Types of Muscle Fibres in Toad Skeletal Muscle

Abstract An attempt has been made to define the types of muscle fibres present in the iliofibularis muscle of Xenopus laevis. Histochemical studies were carried out on cross sections of the muscle. The variation between cells in the content of lipids, DPN‐diaphorase and succinic dehydrogenase was noted, and on this basis the fibres were classified into three main groups. Single, living fibres representing the histochemical groups were selected and their contractile behaviour in response to electrical stimulation and the application of ACh was studied. The findings indicated a relationship between the histochemical and functional properties of the muscle fibres. Fibres with a moderate content of lipid and enzymes (pale fibres) were fast fibres which fatigued rapidly on maintained stimulation. Fibres with a high content of lipid and enzymes (dark fibres) were fast fibres which did not fatigue rapidly. Fibres containing no lipid and no DPN‐diaphorase or succinic dehydrogenase (clear fibres) were slow fibres. Staining of the end‐plates on the fibres showed that fast fibres had end‐plates of varying gross morphology and with high cholinesterase activity, whereas slow fibres had ‘en grappe’ end‐plates with very low cholinesterase activity.