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Abstract
Excitability properties of isolated frog and rat sciatic nerve fibers were examined
using intra-axonal and sucrose-gap recording techniques. Paired stimulation experiments
on rat myelinated fibers indicate that a small proportion (11%; n = 84) of these axons
demonstrate decreased threshold indicative of a supernormal period. In contrast, 81%
(n = 23) of frog axons displayed a supernormal period. A depolarizing afterpotential
was observed in most of the rat and frog fibers having a supernormal period and the
depolarizing afterpotential increased in magnitude and duration during hyperpolarization.
In addition to whole nerve stimulation, a supernormal period could be induced by stimulation
of a single axon via current passage through the recording microelectrode. Brief (2-5
ms) subthreshold depolarizing pulses were followed by a slowly decaying depolarization
and a period of increased excitability that mimicked the supernormal period. A supernormal
period was also observed in the whole nerve preparation using a sucrose-gap technique.
The magnitude and duration of the supernormal period, as measured in the sucrose-gap,
were greater for frog nerve than for rat nerve. Additionally, a larger postspike negativity,
the extracellular equivalent of the intra-axonally observed depolarizing afterpotential,
was present in sucrose-gap recordings for frog nerve than for rat nerve. The results
indicate that the depolarizing afterpotential is an important determinant of the supernormal
period, and that both the depolarizing afterpotential and supernormal period are more
prominent in frog than in rat sciatic nerve.