Single motor unit activity of human intrinsic laryngeal muscles during respiration

Individual motor units in the thyroarytenoid (TA) and cricothyroid (CT) muscles were studied in 10 normal human volunteers during quiet respiration. Both tonic and phasic firing patterns were found in both TA and CT units. The rate of firing was higher during inhalation than during exhalation in phasic TA units and in tonic CT units. Tonically active units had a higher firing frequency than phasically active units in both TA and CT muscles. Phasically active units corresponded with the respiratory cycle, with firing associated with inhalation in both the TA and CT muscles. A variety of firing patterns were found between units in both the TA and CT muscles, and in one subject, units recorded from the same muscle had very different firing patterns. The results suggest that although laryngeal motoneurons are modulated by the respiratory cycle, they do not respond uniformly to respiration.