LARYNGEAL PHYSIOLOGY AND MECHANICS OF PHONATION
INTRODUCTION:
The
larynx is a musculo-cartilaginous structure located at the superior end of
trachea; guarding the entrance to the lower respiratory passages (trachea,
bronchi, and lungs) and houses the vocal cords. The larynx consists of four
basic anatomic components: a cartilaginous skeleton, intrinsic and extrinsic
muscles, and a mucosal lining. These cartilages are connected to other
structures of the head and neck through the extrinsic muscles. The intrinsic
muscles of the larynx alter the position, shape and tension of the vocal folds.
The larynx functions in deglutition (swallowing), respiration (breathing), and
phonation (voice production). The production of voice can be thought of in terms
of three components: the production of airflow, the generation and resonance of
sound and the articulation of voice.
Although
the gross anatomy of larynx has been known since the mid-sixteenth centaury,
details of laryngeal structure are being discussed. Researchers have been
viewing and photographing the vibrating larynx for more than 100 years;
electromyographic and air flow data are continually being published and
constructs of structure and function are constantly being subjected to
revision.
The following is a
description of
The
onset of phonation
The
Bernoulli effect
The
Bernoulli effect applied to phonation
Initiation
of phonation and
Characteristics
of a vibratory cycle
All
the above let us know the manner in which the phonation is initiated and
characteristics; which make human larynx a unique structure.
MECHANICS
OF PHONATION:
There
are just two basic internal laryngeal adjustments that can take place. They are
the force with which the vocal folds are brought together at the midline
termed,
1. Medial
Compression
2. Longitudinal
tension
1) MEDIAL
COMPRESSION
It
is the force with which the vocal folds are brought together at the midline.
2) LONGITUDINAL
TENSION
It
is the extent of stretching force.
These
two adjustments or combinations of them, plus a variable air supply account for
incredible versality of
the human voice.
In
1886, Stroker suggested that the larynx operated much like a single stringed
instrument. In 1892, Woods suggested that the larynx complies with the
fundamental equation of vibrating strings, which states
n=
1/L √T/M
n=
Frequency of vibration
L=
Length of vocal folds
T=
Tension of vocal folds
M=
Mass per unit length
The primary
factor that determine the vibratory rate of a string are mass
and tension in relation to length. Accordingly, a strings vibration may be
doubled by halving its length or by increasing tension or by decreasing mass by
a factor of four. Strings behave in accordance with basic laws of physics, but
the larynx is an aerodynamic structure and only partly complies. The vocal fold
should not be equated with vibrating strings.
These
problems have been recognized by Sonninen (1956). Who stated that the
relationship of factors influencing the pitch of voice can be
represented by the following equation.
f
= c K/M
f
= Frequency of vocal folds
C
= A constant
K
= K + K
Where
K represents inner passive tension of the vocal folds (related to tissue
elasticity) and K represents an inner active tension (longitudinal tension
related to muscle contraction and changes in length of vocal fold)
M
= Mass of vocal fold
Both
pitch and spectral characteristics of voice (voice quality) are dependent upon
1) The
frequency of vocal fold vibration
2) The
pattern or mode of vocal fold vibration
3) The
configuration of the vocal tract
THE
ONSET OF PHONATION:
The
onset of phonation is may be divided into two phases:
1. Prephonation
phase
2. Attack
phase
THE
PREPHONATION PHASE:
The
prephonation phase is the period during which the vocal folds move from an
abducted to either adducted or partially adducted position. When the vocal
folds are viewed prior to the onset of phonation, they are usually seen to in
an abducted position, i.e., the subject is breathing. The duration of
prephonation phase and the extent to which the vocal folds are approximate are
highly variable, depending largely upon the utterance to be emitted.
If
the forces of exhalation are released and the vocal folds approximate or
nearly approximate, they begin to obstruct the outward flow of air and from the
lower respiratory tract and the sub glottal pressure beneath the vocal folds
build up. In addition, the velocity of the air as to it flows the glottal
constriction is raised sharply.
The
extent to which the V.F are approximated is called medial compression which is
brought about by the action of the adductor muscles, which work in pairs or
group to execute movement. The lateral crioarytenoid and arytenoids muscles are
called the adductor of vocal folds.
