Laryngeal muscles are innervated by the vagus nerve [cranial nerve (CN) X] and move the laryngeal skeleton. In turn, this movement changes the width and the tension on the vocal folds so that air passing between the vocal folds causes them to vibrate, producing sound.
The intrinsic laryngeal muscles move the laryngeal framework, altering the size and shape of the rima glottidis and the length and tension of the vocal folds (Figure 28-2). The actions of the laryngeal muscles are best understood when considered in the following functional groups: adductors and abductors and tensors and relaxers.
Muscles and actions of laryngeal muscles.
Rotation of the cricoarytenoid cartilages results in medial or lateral displacement of the vocal folds, thereby decreasing or increasing, respectively, the aperture of the rima glottis.
- Adduction (closing) of the vocal folds. The lateral cricoarytenoid muscles pull the muscular processes anteriorly, rotating the arytenoids so their vocal processes swing medially. When this action is combined with that of the transverse arytenoid muscles, which pull the arytenoid cartilages together, the gap between the vocal folds is decreased. Air pushed through the rima glottidis causes vibration of the vocal ligaments.
- Abduction (opening) of the vocal folds. The posterior cricoarytenoid muscles pull the muscular processes posteriorly, rotating the vocal processes laterally and thus widening the rima glottidis.
Gliding Movement of the Arytenoid Cartilages
Horizontal gliding action of the arytenoid cartilages permits the bases of these cartilages to move side to side. Medial gliding and medial rotation of the arytenoid cartilages occur simultaneously, as do the two lateral movements.
- Adduction of the vocal folds also is aided by horizontal medial gliding of the arytenoid cartilages. This action is caused by bilateral contraction of the lateral cricoarytenoid, the transverse and oblique arytenoids, and the aryepiglottic muscles.
- Abduction of the vocal folds also is aided by horizontal lateral gliding of the arytenoid cartilages. This action is caused by bilateral contraction of the posterior cricoarytenoid muscles.
Length and Tension of the Vocal Folds
The cricothyroid joint is a synovial articulation between the side of the cricoid cartilage and the inferior horn of the thyroid cartilage. This joint enables the thyroid cartilage to tilt back and forth upon the cricoid cartilage, altering the length and tension of the vocal folds.
- Tensing of the vocal folds. The principal tensors are the cricothyroid muscles, which tilt the thyroid cartilage anteriorly and inferiorly, increasing the distance between the thyroid cartilage and the arytenoid cartilage. This movement elongates and tightens the vocal ligaments, raising the pitch of the voice.
- Relaxing the vocal folds. The principal relaxers are the thyroarytenoid muscles, which pull the arytenoid cartilages anteriorly toward the thyroid angle (prominence), thereby relaxing the vocal ligaments.
- Vocalis muscles. Produce minute adjustments of the vocal ligaments, selectively tensing and relaxing parts of the vocal folds during animated speech and singing.
To understand how we speak and sing, we first need to understand how sound is produced, and then we need to understand how that sound is articulated.
Phonation, or the production of sound, involves the intermittent release of expired air coordinated with opening and closing the rima glottis. The length of the true vocal folds and the size of the rima glottis are altered by the action of the intrinsic laryngeal muscles, most of which move the arytenoid cartilages. As the length and tension of the vocal folds change, the pitch of the sound is altered. Generally, the more tense the vocal folds, the faster they vibrate and thus the higher the pitch. The rima glottis is wide when deep tones are produced and narrows to a slit when high-pitched sounds are produced. As a young boy's larynx enlarges during puberty, the vocal folds become both longer and thicker, causing them to vibrate more slowly and thus the voice becomes deeper.
Loudness of the voice depends on the force with which air rushes across the vocal folds. The greater the force of air across the vocal folds, the stronger the vibration, which results in louder sounds. The vocal folds do not move at all when we whisper, but they vibrate vigorously when we yell.
Although the vocal folds produce sounds, the quality of the voice depends on the coordinated activity of many other structures. For example, the pharynx acts as a resonating chamber to amplify and enhance the quality of the sound, as do the oral and nasal cavities and the paranasal sinuses.
Articulation, or the production of intelligible sounds, involves the actions of the pharyngeal muscles (vagus nerve, CN X), the tongue (hypoglossal nerve, CN XII), the muscles of facial expression (facial nerve, CN VII), mandibular movements (mandibular branch of the trigeminal nerve, CN V-3), and the soft palate (CN X). Each of these structures modifies the crude sounds that are produced by the larynx and convert them into recognizable consonants and vowels.
Inflammation of the vocal folds, or laryngitis
, results in hoarseness or inability to speak above a whisper. Overuse of the voice, very dry air, bacterial infections, or inhalation of irritating chemicals can cause laryngitis. Whatever the cause, irritation of the laryngeal tissues causes swelling and prevents the vocal folds from moving freely.