Physiology of Larynx

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Physiology of Larynx
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Three important functions

• The larynx serves three important functions in
  humans. In order of functional priority, they are
  protective, respiratory, and phonatory.
• In humans the protective and respiratory
  functions are compromised in favor of its
  phonatory function.
• The protective function is entirely reflexive and
  involuntary, whereas the respiratory and
  phonatory functions are initiated voluntarily but
  regulated involuntarily.

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• Laryngeal function may be best understood by an
  appreciation of its origin determined by
  primitive needs.
• On an evolutionary scale, as animals migrated
  from an aquatic to a terrestrial existence, a
  major change in respiratory requirements became
  necessary.
• These accomplishments were reflected in certain
  contemporary species of fish that developed
  unique respiratory modifications to allow
  intermittent sojourns on dry land.
• These structures, however, contained no valves to
  prevent the entrance of water when an aquatic
  existence was resumed.

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Structure and function of the larynx viewed
phylogenetically

• The climbing perch possessed a respiratory
  diverticulum located above its gills.
• The most primitive larynx may be found in the
  bichir lungfish. The larynx of this fish consists
  simply of a muscular sphincter to guard against
  the entrance of water.
• The African lungfish Australian lungfish both
  possess, in addition to sphincteric musculature,
  discrete muscle fibers that effectively draw the
  valvular margins apart to produce active
  dilatation.

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Structure and function of the larynx viewed
phylogenetically

• To enhance ventilatory flow requirements through
  the laryngeal aperture, the acquisition of
  lateral cartilages may be noted in certain
  amphibians.
• These lateral cartilages form bars on either side
  of the glottis to which the dilator muscles
  insert .
• To augment the mechanical advantage of these
  muscles, a cartilaginous ring (giving origin to
  the dilator muscles) can be found between the
  glottis and trachea in other higher vertebrates.
  Such a configuration is apparent among reptiles
  such as the alligator

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• The primitive larynx, therefore, basically
  functioned as a simple sphincter to protect the
  lower airway from the intrusion of foreign
  matter.
• Its secondary function, supported by the
  sequential phylogenetic acquisition of the
  cricoarytenoid complex, centers about its role in
  respiration governed by active muscular
  dilatation of the laryngeal aperture.
• The third function of the larynx, phonation (best
  observed in mammals), appears to be a late
  phylogenetic acquisition.

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• From a structural point of view, protective
  function of the adult human larynx is admittedly
  precarious by virtue of its low position in the
  neck .
• Other mammalian species are provided with a
  relatively high-riding larynx, affording it a
  close approximation with structures of the
  posterior nasal cavities.
• The intranarial position of the larynx, securing
  a continuous airway from the nose to the bronchi,
  therefore decreases the risk of pulmonary
  contamination by swallowed matter.

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• It is of some interest that the human newborn
  exhibits similar nasolaryngeal connection by
  approximation of its epiglottis with the
  posterior surface of its palate, thus ensuring
  against aspiration by forming a continuous upper
  and lower airway .
• The observation of obligate nasal breathing in
  the newborn period may be related to this
  anatomic configuration, which is lost between 4
  and 6 months postnatally.

The nasolaryngeal relationship.
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• In adult humans the characteristic flat,
  shield-like configuration of the epiglottis
  serves to direct swallowed food laterally into
  the pyriform fossae, away from the midline
  laryngeal aperture.
• Elevation of the larynx toward the nasal cavity
  during the height of deglutition exaggerates this
  protective function.

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• Aryepiglottic folds act as ramparts to the
  larynx, allowing food to pass on either side of
  the epiglottis along the gutter produced between
  each fold and the lateral pharyngeal wall.
• Primary role of the supraglottic larynx in adult
  humans lies in its protection of the lower
  airway.

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• In the human larynx the ability to perform as an
  effective valve depends on the unique shelf-like
  configuration of its superior and inferior folds
  bilaterally represented
• The false cords, which are located superiorly,
  act as exit valves, preventing the escape of air
  from the lower respiratory tract. When positioned
  by muscular contraction, they seal even more
  tightly as tracheal pressure is increased from
  below.
• On the other hand, the true cords behave as a
  one-way valve in the opposite direction,
  obstructing the ingress of air. Therefore, it is
  not surprising that expectorative functions of
  the larynx remain unimpaired in bilateral
  laryngeal paralysis.

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Cough Reflex

• Cough ejects mucus and foreign matter from the
  lungs and helps maintain patency of the pulmonary
  alveoli. May be voluntary, but more often in
  response to stimulation of receptors in the
  larynx or lower respiratory tract.
• Three phases
• inspiratory- larynx opens wide to permit rapid
  and deep inspiration
• compressive- tight closure of the glottis and
  strong activation of expiratory muscles
• expulsive- larynx opens widely and a sudden
  outflow of air in the range of 6-10 liters/sec.

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• The larynx acts as a transducer during phonation
  converting the aerodynamic forces generated by
  the lungs, diaphragm, chest and abdominal muscles
  into acoustic energy.
• This energy transduction occurs precisely at the
  space between the two vocal folds. However
  subglottic and supra glottic pressures also play
  a role in this transformation of aerodynamic
  energy into sound energy.

