Physiology of the Laryngeal System

Non-Speech laryngeal Functions

• Preventing the entry of foreign bodies– Coughing:

• Violent response by the tissues in the respiratory tract• Mediated by the sensory part of the X nerve• Involves abduction of the VF followed by adduction

and elevation of the larynx• The high Psub expels the irritant out • Could lead to vocal abuse

Non-Speech laryngeal Functions

• Throat Clearing– Not as violent as the cough– involves the same processes as that for the cough– Could lead to vocal abuse– Can also be used as a treatment technique for patients

with weak vocal fold muscles• Abdominal fixation

– capturing air within the thorax in order to facilitate pushing or pulling

– Similar initiatory processes as that used during coughing

Non-Speech laryngeal Functions

• Dilation– process of abducting the vocal folds– important during exertion as the oxygen

requirement is increased– the width during dilation is twice as that

maintained during tidal breathing (8mm)• Swallowing

– The larynx is elevated– Epiglottis sits over the opening– The VF are adducted– Aryepiglottic fold is tense

Laryngeal Functions for Speech

• Two major functions for voice production:– Medial compression: the force with which the vocal

folds are brought closer– Longitudinal tension: extent of the stretching force

• These two functions interact with dynamic air supply, resulting in an increase in sub-glottal pressure

• This increased pressure results in setting the vocal folds into vibration

• Thus, the vibrations modify the air stream passing through the glottis into glottal pulses/voice

Direct Visualization

• http://video.google.com/videoplay?docid=-4623616269806981685&hl=en

How are vocal fold vibrations produced?

• Neurochronaxic Theory (Husson, 1950)– Every new vibratory cycle is initiated by a nerve

impulse transmitted via the X nerve– Frequency of the VF vibration is dependent on the

rate at which the impulses are transmitted

• Problems– The course of the RT and the LT X nerve are

different in length (about 10cm)– VF don’t vibrate without the air stream

How is voice produced?

• Myoelastic-Aerodynamic Theory (Muller, 1843)– Based on aerodynamic and physical principles– The air stream sets the elastic VF in to vibration– Frequency of vibration is dependent on the length

in relation to the tension and mass– Properties of the vocal tract also contribute to the

mode and frequency of the VF– Regulated by the intrinsic muscles

Convergent to Divergent

Onset of Phonation/Vibration

• Pre-phonation Phase– period during which the VF are brought from an

abducted position to an adducted or partially adducted position

– The extent of force applied to maintain adduction is medial compression and this is brought by the adductor muscles (LCA & IA)

– The PCA prevents the arytenoids from sliding forward

• Attack Phase– begins with the vocal folds in the adducted

position and extends through the initial phase of the vibratory cycle

– Complete adduction is not required for initiating phonation, however sufficient Psub is required (3-5 cm H2O)

– Three types of attacks: breathy, simultaneous and glottal

Maintaining Phonation/Vibration

• The Bernoulli effect– After the attack phase, the air stream pushes

itself in an upward direction– The air stream starts to pass through a narrow

constriction i.e., the glottis– As a result the velocity of the air column

passing through the glottis will increase/higher when compared to the velocity of air in the trachea as the cross-sectional area is smaller at the glottis

– As a results, a negative pressure develops perpendicular to the flow of air between the medial edges of the vocal folds

Bernoulli Effect Summarized• Why is negative pressure generated, how is it related to

velocity and why is velocity increased in the first place? – The rate at which a given quantity of mass (in this case air) flows

through a tube is a product of the density of the mass, the velocity of the mass and the cross-sectional area of the tube and this relationship is constant

– Therefore, at the glottis the cross-sectional area is smaller when compared to the trachea; This results in an increase in the velocity to compensate for the decrease in area in order to keep the product constant

– Now, velocity of the air column and Pressure in the tube are directly proportional as well and their product is always a constant

– Therefore, if the velocity of the air column at the glottis will increase as it moves through the glottis, the pressure has to decrease at the level of the glottis in order to keep the product constant

Bernoulli Effect

– Thus, the negative pressure sucks the medial edges of the vocal folds towards each other, resulting in the completion of one cycle

– The cycle (opening and closing of the vocal fold vibration….not adduction and abduction) continues as long as there is adequate Psub in order to push the air stream in an upward direction

Glottal Cycle

• Arrows represent driving air pressure changes • Notice that the vocal folds are always in transition

from open to closed (it is a vibration)

opening phase closing phaseclosed phase

open phase

• Summary of the different phases of VF vibration– Closed phase– Opening phase (longer than closing)– Closing phase

• Changes in these phases are based on the differences in the mode and frequency of vibration

