Neural Control of Swallowing

1
Neural Control of Swallowing
2
Neural Control of Swallowing

• 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.

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Motor Tracts

• The motor cranial nerves participating in
  deglutition include
• Trigeminal Nerve (V) for muscles of mastication
• Facial Nerve (VII) for lip sphincter and buccal
  muscles
• Glossopharyngeal (IX) and Vagus (X) Nerves for
  muscles of the palate, pharynx, esophagus,
  larynx, and respiratory control centers and
• Hypoglossal Nerve (XII) for the extrinsic muscles
  of the tongue.

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CN V Trigeminal Nerve

• 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.

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CN V Trigeminal Nerve

• Damage to the trigeminal nerve can affect the
  eating process.
• Specific signs of trigeminal nerve damage or lack
  of innervation by the trigeminal nerve include
• Absence/loss of bite reflex in children
• Partial/total paralysis of the muscles of
  mastication, affecting mandible movement
• Lock jaw, or trismus, resulting from tonic spasm
  or rigid contraction of the masseter, temporalis,
  or either pterygoid muscles.

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CN VII Facial Nerve

• 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.
• The general visceral efferent fibers innervate
  the lacrimal, submandibular, and sublingual
  glands, as well as mucous membranes of
  nasopharynx, hard and soft palate.

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CN VII Facial Nerve

• The general visceral efferent fibers innervate
  the lacrimal, submandibular, and sublingual
  glands, as well as mucous membranes of
  nasopharynx, hard and soft palate.
• They originate from a diffuse collection of cell
  bodies in the caudal pons just below the facial
  nucleus known as the superior salivatory nucleus.

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CN VII Facial Nerve

• Signals for voluntary movement of the facial
  muscles originate in the motor cortex (in
  association with other cortical areas) and pass
  via the corticobulbar tract in the posterior limb
  of the internal capsule to the motor nuclei of CN
  VII.
• Fibers pass to both the ipsilateral and
  contralateral motor nuclei of CN VII in the
  caudal pons.

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CN VII Facial Nerve

• The portion of the nucleus that innervates the
  muscles of the forehead receives corticobulbar
  fibers from both the contralateral and
  ipsilateral motor cortex.
• 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.

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CN VII Facial Nerve

• Damage to the motor nucleus of CN VII or its
  axons results in a lower motor neuron lesion.
• Paralysis of all muscles of facial expression
  (including those of the forehead) will be
  expressed ipsilateral to the lesion.
• 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.
• Voluntary control of only the lower muscles of
  facial expression on the side contralateral to
  the lesion will be lost.

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CN VII Facial Nerve

• Voluntary control of muscles of the forehead will
  be spared due to the bilateral innervation of
  that portion of the CN VII motor nucleus.
• Upper motor neuron lesions are usually the result
  of stroke.
• Damage to the facial nerve affecting the eating
  process can include
• Decreased salivary production and mucosal
  dryness.

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CN VII Facial Nerve

• Loss of symmetry to mouth, which may droop to one
  side and
• Inability of buccinator muscles to monitor and
  control food during chewing, causing food to slip
  outside molar surfaces and collect between gums
  and cheeks.

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CN IX Glossopharyngeal Nerve

• The general visceral efferent fibers innervate
  the ipsilateral parotid gland.
• Salivation is produced in response to smelling
  food (mediated by the olfactory system).

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CN IX Glossopharyngeal Nerve

• The 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.
• Signals for voluntary elevation and/or dilation
  of the pharynx by the stylopharygeus muscle
  originate in the pre-motor and motor cortex.
• They 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.
• Damage 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.

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CN IX Glossopharyngeal Nerve

• The 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.
• Signals for voluntary elevation and/or dilation
  of the pharynx by the stylopharygeus muscle
  originate in the pre-motor and motor cortex.
• They 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.

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CN IX Glossopharyngeal Nerve

• Damage 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.

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CN X Vagus Nerve

• The 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.
• Specifically, they supply the superior, middle,
  and inferior pharyngeal constrictor muscles.

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CN X Vagus Nerve

• In 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.

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CN X Vagus Nerve

• Signals 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.
• Damage 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.

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CN X Vagus Nerve

• Specifically, difficulty or inability to elevate
  the soft palate on the affected side may result
  in regurgitation of fluids/foods through the
  nose.
• On 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.

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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.

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CN XII Hypoglossal Nerve

• Signals 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.

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Sensory Tracts

• 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 the trigeminal nerve for
  sensations from the face and the facial nerve for
  taste on the anterior tongue the
  glossopharyngeal and vagus nerves for sensation
  from the posterior tongue, palate, pharynx, and
  larynx 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.

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CN V Trigeminal Nerve

• The 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.
• Damage 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.

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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.