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• The requirements of normal phonation are as
  follows
• Active respiratory support
• Adequate glottic closure
• Normal mucosal covering of the vocal cord
• Adequate control of vocal fold length and
  tension.

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Phonation

• It is generally agreed that speech results from
  the production of a fundamental tone produced at
  the larynx and is modified by resonating chambers
  of the upper aerodigestive tract.
• Intelligible speech, therefore, represents the
  combined effect of the larynx, tongue, palate,
  and related structures of the oral vestibule
• The consonants of speech can be associated with
  particular anatomical sites responsible for their
  generation i.e. 'p' and 'b' are labials, 't' and
  'd' are dentals and 'm' and 'n' are nasals.

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• The production of the fundamental tone is due to
  the vibration of the vocal folds against each
  other, generated by the passage of air between
  them. Vocal cord vibrations may be a passive
  phenomenon representing the basis of the
  aerodynamic theory of sound generation.
• Such a theory finds support in the observation
  that the completely paralyzed larynx is capable
  of producing sound, as is the cadaver larynx when
  subglottic pressure is forcefully increased..

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• The aerodynamic theory of sound production
  therefore replaces the neurochronaxic theory
  which incorrectly advanced the notion that the
  central generation of recurrent laryngeal nerve
  impulses produced cord vibrations by active
  contraction of the thyroarytenoid muscles. Each
  vibration, therefore, represented the result of
  beat-by-beat impulses through the recurrent
  laryngeal nerve.

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• The cricothyroid muscle increases fundamental
  frequency by tensing the vocal fold. The vocal
  fold is stretched, elongated, thinned, and
  slightly adducted to the paramedian position as
  the vocal fold is lowered within the larynx.
  These changes reduce the cross-sectional area of
  the vocal fold, reducing vibratory mass and
  increasing fundamental frequency.

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• Vocalis muscle (Voc), on the other hand,
  generates the opposite effect as it loosens and
  thickens the vocal fold. In addition, as it
  increases glottal resistance, it contributes to
  vocal intensity as subglottal pressure is
  increased.
• Vocal control, therefore, is achieved by the
  coordinated efforts of respiratory, laryngeal,
  and articulatory muscles capable of producing
  great variations of tonal qualities
  characterizing the human voice.

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The Glottic Cycle

• The vocal folds alternately trap and release air
  each trap/release is one cycle of vibration. This
  cycle is often referred to as the glottic cycle,
  and it is divided into phases opening phase,
  open phase, closing phase, closed phase
• During the closed phase, the air pressure builds
  up below the vocal folds. When the glottis opens,
  the air explodes through the vocal folds, and
  that's the beginning of the sound wave. The
  strength of that explosion determines the
  loudness of the sound coming directly from the
  larynx.

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• First, the laryngeal muscles position the vocal
  cords in various degrees of adduction and place
  them under the appropriate longitudinal tension.
• Next, muscular and passive forces of exhalation
  cause the subglottic air pressure to increase.
• When this subglottic pressure reaches a point
  where it exceeds muscular opposition, the glottic
  chink is forced to open.
• When the vocal cords start opening from complete
  closure, they open in a posterior to anterior
  direction with the posterior portion of the
  glottis opening first, reaching maximum excursion
  first, and recontacting each other at the end of
  the vibratory cycle prior to the anterior portion
  of the cords.

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• After release of the puff of air there is a
  reduction of subglottic pressure, and the vocal
  cords approximate each other again (myoelastic
  forces of the vocal cords have exceeded the
  aerodynamic forces).
• The myoelastic forces are enhanced because air
  current flowing through a narrow channel exerts a
  negative pressure on the channel walls This is
  the basis of Bernouilli's Principle.
• The vocal cords are thus sucked back together in
  an adducted state until the subglottic air
  pressure can overcome the myoelastic forces of
  the reapproximated cords, and the cycle is then
  repeated.

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Pitch

• The faster the vocal folds vibrate, the higher
  the pitch.
• In general, men's vocal folds can vibrate from 90
  - 500 Hz, and they average about 115 Hz in
  conversation.
• Women's vocal folds can vibrate from 150 -1000
  Hz, and they average about 200 Hz in
  conversation.
• Vocal folds vibrate faster as they're pulled
  longer, thinner, and more taut and vibrate more
  slowly when they're shorter, thicker, and
  floppier.
• The cricothyroid muscle and thyroarytenoid muscle
  coordinate with each other to create different
  pitches

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Swallowing

• During swallowing the sphincters of larynx stay
  contracted preventing aspiration of food into the
  air passage.
• During the pharyngeal stage of swallowing the
  larynx is elevated towards the lower jaw, this
  elevation opens up the cricopharyngeal sphincter
  thus facilitating swallowing.
• The hyoid bone rotates in such a way that the
  greater cornua becomes horizontal, producing a
  backward tilting of the epiglottis towards the
  posterior pharyngeal wall. This movement of hyoid
  bone effectively closes the laryngeal inlet.

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Fixation of the Chest

• This less known function of the larynx is
  important for increasing intra abdominal
  pressure. Closure of the vocal cords achieves
  fixation of the chest necessary to raise intra
  abdominal pressure required for daily activities
  like lifting weights, climbing and even for
  passing urine and stools.

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