of the tongue.
oSpecifically,
they supply
the superior, middle, and
inferior pharyngeal
constrictor muscles.
CN X Vagus Nerve
oIn
the soft palate, they
supply the levator veli
palatini muscles, the
palatopharyngeal
muscles (posterior faucial
pillar), the palatoglossus
muscles (anterior faucial
pillar), and the intrinsic
muscles of the larynx
involved in abduction
and adduction.
CN X Vagus Nerve
oSignals
for the voluntary movement of the muscles
innervated by CN X originate in the pre-motor and
motor cortex and pass via the corticobulbar tract in
the posterior limb of the internal capsule to synapse
bilaterally on each nucleus ambiguus in the reticular
formation of the medulla.
oDamage to the motor portion of the vagus nerve may
contribute to swallowing difficulty because of
decreased functioning of the muscles of the soft
palate and the pharyngeal constrictors.
CN X Vagus Nerve
oSpecifically,
difficulty or inability to elevate the soft
palate on the affected side may result in
regurgitation of fluids/foods through the nose.
oOn examination the soft palate droops on the
affected side and the uvula deviates opposite the
affected side due to the unopposed action of the
intact levator palatini muscle.
CN XII Hypoglossal Nerve
The
somatic motor portion
of the hypoglossal nerve
innervates all the intrinsic
and most of the extrinsic
muscles of the tongue.
It
supplies three of the
four extrinsic muscles of
the tongue including
genioglossus, styloglossus,
and hyoglossus.
CN XII Hypoglossal Nerve
oSignals
for the voluntary
control of the muscles of the
tongue originate in the
motor cortex and pass via
the corticobulbar tract in
the posterior limb of the
internal capsule to synapse
in contralateral hypoglossal
nucleus (1) in the medulla.
Sensory Tracts
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Ascending sensory tracts reflexively evoke motor programs
via the central pattern generator and provide continual
feedback to modulate the descending motor systems.
The sensory cranial nerves participating in deglutition
include:
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the trigeminal nerve for sensations from the face
the facial nerve for taste on the anterior tongue
the glossopharyngeal and vagus nerves for sensation from the
posterior tongue, palate, pharynx, and larynx; and
the glossopharyngeal nerves for posterior taste sensation.
Sensory inputs also arise from neck muscles and joints to
provide information regarding head position which is critical
in maintaining orientation toward the food source,
optimizing swallow efficiency, and allowing for airway
protection.
CN V Trigeminal Nerve
oThe
sensory component of CN V transmits stimuli from the
areas of the scalp, face, nasal cavity, teeth, and mouth,
as well as from proprioceptors of the muscles of
mastication.
oDamage to the sensory branches of CN V can affect the
eating process by causing pain, that can be experienced
as brief, sharp, flashing periods (like that with a
toothache), or slow, methodically spaced periods of pain.
CN VII Facial Nerve
 The
special afferent
components of CN VII
transmit taste sensation from
the anterior 2/3 of tongue,
hard and soft palates.
 Chemoreceptors of the taste
buds located on the anterior
2/3 of the tongue and hard
and soft palates initiate
receptor (generator)
potentials in response to
chemical stimuli.
CN VII Facial Nerve
The taste buds synapse with the first order special
sensory neurons from CN VII which enter the brainstem
and ascend to synapse with the second order neuron
found in the nucleus tractus solitarius--also referred to
as the gustatory nucleus.
o Ascending secondary neurons originating from nucleus
tractus solitarius project both ipsilaterally and
contralaterally to synapse with the third order neurons
of the ventral posteromedial (VPM) nucleus of the
thalamus.
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CN VII Facial Nerve
Tertiary neurons from the thalamus project via the
posterior limb of the internal capsule to the area of
the cortex responsible for taste.
o Damage to the sensory branches of CN VII can cause
temporary/permanent loss of the sense of taste on the
anterior 2/3 of the tongue, as well as a loss of
sensation to the face.
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CN IX Glossopharyngeal Nerve
The special afferent branches of CN IX provide taste
sensation from the posterior 1/3 of the tongue.
o The general somatic afferent branches of CN IX provide
general sensory information from the upper pharynx, and
the posterior 1/3 of the tongue.
o The general sensory fibers of CN IX mediate the afferent
limb of the pharyngeal reflex in which touching the back of
the pharynx stimulates the patient to gag (i.e., the gag
reflex).
o The efferent signal to the musculature of the pharynx is
carried by the special visceral motor fibers of the vagus
nerve.
