The effects of activity on mammalian nerve fibres of low conduction velocity

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Abstract
The effect of previous activity on the responses to single, subsequent maximal stimuli has been studied in vivo in a number of mammalian nerves containing C fibres. The recorded monophasic action potential of all fibre groups, conducting at velocities less than 1∙1 m/s, is increased up to threefold by repetitive stimulation. Nerves conducting at greater velocities show no change, or a diminution of action potential. The enhancement of the action potential is not due to recruitment of nerve fibres, to ganglionic facilitation in sympathetic fibres, or to the building up of spike potentials on the greatly increased negative after-potentials which accompany the phenomenon. It is caused by a hyperpolarization of the nerve after the repetitive excitation. With suitable frequencies of stimulation, 6 to 20/s, the action potential of, for example, the hypogastric nerve increases in voltage during the train of stimuli, and an action potential of twice the ‘resting’ voltage can be maintained for long periods. It is sug­gested that post-tetanic enhancement of action potential is a property common to all mam­malian non-myelinated nerve fibres and is responsible for post-tetanic facilitation in junctional tissues.