Physiology of Larynx
Functions of larynx:
1. Airway protection
Airway protection: is the most important function of human larynx. Larynx in fact acts as a sphincter protecting the lower airway from secretions of the oropharynx. It also protects the airway from spillage of food during deglutition. Larynx acts as a three tier protective mechanism. These are from above downwards: Aryepiglottic fold, ventricular band and vocal cords. These three structures when contracted can effectively seal the lower airway from the contents of the oropharynx.
Contraction occurs from below upwards, first the vocal cord adducts, followed by ventricular bands. Finally the aryepiglottic fold adducts sealing the lower airway completely.
Respiration: Another important function of larynx is to keep the airway open during respiration. This is done by gentle abduction of the vocal cords while the ventricular bands and aryepiglottic folds are fully abducted.
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.
Phonation: 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 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.
The requirements of normal phonation are as follows:
1. Active respiratory support
2. Adequate glottic closure
3. Normal mucosal covering of the vocal cord
4. Adequate control of vocal fold length and tension.
The vibrations of the vocal folds are complex in nature and are known as the glottic cycle. This cycle involves glottic opening and closing at set frequencies determined by the subglottic air pressure. Normal vocal folds produce three typical vibratory patterns:
2. Modal voice
3. Glottal fry
In falsetto or (light voice) the glottic closure is not complete, and only the upper edge of the vocal fold vibrates.
In Modal voice complete glottic closure occurs. This occurs in a majority of mid frequency range voice. During this modal voice production the vocal fold mucosa vibrates independently from the underlying vocalis muscle. This is the basic frequency at which a person phonates. The modal frequency in adult males is 120 Hz while in adult females it is 200 Hz.
Glottal fry is also known as low frequency phonation is characterised by closed phase. This closed phase is long when compared to the open phase. The vocal cord mucosa and vocalis muscle vibrate in unison.
During phonation two vibratory phases occur i.e. open and closed phases. The open phase denotes the phase during which the glottis is atleast partially open, while the closed phase denotes the phase when the vocal folds completely occlude the glottic chink.
The open phase can be further divided into opening and closing phases. The opening phase is defined as the phase during which the vocal folds move away from one another, while during the closing phase the vocal folds move together in unison.
One important physiologic parameter which must be noted during phonation is the mucosal wave. The mucosal wave is an undulation which occur over the vocal fold mucosa. This wave travels in an infero superior direction. The speed of mucosal wave ranges from 0.5 - 1 m/sec. The symmetry of these mocosal waves must also be taken into consideration while studying the physiology of voice production. Any mild assymetry between the two vocal folds must be considered as pathological.
The function of vocal folds is to produce sound varying in intensity and pitch. This sound is then modified by various resonating chambers present above and below the larynx and are converted into words by the articulating action of the pharynx, tongue, palate, teeth and lips.
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.