Electrical properties of normal, denervated and organ-cultured slow fibres of toad cruralis muscles.

Two types of fibres were characterized in the cruralis muscle of the toad using electrophysiological techniques: the slow and the fast fibres. Five to ten slow fibres were easily identified on the inner face of this muscle. The cruralis slow fibres developed the ability to produce action potentials 40 days after the sciatic nerve was transected at the hip level, while the slow fibres of the pyriformis muscle showed a latent period of 17 days after the same surgical treatment. However, when in addition to this procedure the nerve was transected at the point where it enters the muscle but without damaging the slow fibres, the latency was about 20 days. The slow fibres of the cruralis muscle maintained in organ culture developed the ability to produce action potentials in 24 days. During the winter the slow fibres of in vivo denervated cruralis and pyriformis muscles did not develop the ability to produce regenerative responses. More-over organ-cultured cruralis muscles taken from winter toads showed this same inability. These results further support the idea that the excitability of slow fibres is under the control of a neural factor rather than of activity. The seasonal dependence points to the fact that the metabolic state of the muscle is of crucial importance in determining the development of excitability of slow fibres.