Is there an Explicit Correspondence Between Physiological and Morphological Features in Amphibian Twitch Skeletal Muscle Fibres?

The contractile characteristics, fatiguability, and ultrastructure were examined in individual twitch skeletal muscle fibres dissected from the iliofibularis muscle of frog Rana temporaria in order to define if an explicit correspondence exists between physiological and morphological features in different types of muscle fibres. Three groups of twitch fibres were distinguished in the study according to their fatiguability: fatigue resistant (FR), moderately fatigued (MF) and easily fatigued (EF). The ultrastructural features such as regularity of striation, the density of myofibrils, Z-band morphology, mitochondrial and lipid droplet contents, which are commonly used to distinguish skeletal muscle fibre types, are shown to correlate with physiological properties verifying FR, MF and EF fibre groups only in a statistical way. In individual fibres, however, there is no strong correlation between contraction speeds, fatiguability, and ultrastructural features. The conclusion is made that ultrastructural features of individual fibres may not be used to predict the physiological properties attributed to particular fibre type.