A) Abduction of V.Fs
caused contraction of the posterior cricoarytenoid muscles
B) Medial compression
of the vocal folds caused by the contraction of the transversal and oblique
arytenoid muscle.
c) Adduction of the
folds caused by contraction of the lateral crioarytenoid muscle
1) THE
ATTACK PHASE:
The
attack phase begins with the V.Fs adducted or nearly so and extends through the
initial vibratory cycles. The phase is also highly variable in its duration,
depending primarily upon the extent to which the VFs are adducted during the
pre-phonation phase and the manner in which the air stream is released.
Often
the vocal folds are not completely adducted during the prephonation phase;
complete obstruction of the air passage way is not necessary to initiate
phonation. If the glottal chink is narrowed to 3mm, a minimal amount of airflow
ill set the VFs into vibration.
High-speed
laryngeal photography shows that the initial movement in incompletely adducted
VFs is medial ward. Medial movement can be adequately accounted by the
Bernoulli effect.
THE
BERNOULLI EFFECT:
Daniel Bernoulli a 17th centaury
Swiss scientist, recognized the effects of constructing a tube during fliud
flow. The Bernoulli effect states that, given a constant volume flow of air or
fluid, at a point of constriction there will be a decrease in air flow and an
increase in velocity of the flow. He formulated the following aerodynamic law:
d (V
p) = c
d =
density v = velocity
p
= pressure C = a contant
Total
energy is the sum of the kinetic energy and potential energy and in case of
fluid flow, total energy is a constant. So
E
= KE + PE = C
In
certain mechanical systems there is a constant exchange of kinetic and
potential energies. E.g.: in the case of a mass bobbing a spring; at two
extremes of movement the mass momentarily comes to a rest before its movement
changes direction. At the instant
of rest, all the
energy is potential, because no movement is taking place. The energy is stored.
Half-way between the two extremes of displacement, all the energy is kinetic,
because it is here that the velocity of the mass is maximum and the
acceleration (the result of potential energy) is zero.
If
the velocity increases, the energy of movement or kinetic energy must also
increase. If total energy is to be a constant, potential energy must decrease.
In case of fluid flow, Kinetic energy is equal to the product of one-half the
mass or density of the fluid and the velocity of fluid flow squared. The
equation is very familiar
KE
= ½ mv
M
= Density or mass of fluid
V
= Velocity of flow
Potential
energy is pressure (force per unit area). So the total energy is equal to the
sum of kinetic and potential energies, or
E
= ½ MV + P = C
As the velocity of
fluid flow increases, kinetic energy must of necessity increase and potential
energy (pressure must decrease accordingly).
The
following figure is a simple illustration of the Bernoulli effect.
The
tube at the bottom of illustration can be thought of as the trachea. The
constriction represents the larynx and vocal folds, and the larynx portion at
the top, the pharynx and the oral cavity. The same amount of air that enters
from beneath and leave through the top. So the velocity of air flow will be
especially high at the constriction and low in the upper portion.
To
apply the Bernoulli effect to phonation, assume that the V.Fs are nearly
approximated at the instant the air stream is released by the force of
exhalation. The air stream will have a constant velocity until it reaches the
glottal constriction. Velocity will increase as air passes through the chink.
The result is a negative pressure between the medial edges of the vocal folds,
and they will literally be sucked towards one another.
The
Bernoulli effect is of major importance in understanding the vocal mechanism,
especially as it applies to ordinary phonation (Van den berg, 1958a).
INITIATION
OF PHONATION
The
movement of vocal folds as they enter into vibration are shown graphically
As
glottal area reaches a certain critical value, the folds begin to execute
vibratory movements result in a decrease in glottal area. It is to be noted
that the V.Fs undergo, a number of vibrations before they meet to completely
obstruct the air stream.
As
long as sub glottal pressure is adequate, the medial compression of the vocal
folds will overcome and they will be blown apart to release a puff of air into
the supra-glottal area. This somewhat explosive release results in an immediate
but short-duration decrease in sub-glottal pressure. The elasticity of the
vocal fold tissue, along with the Bernoulli effect, causes the V.Fs to snap
back to the midline.
The
nature of initial vibratory cycle may be influcenced by a host of variables,
including the intensity of phonation, the linguistic environment of the sound
to be emitted, the pitch of the voice and vocal habits. This was recognized by
Moore in 1938. He suggested three ways in which the air stream might be
released:
1) Simultaneous
attack
2) The
breathy attack and
3) The
glottal attack
1) SIMULTANEOUS
ATTACK
Here
there is a healthy balance between the respiratory and laryngeal mechanism and
the air stream is released just as the VFs meet at the midline.
