The Coordination of Intrinsic Laryngeal Muscle Activation During Phonatory and Non-Phonatory Tasks

Lesions at different levels of the central nervous system can selectively interfere with laryngeal movements during respiration, vocalization, speech, effort closure and swallow. This suggests that these actions are controlled by complex programs at different levels of the nervous system. During respiration, two laryngeal muscles, the thyroarytenoid (TA) and the cricothyroid (CT) are activated in a reciprocal pattern with the posterior cricoarytenoid. Increased CT activity during inspiration and expiration is phasically related to respiration and most likely driven by the medullary respiratory center (Sasaki and Buckwalter, 1984). During inspiration, the CTs contract to lengthen the vocal folds as they are opened, thus increasing the size of the glottis. The CT is also active during expiration in relation to the degree of positive subglottic pressure (Sasaki and Buckwalter, 1984). Others have observed that the TA activates with the onset of expiration, increments with expiratory action and drops in activity with inspiration onset (Wyke and Kirchner, 1976). Human studies however, have reported that the opposite is true in unanaesthetized subjects with the greatest amount of TA activity immediately preceding inspiration onset (Buchthal and Faaborg-Andersen, 1964). Since the major function of the TA is vocal fold adduction and shortening, it is difficult to reconcile these observations with the opening and lengthening of the vocal folds observed during inspiration (Brancatisano, Collett and Engel, 1983).