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Tetrodotoxin‐sensitive and ‐insensitive action potentials, in myotubes

The study of long‐term cultures of myogenic cells has proven that electrical excitability develops only after the development of electrical coupling between the cells. That is, neither surface contact in itself nor coupling in itself is sufficient to cause excitability to develop in these cells. Following the formation of multinucleated myotubes, several different types of electrical responses develop. Some of the action potentials are sodium‐dependent and are blocked by tetrodotoxin (TTX). Others are dependent upon sodium and possibly calcium and they are not blocked by TTX. Furthermore, these two types of responses may exist in a myotube at the same time. Under some circumstances the kinetics of the two systems are sufficiently different to result in action potentials that have two peaks. Under these conditions the first peak is always of shorter duration and it is always blocked by TTX.