2) THE
BREATHY ATTACK
The
air stream is released before VF adduction is completed and a considerable
quantity of air may be exhaled while folds are being sent into periodic
vibration.
3) THE
GLOTTAL ATTACK
When
the phonation is initiated while the folds are subjected to considerable medial
compression. The voice exhibits onset more sudden than during either the
simultaneous or the breath attacks. The vocal tone is explosive in nature and
the initiation of phonation is called a glottal attack, glottal shock or stroke
of the glottis
(Coup-de-glotte).
CHARACTERISTICS
OF A VIBRATORY CYCLE
GLOTTAL
AREA
To
identify the characteristics of vocal fold vibration, the vibrating VFs needs
to be photographed initially at an exposure rate of about 40000 frames a
second. One or more cycles of vibration are then projected, frame by frame and
the area that comprises the glottis is computed or measured. Graphs of glottal
area as a function of time (or film frames) can be constructed.
The
glottal area has been extracted from each frame and plotted against time. The
vibratory rate of this particular subject was about 168 cycles per second (Hz)
and the film was exposed at a rate of about 4000 frames per second. The opening
phase extended through the first 12 frames; in other words, it occupied one-half
0r 50% of the vibratory cycle. The closing phase extended through the next nine
frames and occupied about 37% of the cycle. The closed phase extended through
the final 3 frames and occupied about 13% of the total cycle. These values are
fairly representative for phonation at conversational pitch and intensity.
OPEN
QUOTIENTS:
Timcke,
von Leden and Moore (1958) measured the glottal wave. They illuminated the
larynx with an advanced ‘Synchrostroboscopic’ technique, and express the relative
amount of durations of the phases of the vibratory cycle in terms of quotients.
Thus, the ratio of the fraction of the cycle during which the glottis is open,
compared with the total duration of the cycle is referred to as the open
quotient (OQ). The larger the open phase, the larger the OQ.
Time
the glottis is open/ duration of the open phase
OQ=
Time of entire
vibratory cycle /duration of the entire vibratory cycle or fundamental period
SPEED
QUOTIENT:
Same
investigators employed high-speed photography of the larynx and measured the
difference in duration between the opening and closing phase. They selected the
ratio between two phases and termed it as speed quotient (SQ). So
Time of abduction or
lateral excursion
SQ=
Time of adduction or medial
excursion
At
same instance, the value of open quotient is 1.0 when the glottis never closes
or when there is no complete glottal closure and Hence the speed quotient
provides additional descriptive information about the vibratory characteristics.
STUDIES
ON OPEN QUOTIENT AND CLOSED QUOTIENT
1)
Frequency, Intensity, and Target Matching Effects on Photoglottographic
Measures of Open Quotient and Speed Quotient
David G. Hanson , Bruce R. Gerratt and Gerald S. Berke
(1990)
Measurements of Open
Quotient (OQ) and Speed Quotient (SQ) were made from
photoglottographic signals of normal male subjects during
phonation. Samples were obtained at spontaneous levels of
fundamental frequency and intensity, and at nine specified frequency/intensity
combinations. OQ increased with fundamental frequency. OQ
change was not significant for change in intensity and there
was no significant interaction between frequency and intensity.
Changes in SQ with variations of frequency and intensity were
not significant. However, SQ did increase significantly when
spontaneous phonation was compared to target matching phonation at
similar frequency/intensity. Changes in both OQ and SQ across comfortable
frequency and intensity ranges were relatively small in
comparison to changes in OQ and SQ reported for pathological phonation.
2) Glottal
open quotient in singing: Measurements and correlation with
laryngeal mechanisms, vocal intensity, and fundamental frequency
(2005)
It explores the
relationship between open quotient and laryngeal mechanisms, vocal intensity,
and fundamental frequency. The audio and electroglottographic signals of 18
classically trained male and female singers were recorded and
analyzed with regard to vocal intensity, fundamental frequency,
and open quotient. Fundamental frequency and open quotient are
derived from the differentiated electroglottographic signal, using the
DECOM (DEgg Correlation-based Open quotient Measurement). As male and female
phonation may differ in respect to vocal-fold vibratory properties, a
distinction is made between two different glottal configurations, which are
called laryngeal mechanisms: mechanism 1 (related to chest, modal, and male
head register) and mechanism 2 (related to falsetto for male and head register
for female). The results show that open quotient depends on the laryngeal
mechanisms. It ranges from 0.3 to 0.8 in mechanism 1 and from 0.5 to 0.95 in
mechanism 2. The open quotient is strongly related to vocal intensity in
mechanism 1 and to fundamental frequency.
