Histochemical properties of intrinsic laryngeal muscles in cats

As one means in elucidating the contractile properties of the intrinsic laryngeal muscles, we examined the histochemical properties of the posterior cricoarytenoid (PCA), thyroarytenoid (TA), lateral cricoarytenoid (LCA), arytenoid (A) and cricothyroid (CT) muscles in cats using the reaction for myosin ATPase following acid and alkali preincubation. The muscle fiber compositions of the intrinsic laryngeal muscles differed not only across the muscles but also across the muscle fascicles within a single muscle. The relative frequency of type-1 fibers was the smallest (9%) in the TA and the largest (45%) in the CT, and that of type-2A fibers was 39-48% in each muscle. The relative frequency of type-2B fibers was the smallest (9%) in the CT and the largest (42-45%) in the TA, LCA and A, and that of type-2C fibers was less than 1.0% in each muscle. Across the muscle fascicles, the TA was the most heterogeneous in muscle fiber distribution. The mediocaudal part of the TA was mainly composed of the muscle fascicles with type-1 fibers, while the laterocaudal and rostral parts of the TA were mostly composed of the muscle fascicles without type-1 fibers. The PCA was the most homogeneous in muscle fiber distribution. Our results demonstrate that the contractile properties of the intrinsic laryngeal muscles differ across the muscles and across the muscle fascicles within a single muscle and suggest that the delicate laryngeal movements are established by the coordinated activities of the intrinsic laryngeal muscle fibers with different contractile properties.