IRA SANDERS, MD; YINGSHI HAN, MD; JUN WANG, MD; HUGH F. BILLER, MD

NEW YORK, NEW YORK

The cricothyroid (CT) muscle increases the length of the glottis during phonation. Its action increases the area of the glottis during respiration. The CT muscle has 3 bellies (rectus, oblique, and horizontal), each of which has a different effect on the movement of the thyroid cartilage in relation to the cricoid. It has long been speculated that some bellies have distinct functions during speech or respiration. To test this hypothesis, we examined 5 CT muscles for the distribution of muscle fibers containing alpha cardiac (AC) myosin. The percentage of muscle fibers was determined in each belly. We found AC muscle fibers in each belly of every CT muscle examined, and the percentage of AC muscle fibers varied dramatically from one belly to another within every muscle. Some CT muscles had a belly that contained as much as 80% AC muscle fibers, while another belly within the same muscle had less than 20%. However, the variations in AC muscle fiber percentage were not consistent, as some CT muscles had most of these muscle fibers in their rectus bellies and some had the highest percentages in the oblique or horizontal bellies. The data suggest that each belly of the CT muscle may function in a different way.