Activity-dependent changes in conduction velocity in the olfactory nerve of the tortoise
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The conduction velocity of the compound action potential in the olfactory nerve of the tortoise is affected by previous activity in two ways. First, there is an inverse relation between conduction velocity and the level of activity as determined by the rate of stimulation. Second, in the active, though not in the resting nerve, the action potential initiated by the second of a pair of shocks is conducted faster than that initiated by the first. The period of supernormal conduction velocity set up in the wake of the first response lasts for about 1 s, and at the peak of the effect conduction velocity is increased by up to 27%.
Aspects of these phenomena were investigated in vitro. The magnitude of supernormality was diminished by ouabain, and by the substitution of lithium for sodium in the bathing medium; it was enhanced when chloride was replaced by sulphate.
Supernormality could not be potentiated by increasing the number of conditioning volleys. The conduction velocity achieved at the peak of the supernormal period was usually as great as, but never greater than the conduction velocity of the resting nerve.
These results, together with the results of computations based on the Hodgkin-Huxley equations, suggest that the dependence of conduction velocity on the rate of stimulation is a reflection of a corresponding dependence of intracellular sodium levels on activity. Two classes of explanation for supernormality are considered, one based on the activation of an electrogenic pump, and the other on a transient increase in sodium conductance. The available evidence is insufficient to distinguish between these two hypotheses.
Key wordsNerve conduction Olfactory nerve Tortoise
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