Download Physiology of Larynx

Physiology of
Larynx
Three important functions
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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.
Structure and function of the larynx viewed phylogenetically
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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.
Structure and function of the larynx viewed phylogenetically
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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
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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The nasolaryngeal
relationship.
•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.
In adult humans the
characteristic flat, shieldlike 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.
Cough Reflex
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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.
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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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.
Phonation
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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.
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.
The Glottic Cycle
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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.
Pitch
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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
Swallowing:
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During swallowing the sphincters of larynx stay
contracted preventing aspiration of food into the
air passage.
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During the pharyngeal stage of swallowing the
larynx is elevated towards the lower jaw, this
elevation opens up the cricopharyngeal
sphincter thus facilitating swallowing.
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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.
Fixation of the Chest
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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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