• Thus producing the voice characteristics– Frequency of the vocal fold vibration– pattern or mode of VF vibration – configuration of the vocal tract

Mode of VF Vibration

• The Anterior 2/3rd is primarily vibrating as it is membranous; the posterior 1/3rd of the folds are cartilaginous

• The horizontal plane has the maximum displacement, with the posterior sections opening first and its moves anterior; the direction is the opposite during closing

• There is some displacement in the vertical plane as well wherein the lower edges of the VF open first and is followed by the upper edges. The same occurs during closing

VF Vibration

• Mucosal Wave– As there is a simultaneous horizontal and vertical

movement, the cover is able to slide over the rest of the layers to generate a wave that moves along the superior surface of the VF and travels about 2/3rd of the way to the lateral edge.

– Will generally dissipate.– Abnormalities usually disrupt this movement and is

the first sign associated with a change of voice quality.

Movement of vocal folds

• Note how the vocal folds open from bottom to top & back to front and close from bottom to top & front to back

Spread of glottal opening

Vertical Phase difference

Frequency of Vibration

F= T/ML• Increase in Tension will increase frequency

• Increase in vibrating Length will reduce fundamental frequency

• Increase in Mass will reduce fundamental frequency

Changes in Vocal Parameters

• Please note that – Pitch is the psychophysical correlate of frequency– Loudness is the psychophysical correlate of

intensity• Optimal frequency: frequency of the VF

vibration that is most optimal for the individuals system (mass and stiffness)

• Habitual frequency: frequency of the VF vibration that is habitually used by the speaker in speech

Changes in Vocal Parameters

• Pitch changing mechanism• Intensity changing mechanism• Changes in vocal Quality• Limits on vocal vibrations

Pitch Changing mechanism

• The Frequency is dictated by:– Tension of the VF– Length of the VF and this is with reference to the

length of VF set into vibration. The length is maximum in the abducted position.

– Mass of the VF and this is not so much to do with the actual mass; rather it is the mass set into vibration.

Vocal Pitch

• F0: Average rate of vocal fold vibration.• Harmonics (integer multiples of the F0) will

contribute to the perception of vocal quality, and will provide the power for vocal tract filtering

• Males: 130 Hz; Females: 220 Hz; overlap observed between subjects and within each subject depending on the content of the utterance.

Pitch Increase

• Changes in the length of the VF results in a change of cross-sectional mass

• Note that the length is already at its maximum during abducted position.

• Tension can be varied by the CT and the TA with help from the PCA. The CT is for increase in tension in a gross manner while the TA is for fine adjustments.

• Thyrohyoid (extrinsic muscle) might be used during high pitches in order to raise the thyroid cartilage and thereby increase the tension of the VF

Pitch Manipulation

• As the length of the VF is increased, there is an increase in frequency of the glottal pulse as well; the VF resemble narrow elastic bands that are stiff and rigid.

• VF approximation (or closure) at the center of the glottis is reduced at high pitches.

• Increase in frequency is associated with increase in Psub (as a means of compensation) but an increase in Psub is not always associated with increase in frequency.

Pitch Decrease

• Our habitual pitch is at the lower end of the frequency range of hearing (70 to 525 Hz).

• Produced by reducing tension and increasing mass per unit length that can be set into vibration.

• Relaxation is due to the recoil forces that will work towards reducing the tension as soon as the muscle contraction inducing the tension ceases.

Pitch Decrease

• A further decrease in tension below the habitual pitch requires active forces whereby the thyroarytenoid (less stiff) lowers the pitch and the LCA helps in medial compression.

• Some extrinsic muscles may be active (e.g., sternothyroid).

• The false cords may also aid in reducing the pitch (inc. mass to the system)

Intensity Changes

• Involves muscular effort and its effect on aerodynamic parameters.

• SPL is the sound pressure level that is used to designate the intensity of the voice generated.

• SPL is proportional to the square of Psub – voice changes by 8 to 12 dB as the Psub is

doubled. • At 3-6 cm H2O pressure, the intensity is about 45-

65 dB SPL• Psub about 20 cm H2O or higher for shouting.

Vocal level modification

• The intensity controlling mechanism is not due solely to Psub changes

• The degree and duration of vocal fold closure dictates the Psub and is therefore responsible for intensity changes.

• The duration of closed phase is higher as intensity is increased; this increases Psub

• Further, increase in medial compression will offer more resistance

Summary

• In summary, intensity changes are associated with increased Psub

• There are several factors inherent to this process that produces the increase in Psub and thus increase in intensity– Degree and Time of vocal fold closure– Glottal resistance– Rate of air flow through the glottis

Quality Changing Mechanism

• Usually the influence of pitch has not been found to dramatically alter quality judgments.

