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What does the word “refractory” in “refractory period” refer to?


What does the word "refractory" in "refractory period" refer to?

I know what it means "refractory period" (both, absolute and relative) in the action potential graph, but I don't understand what the meaning of "refractory" in this context is. What is it refractory for?


From encyclopedia Britannica:

"After repolarization there is a period during which a second action potential cannot be initiated, no matter how large a stimulus current is applied to the neuron. This is called the absolute refractory period, and it is followed by a relative refractory period, during which another action potential can be generated, but only by a greater stimulus current than that originally needed. This period is followed by the return of the neuronal properties to the threshold levels originally required for the initiation of action potentials."

Absolute refractory period = initiation impossible absolutely

Relative refractory period = initiation possible but in greater energy relative to the original required.