Depolarization of the cell membrane

When muscle fibres are immersed in a solution containing a high concentration of potassium ions, they undergo a relatively prolonged contraction called a potassium contracture. The tension produced is related to the potassium concentration in a sigmoidal way as is shown in Fig. 10.1. The membrane potential is of course reduced under these conditions (see Chapter 3), so it seems that depolarization is an adequate stimulus for contraction. Normally this depolarization occurs during the propagated action potential.

95 -71 -58 —46 ^25 mV

Membrane potential

Fig. 10.1. The relation between peak contracture tension and potassium ion concentrations or membrane potential in single frog muscle fibres. From Hodgkin and Horowicz (1960).

Contractures can also be produced by various drugs which depolarize the cell membrane, such as acetylcholine, veratridine and others. Caffeine also causes contractures, but without producing much depolarization; it is apparently affecting a later stage in the coupling process.

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