New perspectives about human laryngeal muscle: single-fiber analyses and interspecies comparisons

BACKGROUND: In companion studies on canine and rodent laryngeal muscle, we observed that (1) muscle fibers in both the canine and rodent posterior cricoarytenoid (PCA) muscles have a slower myosin heavy-chain (MyHC) isoform profile than those in the thyroarytenoid (TA) muscle; (2) the muscle fiber composition of PCA and TA muscles in canines and rodents is complex given the presence of so-called hybrid fibers (fibers coexpressing various combinations of MyHC isoforms); (3) the types and proportions of hybrid fibers are both muscle specific and, in some cases, region specific; and (4) the MyHC isoform profile of canine laryngeal muscle appears to be slower than that of rodent laryngeal muscle, suggesting the possibility that larger mammals have a slower MyHC isoform profile. OBJECTIVES: Given the findings of these companion studies and the fact that very little is known about the MyHC isoform composition of laryngeal muscle fibers, the primary objectives of this study were to determine (1) the types of MyHC isoforms found in the human PCA and TA muscles, (2) if there were regional differences in MyHC isoform composition, (3) if hybrid fibers commonly occur in human laryngeal muscle, and (4) if the MyHC isoform profile of human laryngeal muscle is slower than that of canine and rodent laryngeal muscle. RESULTS AND CONCLUSIONS: The findings of this study clearly demonstrate that both the PCA and TA muscles in humans express 3 types of MyHC isoforms (ie, slow type I, fast type IIA, and fast type IIX MyHC isoforms). At the single-fiber level, there were distinct regional differences and hybrid fibers were a common occurrence. Finally, the data demonstrate that the PCA and TA muscles of humans have a slower MyHC profile than that found in either canine or rodent laryngeal muscle.