Function of the larynx

The larynx is situated at the upper end of the trachea, connecting the lungs to the vocal tract. It has two main functions:

Laryngeal cartilages

The larynx is constructed from nine cartilages. For the purpose of this discussion, however, we will restrict ourselves to a consideration of just three of these:

• thyroid cartilage

• cricoid cartilage

• arytenoid cartilages

The thyroid cartilage is the largest of the laryngeal cartilages and forms the major chamber of the larynx. It is constructed from two relatively flat quadrilateral (four-sided) plates that are fused anteriorly in the midline along most of their length into a characteristic shield shape. The angle at which the plates join is different in males and females. In males it is about 90^o and in females it is nearer 120^o. This so-called thyroid angle is, therefore, more acute in men and it is consequently more noticeable as the Adam’s Apple. Posteriorly each plate is extended both upwards and downwards into horns or cornu. The superior horns attach by ligaments to the hyoid bone, situated under the base of the tongue in the neck, and the inferior horns articulate with the cricoid cartilage beneath.

The cricoid cartilage, therefore, forms the base of the larynx. It is a complete ring of cartilage that extends upwards posteriorly to form a plate. The inferior horns of the thyroid cartilage sit on the cricoid cartilage at this posterior position. This articulation allows the cricoid cartilage to be tilted by an angle of up to 15^o downwards and away from the lower anterior border of the thyroid cartilage. The cricoid cartilage is connected underneath to the upper trachea by the crico-tracheal ligament. We see, therefore, that both the thyroid cartilage and cricoid cartilages are individual cartilages (see Figure 1).

Figure 1. Human Larynx (click for an enlarged image)

The third major cartilage construction in the larynx is actually a pair of cartilages known as the arytenoid cartilages. These are pyramid-shaped and they are situated on the upper posterior part of the cricoid cartilage. One is placed laterally to the left and one laterally to the right. The arytenoids can be moved in rotational and sliding movements that are used to control the movement of the attached vocal cords.

Vocal cords

Two muscles known as thyroarytenoid muscles attach the thyroid cartilage to the arytenoid cartilages. The lower portion of these muscles forms the true vocal cords.

True vocal cords

The vocal cords are approximately 17-22 mm long along their upper edge in adult males and about 11-16 mm long in adult females. In cross-section they are almost triangular in shape. Hence, a better term for them – and the one most likely to be used by speech therapists – is vocal folds, as ‘cords’ incorrectly suggests that they are each a long, slender, thin strand. [Sometimes ‘vocal cords’ is written as ‘vocal chords’ – the term vocal folds also, therefore, neatly avoids this confusion.]

The vocal folds are made of elastic tissue and are connected anteriorly to the inferior edge of the thyroid angle. Posteriorly the folds are connected to the anterior aspect of the arytenoid cartilages. Therefore, the anterior borders of the vocal folds are static, being fixed to the inner surface of the thyroid angle.

As the arytenoids are variously rotated, and slid backwards and forwards, the posterior borders of the vocal folds are either moved apart or moved close together. Air from the lungs passes upwards through the trachea and through the larynx.

abduction

If the vocal folds are held apart as the air passes through then the folds do not vibrate. In this position the vocal folds are said to be abducted and the space between the vocal folds is known as the glottis. When the folds are fully abducted the glottis may be up to 12 mm in width at its widest point.

adduction

In contrast, the vocal folds may be held together as the air passes through, in which case they will vibrate. The vocal folds are said to be adducted when they are held together (see Figure 2).

It is the vibration of the adducted vocal folds that creates the note on which many English speech sounds are produced. This is the human voice.

Phonation is the term given to the vocalization of the airstream by the vibration of the vocal folds. The resulting sound is said to be phonated (see Voice).

pitch

As well as lateral movements that open or close the glottis, the action of the arytenoids and the tilting of the cricoid cartilage can also increase the tension along the length of the vocal folds. The ability to vary the tension of the folds is an important aid to varying the pitch of the voice (i.e. how high or low the note produced by the vibration of the vocal folds appears to be).

False vocal cords

Whereas the lower portion of the thyroarytenoid muscles form the true vocal cords, the superior portion forms the false vocal folds (false vocal cords).

Sometimes known as ventricular folds or vestibular folds, the false vocal folds do not usually play a part in phonation. However, they do close together to form a valve during swallowing. It is also thought that they function to lubricate the true vocal folds. Further, as they contain specialized immune response cells, they appear to play a part in protecting the vocal tract from invasion by infectious organisms (e.g. bacteria, fungi).

false vocal fold voice

If the false vocal folds are pressed together during normal speech this has the effect of obscuring the true vocal folds. The resulting voice can sound harsh and breathy, with a lowered pitch. If this pattern of voice production is used consistently this can lead to a so-called hyperfunctional voice disorder. Specifically, in this case, it is known as plica ventricularis (read more about voice disorders here).