3) ADVANTAGES OF USING OQ AND SQ
1) G.
E. Murty and P. N. Carding (1992)
Recordings by combined
glottography of vocal cord movements in patients with a vocal cord palsy were
compared with a control group. In paralysis of the vocal cord the open quotient
(OQ) is increased and the speed quotient (SQ) decreased. This system may have
potential in the diagnosis and continued assessment of laryngeal abnormalities
as well as providing a permanent objective record in medico-legal cases.
2) Jiang,
J.J.; Shuangyi Tang; Dalal, M.; Chi-Haur Wu; Hanson, D.G (1998)
Measures such as open
quotient and speed quotient calculated from glottographic signals can provide
useful information regarding pathological phonation i.e. in patients with voice
disorders but requires further evaluation before clinical application
MODELS
OF VOCAL FOLD VIBRATION
The models of V.F
vibration is used to provide the representation of the contact area of the VFs,
to evaluate the contributions of the larynx to speech production and for
assessing the role of various tissues, the influence of medial compression and
their longitudinal tension. To be completely successful a model should manifest
all the known properties of the structure or system it represents.
A
SINGLE-DEGREE-OF–FREEDOM MODEL
Described
by Flanagan and Landgray (1967).
In
this model the VFs must move as a single mass toward and away from the midline,
they have nowhere else to go.
They
are noted that the VFs operate as an aerodynamic oscillator and their motion is
a self-determined function of the physical parameters such as sub glottal
pressure, vocal fold tension and vocal tract configuration.
SCHEMATIC OF A
SINGLE-MASS-MODEL OF THE VOCAL FOLDS (AFTER FLANAGAN AND LANDGRAF, (1967)
The
folds are considered as a simple mechanical oscillator of mass, M which
represents the mass of the paired VFs: a spring constant K, Which represents
the vocal tract tension and viscous damping B. Which is due to a condition at
the boundary where the VFs strike one another upon closure i.e. the opposing
surface that the mass of the vocal fold strikes is relatively massless and
mainly fluid or viscous or fluid like.
When
the closing folds meet at t he midline, they give up some of their momentrum,
but because of internal properties of the folds, the tissue tends to be
displace towards the midline. As a result, the glottis is closed for a brief
period of time and at the same time the force acting on the mass of the VFs are
immediately in a direction to open the glottis. The VFs operate automatically.
The
boundary may also be massive or hard and in that case the folds give up their
momentrum instantaneously: The damping, of course, is quit
different under these conditions. In the viscous condition, the folds tend to
mould into one another as they meet. At hard boundary condition, they tend to
rebound. Viscous and hard boundary conditions can be thought of as representing
low and high pitch phonation.
In
the figure,
Ps-
denotes sub-glottic pressure
P1 and P2- acoustical
pressures at the inlet and outlet of the glottal orifice, respectively and
Ug-
Acoustic volume velocity through the glottic orifice
It
is also to be noted that the vibration VFs exhibit considerable vertical phase
difference at low and moderate pitch levels and that they manifest a certain
amount of vertical displacement as they vibrate.
A
TWO-DEGREE-OF–FREEDOM MODEL
Describe
by Ishizaka and Flangan in 1972.
The
figure given below represents several characteristics of oscillation in common
with the VFs
The
VFs are represented by two masses, M1 and M2, which are capable of purely
horizontal motion independently.
Each
mass is thought of as a simple mechanical oscillator with a mass M, a spring
constant K and viscous damping B, as with the single mass model.
These
masses are coupled together by S3, which acts to supply a force on M1 and M2 in
the horizontal direction, by virtue of a difference in their lateral
displacements X1 and X2 respectively.
If
we let Lg represent the length of the glottis, the glottal area A1 and A2
corresponding to the region of M1 and M2 for paired masses (as in the real
larynx) becomes
A= and
The
equilibrium position of the masses is X
The
stiffness exhibited by the spring S and S is due to the longitudinal tension of
the vocal folds.
