Effect of denervation and colchicine treatment of the nerve on electrical and chemical excitability of frog tonic muscle fibre

In order to investigate the role of axoplasmic transport in controlling the acetylcholine sensitivity and action potential generation in frog tonic muscle fibres, a comparison of changes produced in these fibres by denervation or by application of colchicine to the nerve was performed. Extrajunctional cholinergic sensitivity appeared both after colchicine treatment and after denervation. However, action potential generation appeared after denervation but not after colchicine treatment. Insensitivity of the factor controlling action potential generation to colchicine, which is known to interrupt fast axonal flow, and the low rate of its transport (determined by the time difference in action potential occurrence at various lengths of the nerve stump after denervation and coming to about 1.6 mm/day) suggest that the tropic factor responsible for the suppression of action potentials in tonic fibres is moved by slow axonal transport. It is concluded that the action potential mechanism in tonic fibres is controlled by the axonal flow of substances which differ from these controlling the acetylcholine receptors.