Comparative Physiology of the Nervous Control of Muscular Contraction

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CHAPTER 3 Excitation of Muscle: the Action Potential

CHARACTERISTICS OF ACTION POTENTIALS

ALTHOUGH the arrival of nerve impulses is the normal signal which triggers or regulates muscular contraction, a great variety of artificial stimuli may also be used to initiate or modify muscular action. These include mechanical deformations such as sudden pressure or stretch, besides a wide range of chemical treatments due to changes in the concentration of specific ions, and particularly potassium ions, in the external fluid; to changes in oxygen tension and in pH; to small amounts of pharmacological agents, e.g. acetylcholine, veratrine and histamine; to changes in osmotic pressure of the external medium; to changes in temperature, particularly high temperature; to ultrasonic vibrations; to high doses of X-rays; and to electric currents. In almost all cases the stimulus can be shown to lead to the production of electrical events in the muscle. The changes in electrical potential can be recorded with the aid of surface electrodes from undamaged muscle or between an intact and a cut or damaged surface, or with the aid of intracellular electrodes inserted into single muscle fibres. In the second case the demarcation potential is also recorded and in the third the resting potential. Experiments using the intracellular method make it evident that the electrical changes associated with contraction always take the form of a reduction in the magnitude of the resting potential.

The membrane potential changes are entirely responsible for the potentials observed with external leads, except for occasional artifacts due to resistance changes and to movements.

Three general classes of change are observed as follows: (1) Quick, large depolarizations returning rapidly to the resting level; in a series (train) of these potential changes successive events are similar; they do not summate. (2) Smaller, often very small, quick potentials returning fairly rapidly to the resting level; these potentials summate to a greater or lesser

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