Myosin isoform composition of the human medial and lateral pterygoid muscles.

The medial and lateral pterygoid muscles are different in structure as well as in function. The medial pterygoid muscle is concentrically active during jaw closing, and the superior head of the lateral pterygoid muscle is eccentrically active during jaw closing, while its inferior head is concentrically active during jaw opening. Architecturally, the medial pterygoid can deliver higher forces than the lateral pterygoid. We investigated whether these differences are reflected in the myosin heavy-chain (MyHC) composition and the fiber cross-sectional area (f-csa) of these muscles. The pterygoid muscles from eight cadavers were investigated by means of monoclonal antibodies against different isoforms of MyHC. The proportions of pure MyHC type I fibers did not differ significantly among the muscles (32% in medial pterygoid, 34% in superior head, and 36% in the inferior head of the lateral pterygoid), nor did the total proportions of pure MyHC type IIA and IIX fibers (16% in medial pterygoid, 26% in the superior head, and 19% in the inferior head of the lateral pterygoid). The mean f-csa of type I fibers was 1315 microm2, which did not differ significantly among the muscles, and was significantly larger than the f-csa of type IIA fibers. The relative proportions of hybrid fibers, which expressed more than one MyHC isoform, were 52% in the medial pterygoid, 40% in the superior head, and 45% in the inferior head of the lateral pterygoid and did not differ significantly among the muscles. The most abundant hybrid fiber types found were fibers expressing MyHCs-cardiac alpha+IIA and MyHCs-cardiac alpha+I+IIA. Significant regional differences were found in the proportions of MyHC type I fibers in the medial pterygoid and in the inferior head of the lateral pterygoid. Although the form and function of the muscles are different, we conclude that this is not reflected in their myosin isoform composition.