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CN IX Glossopharyngeal Nerve
Damage to the sensory branches of CN IX may result in the
inability to discriminate taste sensations on the posterior
1/3 of tongue.
o Loss of sensitivity in the soft palate and posterior part of
tongue may result in reduced or absent gag reflex.
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CN X Vagus Nerve
The special afferent component of CN X is a very minor
component.
o It provides taste sensation from the epiglottic region.
o However, the general visceral afferent component
provides information from the larynx and the esophagus.
o Damage to the sensory branches of the vagus nerve may
affect laryngeal sensation to food/liquid penetration.
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Central Pattern Generator
Swallowing, like sneezing and orgasm, is a fixed action
pattern.
o It is involuntary and stereotyped, but typically has a stimulus
threshold that must be reached by specific “key stimuli”
before it is triggered and its expression does not require
previous learning.
o It is different from a simple reflex in that it can not be elicited
by isolated nerve activation (e.g., gag reflex) but must
instead conform to a highly codified stimulus pattern that
produces a behavioral sequence of more elementary motor
acts.
o Different individuals produce almost identical behavioral
responses to specific key stimuli, and once initiated, fixed
action patterns continue until completion.
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Central Pattern Generator
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For swallowing, the fixed action potential is triggered by
stimulation of several receptors.
Once triggered, pharyngeal swallowing behavior involves
a complex sequential activation of at least 10 different
muscle groups.
Both sensory and motor information are necessary for the
initiation of the pharyngeal swallow.
Sensory input involved in the initiation in the swallow comes
from CNs V, VII, IX, and X.
Information about motor movement comes from the muscle
spindles in the tongue via the CN XII.
Central Pattern Generator
The swallowing response is elicited from an interneuronal
network of dorsal and ventral reticular bodies that comprise
the central pattern generator.
 The interneurons of the central pattern generator mediate
interactions between motor and sensory nuclei.
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Central Pattern Generator
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The dorsal interneurons (in
blue) initiate and program
(spatially and temporally)
swallowing behaviors.
The ventral interneurons (in
red) distribute the excitation
to the swallowing motor
nuclei.
Let’s see how this works.
Input Functions
Receptor fields on the posterior tongue (CN IX), fauces,
tonsils, velum (CN IX), laryngeal vestibule and ventricle (CN
X), as well as the mucosa of the valleculae and pyriform
recesses (CN X) and the salivary glands (CN VII) are
stimulated by the presence of the bolus.
o They send sensory information via their respective fasciculi to
the cell bodies comprising the nucleus tractus solitarius (NTS).
o The NTS, located in the dorsal medulla, is comprised of the
cell bodies of the sensory neurons of the facial (VII),
glossopharyngeal (IX), and vagus(X) nerves clustered in a
long single group.
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Input Functions
In addition to receiving sensory input from oropharyngeal
receptors, it receives excitatory motor input from structures
involved in motor control, including the motor and premotor
cerebral cortices, via cortico-reticular pathways.
o The input information arriving at the NTS from various sensory
receptors and motor structures is “summed” and if stimulus
threshold is reached by these “key stimuli” then the NTS
organizes the pre-programmed sequential spatial-temporal
sequence of swallow and sends this information to the nucleus
ambiguus (NA) to execute the specified motor sequence.
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Input Functions
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Threshold of stimulation depends on the frequency of the
stimulus, suggesting that when the correct excitatory code
is carried by the descending corticobulbar tract and the
peripheral sensory inputs, swallowing is elicited.
Corticobulbar input is thought to influence only the
duration and intensity of muscle activity pre-programmed
by the NTS for involuntary swallow behavior.
Indeed, if the dorsal medulla is destroyed, electrical
stimulation of specific cortical sites involved in swallowing
input will not trigger a swallow.
Moreover, direct isolated stimulation of any of the cranial
nerve nuclei DOES NOT evoke swallowing.
Output Functions
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The nucleus ambiguus (NA) consists of the cell bodies of the
motor neurons of the glossopharyngeal (IX) and Vagus (X)
nerves clustered in a single group.
It connects with the trigeminal (V), facial (VII), and
hypoglossal (XII) motor nuclei.
All efferent information is sent via the NA to the striated
muscles of the pharynx, larynx, and upper esophagus.
Specifically, this ventral brainstem area coordinates
efferent impulse flow by way of:
o CNs V, X, and XII to the muscles of the oropharynx;
Output Functions
by way of CN X to the muscles of the hypopharynx;
o by way of CNs V and XII to the extrinsic muscles of the
larynx; and
o by way of CN X to the intrinsic muscles of the larynx and
esophagus.