If
the masses are displaced from their equilibrium position x by distance x – x
and x – x the restoration force is equal to S (x-x) and S (x-x).
TWO-DEGREE-OF–FREEDOM
MODEL OF THE VOCAL FOLDS (AFTER ISHIZAKA AND FLANAGAN, 1972)
In
the above figure, the resistance r1 and r2 represents dashpots. In the larynx,
the dashpots r1 and r2 function to decrease the velocities of the masses M1 and
M2 due to the restoration forces of S and S.
DRAWBACKS:
It
is t be noted that
1) Restoration
forces are not linearly proportional to the displacement.
2) The
vibratory pattern of the VF is not sinusoid and
3) Under
certain conditions the system can become unstable.
In
this model, air flow through the trachea is shown as Vt. As the constriction at
M and M is reached, the velocities increase and so the restoration forces are
complemented by the important Bernoulli effect.
THE
SIXTEEN-MASS MODEL:
A
sixteen-mass model of the larynx was described by Titze in 1973 in an attempt
to stimulate human like speech that would
1) Phonate
in at least two resisters.
2) Provide
sufficient flexibility for pathological studies.
3) Be
capable of stimulating transient responses of VFs such as moderate cough or
voice breaks.
4) Be
regulated by parameters that have direct physiological correlates and
5) Increase
the naturalness of utterances
Titze’s
model attempts to simulate various observed vocal fold behaviours, including
vertical and horizontal motion of the vocal folds, and horizontal and vertical
phase differences. Each VF consists of two portions that behave differently
during oscillations. They are the mucous membrane and the vocalis muscle, which
are tightly coupled to the vocal ligament. But this coupling is not constant;
it is altered during pitch changes that are accompanied by changes in tension
and length.
THE
SIXTEEN-MASS MODEL OF THE VOCAL FOLDS BY TITZE (1973)
The mucous membrane
has been observed at high pitch phonation (that of a female singer) to collect
in 8 nodal regions in a standing wave pattern. This pattern and some
mathematical considerations lead Titze to sub-divide the Mucous membrane and
vocalis-vocal muscle ligament masses into 8 seprate masses. The model then
consists of 16 masses which are allowed
to move in a direction perpendicular to air flow (in a lateral direction and in
the direction of this flow. No motion in a longitudinal direction is
considered. The sixteen-mass model is shown in the figure.
Titze
identifies three general categories of forces that act upon the VFs. They are
Internal
forces
External
forces and
Dissipative
force
INTERNAL
FORCES
Internal
force refer to nearest neighbour forces only, with maximum four nearest
neighbour forces acting on a given particle. They are the restoring force which
are space dependent and take the general
form,
Stress
= R Strain (Hook’s law)
EXTERNAL
FORCES
These are the gravity
and aerodynamic force.
DISSIPATIVE
FORCE
It includes losses
associated with glottal flow, losses in the vocal tract and losses in the vocal
tissues. The damping factor of the system is variable, depending
upon whether the VFs are abducted or adducted.
For quantitative
inputs on elasticity to the model, Titze makes use of the following information
from Van den Berg (1960):
1) The
maximum strain exhibited by the vocal ligament as about 30% of the related
strength. To a first-order approximation, the stress-strain curve is
exponential. After maximum strain, the ligament is indistensible and
behaves like a conventional string.
2) Relaxed
muscular tissue reaches this point at 50% of relaxed length.
3) Active
stress supported by the vocalisnvaries continuously from zero to about 10g/mm
(van den Berg, 1958).
By varying the active
muscular tension from zero to slight, we observe that the tension
supported by the ligaments decreases with muscle activity unless the strains
are very high.
The mucous membrane
supports little tension when not engaged in vibration.
In motion, it is displaced considerably, and
it is out of phase with the rest of VFs, so it generates lateral strains
between the particles. Titze assumes exponential elastic behavior to be
exhibited by the mucous membrane (no actual information of elasticity is
available). Spring constants K1 and K2 depend upon registeration; active
muscle involvement increase resistance to lateral deformation.
