SPECIFIC EXCITABILITY OF THE ENDPLATE REGION IN NORMAL AND DENERVATED MUSCLE

Abstract

Properties of the endplate and endplate-free regions were investigated in single nerve-muscle fibre preparations of the M. adductor longus and in isolated sartorius muscles of Australian frogs (Hyla aurea). Acetylcholine, nicotine and caffeine set up impulses by depolarizing the muscle membrane at the endplate region. None of the drugs appreciably depolarizes endplate-free parts of muscle, or sets up impulses there. K initiates impulses at the endplate region only, but no striking difference could be detected between the depolarizing action at or off the endplate. Curarine opposes the depolarization and excitation caused by the drugs, except K. The sensitivity at the endplates to acetylcholine, nicotine and caffeine in chronically denervated muscles (3-11 wks.) is increased from 1,000 to 100,000 times and is diminished by curarine. The selective depolarization of the endplate region and the lack of depolarization at the endplate-free regions pertain also to denervated muscle. No appreciable potential differences are found in normal uninjured muscle along the outside of isolated fibres. No evidence was obtained of potential gradients around the neuromuscular junction.