Microelectrode recording during swallow demonstrate that
ventral interneurons of the NA discharge at specific times
during the pharyngeal and esophageal stages of swallow.
The first detectable action is contraction of the mylohyoid
muscles, preceding all other muscle contractions by 30-40
ms, to elevate the larynx.
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Output Functions
oThen in sequence, there is activation of the posterior tongue
(continues to move back toward pharynx), the superior
constrictor muscles, the palatopharyngeus (elevates pharynx
and larynx and closes nasopharyngeal isthmus) and the
stylohyoid and the geniohyoid muscles, which move the
larynx up and forward.
oPharyngeal constrictors fire in overlapping order.
oThe cricopharyngeus dilates and esophageal peristalsis
commences at a velocity of between 2-4 cm sec.
oDirect stimulation of the NA or other ventral motor nuclei
does not evoke swallowing.
Output Functions
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Instead only contraction of individual muscles is produced.
This is because swallowing is a sequential pattern of
muscle contraction established by the NTS.
The ventral regions require input from the dorsal medulla
to complete a swallow.
There are also cross connections between the CPGs on the
right and the CPGs on the left side of the brainstem.
Therefore, there is bilateral symmetry of pharyngeal
swallow and either side of the brainstem can coordinate
the pharyngeal and esophageal phases.
Cortical Involvement
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Although the brainstem alone
can excite muscle contraction
similar to swallowing, the
cortex has significant control
over the initiation of
swallowing and the level of
neuromuscular activity of
volitional swallowing.
Cortical Involvement
The “swallowing cortex” is a
discrete area located in the
supplemental motor area,
anterior to M1.
o It is important for timeordered organization of
movements, especially in
sequential performance of
multiple movements.
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Cortical Involvement
It is important in initiation
of voluntary movements.
o Other cortical sites
involved in swallowing are
the bilateral anterolateral
regions of the premotor
cortex.
o These areas are believed
to coordinate the
sequence of tongue and
facial movements.
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Cortical Involvement
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The primary motor strip (M1)
controls execution of specific
body parts, with tongue,
mouth, eye, hand, arm, head,
trunk, torso, and lower limbs
represented in a caudalrostral fashion along the
precentral gyrus.
Cortical Involvement
o Research
shows that the
insula, in particular the
anterior insula (AI), is
involved in the coordination
of the interaction of oral
musculature, gustation and
autonomic functions.
Cerebellum
o Research
on swallowing and the cerebellum is minimal.
o Most documented studies are case studies that involve
widespread lesions and not isolated cerebellar lesions.
o PET results in normals have indicated that there is specific
representation of the pharyngeal/esophageal stages of
swallowing in left cerebellar hemisphere, and that the whole
cerebellum is involved in the coordination, sequencing, and
timing of the swallow.
o It is thought the cerebellum integrates proprioceptive,
vestibular, and motor planning information and then
communicates with the cerebral cortex to produce smooth
synergistic movements.
Biomechanics of Bolus Flow
The duration and characteristics of each phase of swallow
depends on the type and volume of food being swallowed.
o Therefore, there are many types of normal swallows that
occur predictably based on the characteristics of the food
swallowed and voluntary control.
o Moreover, the frequency of deglutition varies with activity:
we swallow more when eating; and we swallow less when
sleeping.
o Mean deglutition frequency is approximately 580 swallows
per day.
o During sleep, periods of 20 minutes or may pass when no
swallow occurs.
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Volume Effects
o Changes
in bolus volume create the greatest systematic
changes in the oropharyngeal swallow.
o Small volume swallows, such as saliva, of 1 to 3 ml,
produce sequential swallow phases (oral phase, followed
by pharyngeal swallow, pharyngeal, and esophageal
phases).
o Large volume swallows, as in cup drinking, of 10 to 20 ml,
produce simultaneous oral and pharyngeal phase activity
in order to safely clear the large bolus from both the oral
cavity and the pharynx.
Volume Effects
o As
bolus volume increases, the timing of the tongue base
retraction to contact the anteriorly and medially moving
pharyngeal walls occurs later in the swallow.
o The tongue based and pharyngeal walls will not move
toward each other and make contact until the tail of the
bolus reaches the tongue base.
