Desensitization to Acetylcholine of Frog Tonic Muscle Fibres

IT is known that depolarization of the postsynaptic membrane of muscle fibres caused by acetylcholine gradually decreases as a result of its prolonged action: after some time the membrane protential may attain its initial level despite the presence of acetylcholine. This decrease in the sensitivity of the postsynaptic membrane—desensitization—has been investigated either in phasic (fast) muscle fibres of the frog or in mammalian muscle fibres 1–3 . Neither type of muscle fibre develops permanent contraction in response to the action of acetylcholine. We report here findings relating to desensitization in tonic (slow) muscle fibres of the frog which are known to be responsible for acetylcholine induced contraction of the whole muscle.