Slow tonic muscle fibers in the thyroarytenoid muscles of human vocal folds; A possible specialization for speech

Most of the sounds of human speech are produced by vibration of the vocal f olds, yet the biomechanics and control of these vibrations are poorly under stood. In this study the muscle within the vocal fold, the thyroarytenoid m uscle (TA), was examined for the presence and distribution of slow tonic mu scle fibers (STF), a rare muscle fiber type with unique contraction propert ies. Nine human TAs were frozen and serially sectioned in the frontal plane . The presence and distribution pattern of STF in each TA were examined by immunofluorescence microscopy using the monoclonal antibodies (mAb) ALD-19 and ALD-58 which react with the slow tonic myosin heavy chain (MyHC) isofor m. In addition, TA muscle samples from adjacent frozen sections were also e xamined for slow tonic MyHC isoform by electrophoretic immunoblotting. STF were detected in all nine TAs and the presence of slow tonic MyHC isoform w as confirmed in the immunoblots. The STF were distributed predominantly in the medial aspect of the TA, a distinct muscle compartment called the vocal is which is the vibrating part of the vocal fold. STF do not contract with a twitch like most muscle fibers, instead, their contractions are prolonged , stable, precisely controlled, and fatigue resistant. The human voice is c haracterized by a stable sound with a wide frequency spectrum that can be p recisely modulated and the STF may contribute to this ability. At present, the evidence suggests that STF are not presented in the vocal folds of othe r mammals (including other primates), therefore STF may be a unique human s pecialization for speech. Anat; nec 256:146-157, 1999. (C) 1999 Wiley-Liss, Inc.