Propagation Of The Endplate Potential

One obvious question is whether the EPP is propagated in an all-or-nothing fashion like the action potential. Figure 6 shows a simple experiment designed to answer this question. In this experiment, the motor axon is electrically stimulated, and multiple intracellular recordings are made from the muscle cell at 1-mm intervals from the end plate. There is a large EPP (Vo) at the region of the motor end plate; however, at more distant regions from the end plate, the amplitude of the EPP becomes smaller. In fact, the decay is an exponential function of distance. This indicates that the EPP is not propagated in an all-or-nothing fashion; rather, it spreads with decrement, just as weak, artificially produced hyperpolarizations or depolarizations (subthreshold) spread with decrement along a nerve axon. As shown in Fig. 6, an agent such as curare is used so that the EPP is reduced in size and fails to trigger an action potential. When the EPP triggers an action potential (as occurs in the absence of curare), the action potential propagates without decrement along the muscle cell.

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