An investigation has been made of the action potentials of the muscles of the crayfish, Cambarus clarkii. It was found that the polarity of these potentials was in many instances the reverse of that expected.

It is shown that ‘positive’ potentials cannot be explained by injury to the active tissue, by the complex structure of the muscles, or by the direction of the muscle fibres.

Using muscles in which all extraneous connexions could be rigorously avoided, the muscle action potentials were invariably found to be negative.

Direct evidence is presented for the local non-conducted nature of the crustacean muscle action potentials. An hypothesis is offered to explain the monophasicity and polarity of these action potentials on the basis of multiple unilateral innervation of the muscle fibres.

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