4 Physiology of Phonation

The capacity to produce complex phonatory behavior appears to be a recent evolutionary phenomenon limited to human beings.1 Phonation, or the production of voice, involves a power source, oscillator, and resonance chamber, each with different anatomical parts and specialized roles. Together, these three subsystems produce sound perceived as voice.

Power Source

Air is the power source of voice. Coordinated functions of the diaphragm, abdominal and chest muscles, lungs, and chest cavity work in concert to inspire air, filling the pulmonary reserve. This is followed by the movement of air superiorly toward the vocal folds. In expiration, as air flows past adducted vocal folds, the folds are set into motion leading to their vibration.²

Oscillator

Although almost all body systems can affect the voice, the larynx is the most sensitive and expressive component of the vocal mechanism.³ The four main parts of the larynx involved in phonation are the skeleton, mucosa, intrinsic muscles, and extrinsic muscles.

Laryngeal Anatomy and Structure

Skeleton

The main cartilages of the larynx that have importance in phonation are the cricoid, thyroid, and arytenoid cartilages.

Musculature of the Vocal Fold

The intrinsic laryngeal muscles are paramount in the functioning of the vocal folds as they control the position and shape of the folds along with the elasticity and viscosity of each layer.⁴

The fibers of the thyroarytenoid muscle run parallel to the vocal ligament. The part of the muscle that borders the vocal ligament is called the vocalis muscle. Contraction of the thyroarytenoid muscle lowers, shortens, adducts, and thickens the vocal fold, bringing the arytenoid and thyroid cartilages closer. The body of the vocal fold is actively stiffened, and the cover and transition are passively slackened, resulting in rounding of the vocal fold edge.^3,4 When the thyroarytenoid muscle is activated, the length and tension of the vocal ligament are decreased, lowering the pitch of the voice. The vocalis muscle can provide fine control of the tension in the vocal ligaments enabling rapid variation in the pitch. Depending on tension in the rest of the thyroarytenoid, contraction of the vocalis may result in either raising or lowering of the vocal pitch.

The posterior cricoarytenoid muscle abducts, elevates, elongates, and thins the vocal fold by rocking the arytenoid cartilage posterolaterally.³ The vocal fold elongates and thins. The size of the rima glottidis, the opening between the vocal folds, is increased. The lateral cricoarytenoid muscle adducts, lowers, elongates, and thins the vocal fold, making the edge of the vocal fold sharp while passively stiffening all layers.⁴

The interarytenoid muscle serves to adduct the cartilaginous portion of the vocal folds altering the position of the vocal folds, but has little influence on the stiffness of the membranous section.

The extrinsic muscles of the larynx, mainly the strap muscles, serve an important function of preserving the position of the larynx in the neck. The extrinsic muscles are responsible for maintaining a stable laryngeal skeleton to ensure the intrinsic musculature can work effectively.³

Innervation of the Larynx

The vagus nerve (cranial nerve [CN] X) provides sensory and motor innervation to the larynx. The two main nerves supplying sensory and motor innervation to the larynx are the superior laryngeal nerve (SLN) and the recurrent laryngeal nerve (RLN). The SLN is divided into the internal and external branches. The internal branch carries sensory fibers (pain, touch, and temperature) from the mucosa superior to the glottis and provides secretomotor fibers to the same area. The external branch provides motor innervation to the cricothyroid muscle.

The RLN contains motor fibers that innervate all of the intrinsic muscles of the larynx except for the cricothyroid muscle as well as sensory innervations to the infraglottis, subglottis, and trachea.

Structure of the Vocal Fold Edge

The vocal folds are two infoldings of mucous membrane stretched horizontally across the larynx. The most important aspect of voice production is vibration of the vocal folds that converts aerodynamic energy into acoustic energy. The nature of the sound produced is primarily dictated by the condition of the vocal folds, or the vibrators.⁴

The unique properties of sound from the larynx are determined by the inherent properties and manipulation of the vocal folds. The cross-section of the vocal fold reveals a five-layered structure, with each layer having a different mechanical property (Fig. 4.1). The outer four layers are controlled passively and the innermost layer is controlled both actively and passively.^3,4 Functionally, the vocal fold acts as three separate layers consisting of the cover (epithelium and Reinke space), the transition (intermediate and deep layers of the lamina propria), and the body (vocalis muscle). The vibratory margin of the vocal fold consists of a stratified squamous epithelium with the purpose of maintaining the shape of the vocal fold and withstanding the trauma of vocal fold contact.³ The superficial layer of the lamina propria, or Reinke space, consists of loose fibrous components and a matrix of elastic fibers. The rheological properties of this layer allow for dynamic spectrum of sound production. The intermediate and deep layers of the lamina propria constitute the vocal ligament, with the intermediate layer composed mainly of elastic fibers and the deep layer made up primarily of collagenous fibers.⁴ The thyroarytenoid or vocalis muscle constitutes the body of the vocal fold.³

Physiology of Vocal Fold Vibration

Three important steps must happen before the production of voice. The first two steps are development of tension in the vocal folds and adduction to the midline (also known as the phonatory attack phase). This is accomplished by the lateral cricoarytenoid and interarytenoid muscles. The final step is the production of airflow from the lungs.

While there are several models describing vocal vibration in various amounts of detail, the myoelastic-aerodynamic theory of phonation provides an appropriate description of the basic forces involved in voice production.¹⁰ As air emerges from the lungs, the pressure in the subglottis increases. Once the subglottic pressure exceeds the myoelastic tension between the vocal folds, the lower lips of the vocal folds separate followed by the upper lips. The myoelastic tension is determined by both the volitional contraction of the laryngeal musculature and the intrinsic elastic properties of the vocal folds. Once the vocal folds are completely separated, air escapes superiorly and the subglottic pressure subsequently drops. When the myoelastic tension exceeds the subglottic pressure, the vocal folds will again approximate. This is assisted by aerodynamic forces known as Bernoulli principle. As air passes through a narrowed opening, such as the glottis, an increase in speed of airflow results in a decrease in pressure, which draws the vocal folds together. Once the vocal folds close, the subglottic pressure begins to increase pushing the vocal folds apart again. This completes one glottic cycle.^2,7 This entire sequence is illustrated in Fig. 4.2. The number of cycles that occur per second determines the frequency in hertz (Hz) or pitch of the sound produced.

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