Properties of postsynaptic channels induced by acetylcholine in different frog muscle fibres.

Skeletal muscles in the frog are composed of two distinct classes of muscle fibre: fast muscle fibres capable of propagating action potentials and twitches, and slow muscle fibres normally unable to generate action potentials or twitches. In addition, amphibian muscles contain a spectrum of 'intermediate' fibres whose structural and functional properties lie between those of fast and slow fibres. Much is now known about the characteristics of the channels opened by the transmitter acetylcholine (ACh) acting on the membrane of fast fibres, but the molecular action of ACh on the other fibre types is only poorly understood. We report here the existence of a muscle in the mandibular arch of the frog in which most, if not all, the fibres are multiply innervated and are capable of eliciting action potentials. We also report that the channels induced by the transmitter on the synaptic membrane of fast, slow and submaxillaris muscle fibres differ in their lifetimes and conductances.