Viscosity Effects
Thin liquids are easily deformed and move readily in
response to gravity and compression.
o Thin liquids will pass most easily through narrow sites in
transit.
o Thus agility and coordination must be adequate to control
the bolus and time its transit through the oral and
pharyngeal chambers while protecting the airway.
o In contrast, thicker foods move more slowly in response to
gravity and compression.
o Normal swallowing transit times are slower on thicker
food.
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Viscosity Effects
The more viscose the bolus, the less agility and control
required, and the more forgiving when timing of swallow
and coordination of transit are impaired.
o However, as bolus viscosity increases, adequate transit
becomes more reliant on strength and constriction--the
pressure generated by the oral tongue, tongue base, and
pharyngeal walls increases and muscular activity
increases.
o As the bolus becomes less deformable, it is less likely to
pass through narrow sites in transit, and may lodge above
them instead.
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Viscosity Effects
Valve functions, such as VP closure, upper esophageal
opening, and laryngeal closure also increase slightly in
duration as viscosity increases.
o Thicker foods also heighten sensory awareness of food.
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Gustatory Effects
Despite the many substances we seem to taste, there are
basically only four primary taste fundamentals: sour, salt,
bitter, and sweet.
o The stimuli that the brain interprets as the basic tastes—
salty, sour, sweet, and bitter (possibly umami—a
glutamate), are registered via a series of chemical
reactions in the taste cells of the taste buds.
o We perceive all taste qualities all over our tongue,
although there may be increased sensitivity to certain
qualities in certain areas.
o Each of the four primary tastes is caused by a different
response to different chemicals.
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Gustatory Effects
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Certain regions of the tongue react more strongly than
others to certain taste sensations although individual taste
cells are not programmed or “tuned” to respond to only
one kind of taste stimulus.
Flavor is a complex mixture of sensory input composed of
taste (gustation), smell (olfaction), and the tactile sensation
(chemical irritation) of food as it is being munched
(mouthfeel).
Our taste system also provides information on the intensity
and pleasantness (or unpleasantness) of taste as well.
Neurons in the taste pathway record these attributes
simultaneously, responding to touch and temperature
stimuli as well.
Gustatory Effects
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Food preferences can be influenced by many factors, such
as physiologic status, food context, familiarity, and
environment.
There are three cranial nerves that supply taste buds: the
facial, glossopharyngeal, and vagus nerves.
Chemical irritation for mouthfeel is due to trigeminal
stimulation, although the taste cranial nerves also perceive
irritation.
Taste thresholds remain quite robust with aging, but loss of
olfaction with aging is another story.
Gustatory Effects
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We begin to lose our sense of smell by age 40, with
significant, gradual decrements occurring each decade
thereafter, reaching up to 70% loss by age 70.
When older adults complain that food doesn’t seem to
“taste” right, it is most likely the loss of smell (which
diminishes flavor).
The threshold for taste varies for each of the primary
tastes.
Bitter substances have the lowest threshold--maybe a
protective function.
The threshold for sour substances is somewhat higher than
for bitter.
Gustatory Effects
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The thresholds for salty and sweet substances are about the
same and higher than both bitter and sour substances.
For taste receptor cells to be stimulated, the substances we
taste must be in a solution of saliva so they can enter taste
pores.
Pleasant tasting foods cause the secretion of large
quantities of saliva.
Foods with unpleasant tastes tend to decrease saliva flow.
Foods with strong acid content, noxious substances, or
extremely dry foods bring about a very watery saliva
secretion.
Moist foods and those with larger particles tend to elicit
saliva with a sticky, thick base.
Gustatory Effects
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With age, taste and smell intensity are reduced, which
may contribute to loss of interest in nutritious foods.
Some medications, such as tetracycline (antibiotic), lithium
carbonate (an antipsychotic), penicillamine (an
antiarthritic), and captorpril (an antihypertensive) can
result in an unpleasant metallic taste in the mouth.
Basic Forces of Eating
o There
are five basic forces involved with eating.
o Compression is the deforming of food using force, such as
between the tongue and palate.
o Adhesiveness is the attraction of food and an external
surface, such as food sticking to the palate.
o Tensile refers to extension of foods under force, such as the
effects of the pharyngeal muscles on the bolus.
o Shear refers to the cutting of food into pieces by forces that
are not directly opposing, such as lateral movement of the
molars during chewing.
Basic Forces of Eating
Fracture is the breaking of food by two directly
opposing forces, such as the incisors biting through a
cookie.
o These forces are used in varying degrees, depending
upon the nature of the food and its position within the
oral/pharyngeal/esophageal continuum.
o