Computer stimulation
of this model yields glottal shapes,spectra, air-flow cahracteristics and
velocity function. That show promising approximation to the data available on
human speech. The overall system of Titze consists of
a 16-mass model of the vocal folds, 18-section cylinder tube, approximation of
the pharynx and mouth, 12-section approximation of the nasal tract. The model
is capable of producing vibrations and related pressures that closely resemble
those of human speech. The parameters used to control the stimulation have
direct physiological correlates, for example, subglottic pressure, muscular
tensions and articulatory movements of tongue and jaw. With this model,
phonation is possible in atleast two registers, transient responses of the
vocal system can be simulated and with it some pathologies can be studied.
In
1975, Titze and Strong treated the vocal folds as a continuum rather than a
system of discrete masses and springs, and they applied viscoelastic properties
of the vocal fold tissue to their model. The effects of tissue viscosity and
incompressibility were incorporated to account for coupling between horizontal
and vertical motion when vertical phase differences occur, a limitation
inherent in earlier finite-element models.
In
1979, Titze and Talkin were able to model the effect of various laryngeal
configurations on phonation, taking into account the curved boundaries of the
V.Fs and their visco-elastic properties. They found fundamental frequency to be
affected primarily by vocal fold length. i.e. Fundamental frequency
is controlled by longitudinal stress in the muscle layers. They also found that
sub-glottal pressure is not a major control of Fundamental frequency.
A
FINITE ELEMENT MODEL OF VOCAL FOLD VIBRATION
Given
by Alipour, Berry and Titze (2000)
A
finite-element model of the VF is developed from basic laws of continuum
mechanics to obtain the oscillatory characteristics of the VFs. The model is
capable of accommodating inhomogeneous, anisotropic material properties and
irregular geometry of boundaries. It has provisions for asymmetry across the
midplane, both from the geometric and tension point of view, which enables one
to stimulate certain kinds of voice disorders due to vocal fold paralysis. It employs
the measured viscoelastic properties of the VF tissues.
CO-ORDINATION
OF THE RESPIRATORY AND LARYNGEAL SYSTEMS IN PHONATION
INTRODUCTION
Speech
production requires the co-ordination of several articulatory systems: the
respiratory system, the larynx, the velum, the lips, the tongue and the jaws.
Reflex effects produced by afferents from the respiratory system appear to be
important to
such co-ordination.
Other laryngeal reflexes, however, those that protect the airway against the
entry of foreign material, would be highly disruptive if they occurred during
vocalization. They have to be controlled in some manner.
A
more subtle co-ordination of functions must exist within the system that we use
for both respiration and phonation. Breathing and vocalization are not mutually
exclusive, but are interdependent. The co-ordination of movements for these two
purposes and the integration of reflexes that serve them constitute an
important problem in regulatory physiology. Since the larynx is not only a
respiratory valve, but also the organ of speech and song,
RESPIRATORY
PHYSIOLOGY OF THE LARYNX
In
this early evolution, the
larynx arose as a respiratory valve that contributed critically to the
emergence of air breathing in amphibians and reptiles (Bartlett, 1989).
In humans and other mammals it has undergone further evolutionary development
in relation to vocalization. But it retains important respiratory even in these
species.
LARYNGEAL
BREATHING MOVEMENTS
The
larynx is not just an open or closed valve, however, even if we continue to
view it as a respiratory rather than a vocal organ. The larynx acts as a
variable resister, which regulates air flow in and out of the lungs. Most
variations in laryngeal resistance occur at the level of vocal folds i.e.
contraction of posterior cricoarytenoid muscle (PCA) abducts the folds and
lowers the resistance. Whereas, contraction of thyroarytenoid (TA) and other
vocal fold adductors narrows the glottic slit and raises resistance (Proctor,
1964, 1980; Sasaki and English, 1984).
The
cricoarytenoid (CT) muscle has complex action, tilting the thyroid cartilage
ventrally and caudally with respect to the cricoid, thus lengthening and
slightly adducting the vocal folds (Arnold, 1961). Simultaneous CT
and PCA activation renders the glottis airway slightly larger than during PCA
activation alone (Horiuchi and Sasaki, 1978; Konard and Rattenborg,
1969). Extrinsic mechanism may also alter laryngeal resistance in some
circumstances (Fink, 1974, 1975; Fink et al, 1956). But most
evidence indicates that intrinsic muscle activity is much more important (Brancatisano
et al, 1983, 1984). The balance between adductors and abductor
activities varies greatly. Considerable relative adduction of vocal folds may
result from relaxation of PCA muscle even in the absence of adductor muscle
activity.
