The normal stimulus for the contraction of a skeletal muscle fibre in a living animal is an impulse in the motor nerve that innervates it. In the twitch muscles of vertebrates with which we are concerned in this chapter, this nerve impulse leads to a propagated action potential in the muscle fibre, which is then followed by a twitch contraction. The time relations of the action potential and twitch tension in a single muscle fibre are shown in fig. 20.1.

The sequence of these events is shown schematically in fig. 18.1. We have examined stages 1 to 4 of this sequence (the excitation processes) in earlier chapters of this book, and stage 6 (contraction) in chapters 19 and 20. Here we consider how excitation of the muscle fibre membrane initiates contraction of the myofibrils in the interior of the fibre. This constitutes stage 5 of fig. 18.1, the excitation–contraction coupling process. In terms of fig. 20.1, then, how does the action potential produce the contraction?

Excitation–contraction coupling

The importance of depolarization of the cell membrane

When muscle fibres are immersed in solutions containing a high concentration of potassium ions, they undergo a relatively prolonged contraction known as a contracture. Contractures can also be produced by various drugs, such as acetylcholine, veratridine and others. Kuffler (1946) showed that many of these substances produce depolarization of the cell membrane; furthermore, if the substance was applied locally the resulting contracture was limited to that part of the muscle fibre where depolarization occurred.