Induction of the action potential in innervated slow muscle fibers of the frog. Effects of tetrodoxin, vincristine and colchicine

Experiments were made to induce action potentials in innervated slow muscle fibers of Rana temporaria. Drugs were applied to the sciatic nerve or its spinal roots by one of the following methods: implantation of a silastic cuff containing tetrodotoxin (TTX); subepineural injection of TTX combined with subcutaneous injections of vincristine; and epineural application of colchicine or vincristine. Twelve to 50 days later slow fibers responded to direct or indirect stimulation with an action potential. A substantial number of these fibers were innervated by 2 or 3 slow motor axons. It is concluded that these effects of TTX and colchicine (vincristine) are due to inhibition of the axoplasmic transport of an unidentified substance. The results support the hypothesis that the inexcitability of normal slow fibers is due to a 'trophic' effect of their motor axons.