Adductor
activity is clearly present in some circumstances. But the most important and
consistent mechanism underlying the respiratory movements is the phasic
activity of the intrinsic abductor muscles, the PCAs (Brancatisano et
al, 1984).
During
quick breathing in humans, inspiration and expiration are fundamentally
different in mechanical and energetic terms.
1) INSPIRATION:
It
is powered by the diaphragm and other muscles of the ventilatory bellow
and the rate of inspiratory airflow is determined by the force generated by
these muscles and by the resistance and elastance of the system. The larynx may
influence inspiration to the extent that influences resistance (Bartlett
et al, 1973; England et al, 1982). Normally the vocal folds are widely
separated during inspiration and dynamic passive collapse of the laryngeal
airway is prevented by the cricoid ring. So laryngeal resistance is low and has
little influence on inspiratory flow (Brancatisano, et al, 1983; England et
al, 1982).
2) EXPIRATION:
In
contrast, here the energy is recovered from potential energy stored in the
system during previous inspiration and its respiratory flow is determined by
the recoil pressure of the respiratory system and its resistance rather than
the on going muscle activity. In most circumstances, there is a remarkable
matching between the duration of respiratory flow and the time before the next
inspiration, so there is no end respiratory pause. (Brody, 1954;
Gautier, et al., 1973)
LARYNGEAL
AFFERENTS AND THEIR REFLEX ACTIONS:
The
larynx has an unusually rich endowment of sensory nerve endings. The superior
laryngeal nerve (SLN) is the main source of laryngeal afferent activity; the
recurrent Laryngeal nerve (RLNs) contain a few sensory nerve fibers (Sampson
and Eyzaguirre, 1964; Suzuki and Kirchner, 1969), but the overwhelming
majority run in the internal branch of superior laryngeal nerves (SLNs) (Mathew
et al., 1984; Widdicombe, 1986; Wyke and Kirchner, 1976).
Bartlett (unpublished)
Davis and Nail (1987) identified that animals such as Rabbits and Snakes
who have little vocal ability are also innervated by large number of sensory
nerve fibers in the larynx. But these nerves may be important for
breathing and airway protection as well as for vocalization.
Widdicombe
(1986) have said that, though it’s a common
experience very little formal study has been done to evaluate the perception of
sensation arising in the larynx. Many investigators studied the responses of
laryngeal receptors to a wide range of stimuli, chiefly by recording the
activity of single afferent fibers in the SLN. But the results of these studies
were confusing. Because of inherent complexity of the receptors and their
responses and to the use of different classification schemes by different
investigators.
MECHANORECEPTORS:
They
respond to mechanical pressure or deformation of the receptor and adjacent
tissue. These are the most thoroughly described general category of laryngeal
receptors.
Sampson
and Eyzaguirre (1964) identified ‘touch’
receptors, which responded to light mechanical stimulation of epithelium and
‘deep’ mechanoreceptors, which they suggested were in the laryngeal muscles or
joints.
Boushly
and associates (1974) also identified two groups of receptors, but
used different criteria. Their ‘Group 1’ fibers had no or little spontaneous
activity and adapted quickly to sustained mechanical stimuli. While ‘Group 2’
units were spontaneously active and adapted slowly and incompletely to
sustained stimuli.
Bradlev
(2000) used anatomical, behavioural and
neurophysiological techniques to examine the receptors responsible for
initiating these responses. Recordings from afferent fibers innervating
laryngeal mechanoreceptors have revealed that some of them are spontaneously
active whereas others are silent until stimulated. Laryngeal mechanoreceptors
respond to stimulation with either a rapidly adapting or a slowly adapting
response pattern.
Davis and
Nail (1987) used a similar binary
classification scheme. He identified ‘Silent’ and ‘tonic’ receptors from among
the SLN afferent fibers. Which could be stimulated by light touch of the
laryngeal mucosa in the Cat and Rabbit. The dynamic sensitivity of these
receptors from various locations would ensure a high probability of the
movement of a foreign particle into the larynx. These observations led
Davis and
Nail (1987) to suggest that the
dynamic sensitivity of touch-sensitive SLN afferents was appropriate for the
detection of the ingress of foreign particles and the provocation of defensive
reflexes.
Esaki,
Umezaki, Takagi and Shin (1997) demonstrated
that highly sensitive mechanoreceptors and polymodal receptors exist in the
laryngeal mucosa of Cats and they are particularly numerous in the laryngeal
surface of the epiglottis and arytenoids region and uncommon in the vocal fold.