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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.
• 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.
• 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.
• The general somatic afferent branches of CN IX
  provide general sensory information from the
  upper pharynx, and the posterior 1/3 of the
  tongue.
• 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).
• 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.
• 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.
• It provides taste sensation from the epiglottic
  region.
• However, the general visceral afferent component
  provides information from the larynx and the
  esophagus.
• 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.
• 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.
• 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.
• 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

• 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.

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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

• 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.

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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.
• They send sensory information via their
  respective fasciculi to the cell bodies
  comprising the nucleus tractus solitarius (NTS).
• 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.
• 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

• 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.

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Output Functions

• The 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
• CNs V, X, and XII to the muscles of the
  oropharynx

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Output Functions

• by way of CN X to the muscles of the hypopharynx
• by way of CNs V and XII to the extrinsic muscles
  of the larynx and
• by way of CN X to the intrinsic muscles of the
  larynx and esophagus.
• Microelectrode recording during swallow prove
  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

• Then 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.
• Pharyngeal constrictors fire in overlapping
  order.
• The cricopharyngeus dilates and esophageal
  peristalsis commences at a velocity of between
  2-4 cm sec.
• Direct stimulation of the NA or other ventral
  motor nuclei does not evoke swallowing.

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Output Functions

• 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.

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Cortical Involvement

• 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.

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Cortical Involvement

• The swallowing cortex is a discrete area
  located in the supplemental motor area, anterior
  to M1.
• It is important for time-ordered organization of
  movements, especially in sequential performance
  of multiple movements.

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Cortical Involvement

• It is important in initiation of voluntary
  movements.
• Other cortical sites involved in swallowing are
  the bilateral anterolateral regions of the
  premotor cortex.
• These areas are believed to coordinate the
  sequence of tongue and facial movements.
•  

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Cortical Involvement

• 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 caudal-rostral fashion along the
  precentral gyrus.
•  

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Cortical Involvement

• 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.

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Cerebellum

• Research on swallowing and the cerebellum is
  minimal.
• Most documented studies are case studies that
  involve widespread lesions and not isolated
  cerebellar lesions.
• 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.
• It is thought the cerebellum integrates
  proprioceptive, vestibular, and motor planning
  information and then communicates with the
  cerebral cortex to produce smooth synergistic
  movements.

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Biomechanics of Bolus Flow

• The duration and characteristics of each phase of
  swallow depends on the type and volume of food
  being swallowed.
• Therefore, there are many types of normal
  swallows that occur predictably based on the
  characteristics of the food swallowed and
  voluntary control.
• Moreover, the frequency of deglutition varies
  with activity we swallow more when eating and
  we swallow less when sleeping.
• Mean deglutition frequency is approximately 580
  swallows per day.
• During sleep, periods of 20 minutes or may pass
  when no swallow occurs.

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Volume Effects

• Changes in bolus volume create the greatest
  systematic changes in the oropharyngeal swallow.
• 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).
• 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.

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Volume Effects

• 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.
• 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.

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Viscosity Effects

• Normal swallowing transit times are slower on
  thicker food.
• Thin liquids are easily deformed and move more
  readily in response to gravity and compression.
• 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.
• Thin liquids, being almost completely deformable,
  will pass most easily through narrow sites in
  transit.
• Thicker foods move more slowly in response to
  gravity and compression.

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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.
• Nonetheless, 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.

53
Viscosity Effects

• Valve functions, such as VP closure, upper
  esophageal opening, and laryngeal closure also
  increase slightly in duration as viscosity
  increases.
• As the bolus becomes less deformable, it is less
  likely to pass through narrow sites in transit,
  and may lodge above them instead.
• 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.
• The stimuli that the brain interprets as the
  basic tastessalty, sour, sweet, and bitter
  (possibly umamia glutamate), are registered via
  a series of chemical reactions in the taste cells
  of the taste buds.
• We perceive all taste qualities all over our
  tongue, although there may be increased
  sensitivity to certain qualities in certain areas.

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Gustatory Effects

• Each of the four primary tastes is caused by a
  different response to different chemicals.
• 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.

56
Gustatory Effects

• Neurons in the taste pathway record these
  attributes simultaneously, responding to touch
  and temperature stimuli as well.
• 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.

57
Gustatory Effects

• 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 doesnt 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.

58
Gustatory Effects

• 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.

59
Gustatory Effects

• 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.

60
Basic Forces of Eating

• There are five basic forces involved with eating.
• Compression is the deforming of food using force,
  such as between the tongue and palate.
• Adhesiveness is the attraction of food and an
  external surface, such as food sticking to the
  palate.
• Tensile refers to extension of foods under force,
  such as the effects of the pharyngeal muscles on
  the bolus.
• 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.

61
Basic Forces of Eating

• Fracture is the breaking of food by two directly
  opposing forces, such as the incisors biting
  through a cookie.
• These forces are used in varying degrees,
  depending upon the nature of the food and its
  position within the oral/pharyngeal/esophageal
  continuum.