The characteristics of muscle fibres in the submaxillaris muscle of the frog were studied using electrophysiological and anatomical techniques. The muscle fibres were capable of eliciting action potentials and their passive membrane properties were similar to those of fast muscle fibres. Composite end-plate potentials, due to polyneuronal innervation, were observed in most muscle fibres. Acetylcholinesterase staining of the whole muscle revealed multiple end-plate areas, and several end-plates of different size and shape were observed in single teased fibres. It is concluded that the submaxillaris muscle of the frog is composed of a distinctive population of muscle fibres (intermediate muscle fibres) with electrical properties like those of fast muscle fibres and multiple innervation like that of slow muscle fibres.
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