Detailed morphology of the stapedius muscle of the rat. An integrated light microscopical, morphometrical, histochemical, immunohistochemical and electron microscopical study in relation to function.

The stapedius muscle of the rat, a very small muscle, appeared to have several atypical morphological characteristics. The muscle fibres were found to be irregularly shaped and composed of relatively small bundles of myofibrils. Mitochondria were distributed irregularly, i.e. in long strands, between the bundles of myofibrils. Enzyme histochemically, most fibres could be classified as fast oxidative glycolytic fibres (IIA). Only 5% of the fibres appeared to be slow twitch fibres; however, these fibres also had a high glycolytic capacity and therefore could not be classified as the classical slow twitch fibres. The immunohistochemical staining with monoclonal anti-heavy chain myosin isotypes showed discrepancies when compared to the enzyme histochemical results. Moreover, different adult heavy chain myosin isotypes appeared to coexist in one single muscle fibre. The results suggest that the muscle has an isometric contraction pattern and that it can contract rapidly with great endurance. This endows the muscle with properties suitable for a function in the prevention of noise-induced hearing loss and also in impact noise over longer periods of time.