Effects of nerve impulses on threshold of frog sciatic nerve fibres.

Abstract

The firing thresholds of single myelinated fibers of frog (Rana pipiens) sciatic nerves were monitored as a function of impulse activity in the fiber. The threshold was given by the number of coulombs in current pulses that excited a particular fiber half the time when delivered to the whole nerve. Threshold was tracked by a device that incrementally decreased the number of coulombs in the current pulse whenever the fiber responded and increased the pulse if it did not respond. There was a pattern to the after-oscillations of threshold following activity. The fibers were briefly refractory, transiently superexcitable for about 1-1.5 s and then entered a phase of raised threshold or depression that lasted for many minutes. Activity produced little change in the threshold curve during the refractory period. After an impulse, superexcitability reached a maximum within 7-20 ms. This peak was larger as the number of impulses in a preceding burst increased and as the intervals between the impulses became briefer. The depression phase was marked by the interaction between build-up, which required as long as an hour or more for the threshold to be completely restored to resting level. These 2 mechanisms, 1 causing build-up and the other recovery, led to formation of dynamic equilibria. The processes associated with superexcitability interact with those producing depression. In active fibers showing raised thresholds, impulses are followed by a relative superexcitability that persists for at least as long as an absolute superexcitability (with threshold below the resting level) which can be measured in the same fiber at rest. The duration of the superexcitable phase interpreted as a relative change in excitability was roughly the same regardless of the level of depression. The magnitude of the oscillation in threshold was 5-10 times larger than the gray region (the range of stimuli for which response is probabilistic). At regions of low conduction safety such as axonal branches, where weak forces can influence whether an impulse will pass, such pronounced and long-lasting after-effects of firing can be expected to modulate conduction of nerve impulses. Two implications are drawn: the static connectivity of an axon, as determined by its anatomy, will in general differ from its dynamic connectivity as defined by the subset of its branches that conduct each impulse through the arbour to synaptic endings; the temporal pattern of firing in the axonal trunk will produce trhoughout the teledendron a distributed time function of local thresholds that reflects the firing patterns, suggesting that the messages encoded in the pattern of firing may be resolved by variation of connectivity according to message.