Flexion and Extension : Studies Employing Muscles of the Shank in Amphibians

Abstract

Tibialis anticus and peronaeus longus of the frog have been examined by optical and electron microscopy. Sodium and potassium concentrations of these muscles have been determined. Double myograms have been used to compare physiological and pharmacological properties of flexor and extensor and to demonstrate various patterns of convulsion and their modification by depressants. Contractures have been investigated in the isolated organ bath. Depolarizing agents have been used to compare the proportions of slow fibres in the gastrocnemius of frogs and toads. Tibialis anticus, a flexor, has been shown to be heterogenous, consisting of twitch and slow fibres, and peronaeus longus to be homogenous, containing only twitch fibres. The twitch fibres of the flexor are shown to be fast compared with the twitch fibres of the extensor and their resting potentials, as deduced from potassium concentrations, are found to differ, Evidence is collated to evince the principle that a flexor is heterogenous to effect the functions of rapid withdrawal and of maintaining that withdrawal, whereas an extensor is homogenous since it merely exerts a thrust. Analogies between amphibian and mammalian muscle fibres are discussed.