• A combination of the vibratory characteristics, vocal tract shape and configuration dictate voice quality (e.g., length, ratio of oral to pharyngeal cavity size, diameters of the cavities).

Valving for Speech• Laryngeal mechanics for different speech

sounds:– Voiceless consonants (first sounds of: pad, tad, cad)

• Laryngeal valve open/not set in vibration• Airflows unobstructed• Restricted partially by tongue or lips

– Voiced Consonants• Laryngeal valve is set in vibration• Air flow is obstructed• There is obstruction in the superior regions as well

Limits of the VF vibration

• Falsetto: – vibration of the VF free borders and the rest are

firm and non-vibratory – appear stiff, long, very thin and bow shaped– indicates the heightened activity of the CT

Limits

• Laryngeal Whistle– may not be produced by VF vibrations. rather

may be produced by the air passage through the vocal tract

– very high Psub (about 30cm H2O)

• Glottal Fry (Pulse register)– Lowest pitch possible– Long, inefficient closed phase– very low due to incomplete, or poor closure Psub

Voice Disorders

• Location and Size of the pathology• Glottal Incompetence• (A)symmetry of VF movement• Uniformity within each fold• Structural differences with the layers• Mass and Stiffness of each layer.• Interference of vibration.

Age and Sex Differences in the Larynx: Maturation

Infant Larynx

• Differences in the shape, relative size, and position when compared to an adult– Lower border of the cricoid between the 2nd and

the 3rd cervical vertebrae (rostral to its eventual location)

– Epiglottis is in contact with the soft palate permitting the infant to breathe and nurse simultaneously

– Thyroid and hyoid in direct contact, thus no anterior space

Young Larynx

• Immediately after birth, the changes in the vertebral column and in the relationship between the base of the skull and column lowers the larynx position.

• At age 5, the lower border of the cricoid cartilage is at C6 and by 15-20 years, it is at C7.

• After this, there is a slow descent with age.

Four voicing signals in the neonate

• Birth Signal– short duration (1 sec.)- vowel pattern

• Pain Signal-– long in duration, high-pitched– strained harsh quality

• Hunger signal– rising & falling of pitch; glottal signal

• Pleasure Signal– more nasal, pitch variability, glottal quality

Young Larynx

• No discernable changes in the infants and the larynx of a small child (toddler) with respect to the sex of the child; thus similarity in voices.

• No significant differences in the pitch or the pitch range.

• General shape remains essentially unchanged until puberty.

• During puberty, the cartilaginous structures grow rapidly.

Changes in Vocal Folds

• At birth, the VF are about 2.5 to 3 mm in length.

• By one year, the length is about 5.5 mm.• Little sex differences in the vocal fold length

until the age of 10.• In males, the post-pubertal VF length reaches

about 17 to 20 mm while it is about 12.5 to 17 mm in females.

Changes in Vocal Folds with age

• The mucosa in the newborn is very thick.• Vocal ligament is not developed until the age of

4.• By age 16 the inner structures are the same as

that of adults.• The layer structures mature during adolescence.

Until then, the inter and deep layers are not differentiated till 6.

• Thus voice is associated with both change in the VF length and the inner structures of the VF mucosa.

Changes in the Vocal Folds with age

• Elastic fibers become fragmented with age, especially in males.

• Density of the vocal ligament decreases.• Loss of muscle tissue.• Increase in connective tissue stiffness within

the body of the VF• Folds more prone to inefficient closure,

producing a variety of voicing problems

Aging Larynx

• Vocal pitch lowers at a rate that is approximately parallel to the laryngeal growth.

• By middle age, the pitch level begins to rise.• This may be due to the deterioration of the

muscle tissue and an increase in connective tissue stiffness in the VF.

• Also, ossification of the thyroid and cricoid cartilage (in the early 20’s).

Aging Larynx

• By 65 years, the entire framework, except for the elastic cartilage is more bony.

• The valving effectiveness is poor in men over 75.

• No major changes in the airway resistance in women (its generally higher in females anyways).

Thyroid Angle

• In infants the laryngeal cross section approximates a semicircle

• The angle (relative to the axis of the vetebral column) is essentially the same for both males and females till puberty.

• By adulthood, the angle is about 84 degrees in males while it is about 92.5 degrees in females.

Summary of Gender and Age

• Prepubescent– minimal, similar pitch

• Puberty– male v.f.’s enlarge 8-10mm & females 4mm– thyroid angle is 90 ° in males and 120 °in

females.– v.f.’s enlarge in both female & male– epiglottis flattens– v.f. mucosa becomes less transparent– tonsils & adenoids atrophy

Fundamental Frequency changes