Gozaine
and Clark (2005) identified two flow receptors, a drive
receptor, a frequency-following receptor and frequency-nonfollowing receptor in
decerebrated cats. Both flow receptors fibers were almost silent during the
phonation phase and reached the maximum activity during after vocalization
during the inspiratory phase. The drive receptor was active during all four
airway maneuvers and was most active during tracheal occlusion. It also kept a
high level of activity during the phonatory phase. Suggesting a role in the
modulation of vocalization and respiration. The next two receptors, a
frequency-following receptor and frequency-nonfollowing receptor, were active
only during phonatory phase and were totally inactive during the airway maneuvers, suggesting
a role during the vocalization
behaviour.
CHEMORECEPTORS:
Some
laryngeal receptors respond dramatically when water or other fluids are placed
in the laryngeal lumen. The characteristic response is immediate, intense
activity, which adapts only to a small extent and is sustained until the
offending liquid is washed out of the larynx with saline or some other
non-stimulating fluid. The chemical basis for the activation of these ‘Water
receptors’ varies with species. But seems to depend on the removal of chloride
ion from the epithelial surface in dogs and rabbits (Boggs and Barlett,
1982; Boushey et al, 1974; Shingai, 1977, 1979).
The
classification of receptors by stimulus modality is somewhat arbitary as some
receptors respond to both medical and chemical stimuli. As reported by Boushey
et al (1974) and Davis (1986), the activity of some ‘Group 2’ or ‘tonic’
mechanoreceptors is inhibited by CO2 concentration as low as 3%, thus
suggesting that the discharge is modulated by the CO2 in expired air with every
breath
REFLEX
RESPONSES:
Barlett et
al, 1981; Szereda-Przestaszewska and Widdicombe (1973) said that the
most prominent and characteristic reflex response to laryngeal mucosal
stimulation is immediate closure of glottis. This response protects the lower
respiratory tract from contamination with undesired materials. Sustained glottic
closure is incompatible with breathing. However, and it is not surprising that
protective closure of the glottis is often brief or incomplete and is
co-ordinated with the breathing cycle.
Weak
mechanical or chemical stimulation of the laryngeal epithelium exaggerates the
expiratory adduction of the vocal folds, but may not result in complete airway
occlusion. More intense stimulation causes apnea or glottic closure, often
interrupted by coughing (Jimenez-Vargas et al; 1962;
Szerecla-Przestaszewska and Widdicombe, 1973).Apnea is particularly
stricking in neonatal animals, whereas coughing becomes more prominent with
maturation. The constellation of responses increased respiratory tract mucus
secretion (Phipps and Richardson, 1976), Bradycardia (Tomori
and Widdicombe, 1969), Bronchoconstriction (Boushey et al,
1972, Nadel and Widdicombe, 1962; Tomori and Widdicombe, 1962) and
increase breath pressure (Nadel and Widdicombe, 1912, Tomori and
Widdicombe, 1969; Cobert, Kerr and Prys-roberts, 1969). Intralaryngeal
CO2 has been shown to inhibit breathing (Bartlett, et al, 1990; Boushey
and Richardson, 1973). Recent analysis of this response suggests that
inhibition of tonically active mechanoreceptors is probably responsible (Bartlett
and Knuth, unpublished).
Reix P, St-Hilaire M, Praud JP (2007)
reported that from a clinical standpoint, laryngeal sensitivity is essential
for both preventing foreign substances from entering into the lower airway and
for finely tuning upper airway resistance and further said that from a
physiological standpoint, many mechanisms pertaining to reflexes originating
from laryngeal receptors are yet to be fully understood.
CONCLUSION:
In
summary, it is clear that we must alter our view of the relationship between
laryngeal afferents and larynx from a strict servo control mechanism to a
system in which the afferents play a more advisory/interventional role. Under
most normal circumstances, their information may or may not be utilized to
influence airway or phonatory function. Under critical situation, their
information becomes paramount and wisely takes over control in order to ensure
the body’s own ABC priority role.
References:
1) Vocal
Fold Physiology- Frontiers in Basic Science - Titze, I.R
2) Speech
and hearing science, Anatomy and Physiology – Willard R. Zemlin
3) Clinical
Examination of voice- M. Hirano
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