Two separate processes are involved in vocalisation: 1. Phonation 2. Atriculation.
Process # 1. Phonation:
The whole process is dominated and co-ordinated by speech centre in the cerebral cortex, damage to which will lead to permanent aphasia.
Phonation is due to vibration of vocal cords which are folds stretched along the lateral wall of the larynx between thyroid and arytenoid cartilages. These are positioned by and their tension is regulated by the intrinsic muscles of the larynx.
The relevant intrinsic muscles (Fig. 8.57) and their mode of action are:
i. Thyroarytenoid muscle is made of many small slips of muscle fibres controlled separately by different nerve fibres. The contraction of these muscle fibres independently of each other are responsible for control of shape of vocal cord, thick or thin or with sharp and blunt edge during phonation of different types (Fig. 8.57).
ii. Posterior crico-arytenoid muscle pulls the aretenoid cartilages away from thyroid cartilage and thereby stretches the vocal cords increasing their tension.
iii. The transverse arytenoid muscle pulls the arytenoid cartilages together and thus draws the vocal cords towards each other.
iv. The lateral crico-arytenoid muscle pulls the arytenoid cartilages forwards and increases the gap between the vocal cords to allow easy respiration.
The vocal cords vibrate laterally because the pressure of air from below pushes the vocal cords apart. The rapid flow of air between their margins creates a ‘negative pressure’ which approximates the cords towards each other. The cycle is then repeated.
Changes in frequency of vibration of vocal cords are effected by:
i. Alteration in tension of the cords.
ii. Alternation in thickness of the vibrating edge.
Tension of the vocal cords is regulated by intrinsic laryngeal muscles described above e.g., posterior cricoarytenoid and transverse arytenoid muscles which increase and lateral crico-arytenoid muscles which decrease the tension of the vocal cords.
Thickness of the vibrating edge is regulated by the muscle fibres of the thyroarytenoid muscles. It has been mentioned that groups of muscle fibres in this muscle may contract independent of each other—one group producing thin edge and the other group producing thick edge of the vocal cords. An extrinsic muscle of the larynx also has got a role to play.
Elevation of the larynx increases the tension and depression of the larynx decreases the tension of the vocal cords.
Process # 2. Articulation and Resonance:
The three major organs for articulation are:
iii. Soft palate.
The buccal cavity, the nose and nasal sinuses, the pharynx and the chest cavity—they all act as resonators and determine the quality of the voice. Thus if the nose is blocked due to cold—the quality of voice is appreciably altered.