Depolarization block and phase II block at the neuromuscular junction

The effects of repeated doses of decamethonium or succinylcholine in muscles of the cat, dog, and rabbit have been examined. In particular, the relation of degree of neuromuscular block to intensity of the electrical change at the end-plate region has been found to be more consistent when the peak spatial gradient of depolarization is used as a measure of electrical effect than when the peak depolarization is used; the reason for this difference is discussed. A plot of twitch height against electrical change provides a convenient frame of reference for following the development of phase II block quantiatively. Examples presented show that the extent and kinetics of phase II block can vary considerably among species or among muscles in a given species.