Neurologic control of the larynx

Abstract

The larynx is a multifunctional organ. It is a valve that plays key roles in breathing, swallowing, and speech, as well as defecation and stabilization of the thorax during heavy lifting. The larynx is also a sensory organ, densely supplied with receptors that are activated by a variety of mechanical and chemical stimuli. Neural control of the larynx is, therefore, complex and varies according to function. Protection of the airway is the most primitive function of the larynx. It closes tightly during a swallow and in response to noxious stimuli. Laryngospasm may result from saturation of the pathways that stimulate laryngeal closure. The larynx opens and closes during breathing to control airflow. This activity is controlled by central pattern generators in response to respiratory demand, but can be overridden by voluntary activity. The laryngeal valve is also very active during a cough: opening widely during the inspiratory phase, closing tightly during the compressive phase, and then suddenly opening in the expulsive phase. As with respiratory motion, cough may be reflexive, or voluntary. Voice production requires appropriate approximation of the vocal folds to achieve oscillation during exhalation. Pitch is controlled by intricate control of the length, tension, and thickness of the vocal folds, in precise coordination with respiratory muscles. Speech also requires precise coordination between the actions of laryngeal muscles, respiratory muscles, and the muscles of articulation: lips, tongue, palate, jaws, and pharyngeal constrictors. Laryngeal motor neurons are located in the nucleus ambiguous. In nonhuman mammals, phonation is controlled at subcortical levels. This system is also present in humans. But humans, and humans alone, have direct cortical control of laryngeal muscles. The complex actions of the small muscles in the larynx are quite vulnerable to distortion by peripheral nerve injury.