THE ALL‐OR‐NONE PRINCIPLE

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Abstract
Summary.: 1. The all‐or‐none character of the heart beat is briefly considered, and, in contrast, some processes which are graded. A theoretical discussion follows emphasising the relation between an all‐or‐none process and a refractory phase following excitation. Some chemical analogies are mentioned.2. The difficulties surrounding the question of conduction with a decrement are discussed and the necessity for observations on single physiological units.3. The all‐or‐none character of the nervous impulse, which the older macroscopic observations strongly supported, is now placed beyond doubt by the investigation of action potentials in single nerve fibres.4. The case of skeletal muscle is not so easily settled, but the conclusion appears to be that its contractile process is all‐or‐none. Apparent exceptions and anomalies are discussed.5. Among vertebrate smooth muscles and various invertebrate muscles there is evidence of all‐or‐none behaviour in certain types at least. But it would be premature to conclude that they are all similar.

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