Distribution of potentiation following short high frequency bursts to motoneurons of different rheobase.

Potentiation of composite EPSPs has been studied at group Ia fiber/alpha motoneuron connections using a short burst of conditioning stimuli [32 shocks with 6 ms interstimulus interval (ISI)]. Potentiation reached its peak (range 1.2-2.0 X control value) 100-150 ms following the burst. Potentiation decayed slowly and was still present 2 s after the burst. High frequency burst stimulation of a nerve to a synergist muscle did not potentiate the response to stimulation of the homonymous nerve. Three independent sets of measurements suggest that the time course of decline of potentiation depends on the amount of transmission during the potentiated state. First, connections on high rheobase motoneurons exhibited more peak potentiation than those on low rheobase motoneurons but potentiation decayed more rapidly in the former than in the latter. Second, increasing the frequency of the conditioning burst enhanced peak potentiation but the rate of decay of this potentiation also increased. Third, potentiated EPSPs exhibited more low frequency depression than unpotentiated ones at the same connection suggesting that low frequency stimulation during the potentiated state could elevate the rate of decay of potentiation. The short burst paradigm could cause peak potentiation similar in magnitude to that evoked by long, high frequency trains (studied here at the same connection) but with much less of an increase in latency and rise time of the potentiated EPSP. The magnitude of potentiation was unrelated to changes in EPSP latency and rise time. These findings indicate that potentiation can act to modulate EPSP amplitude under conditions of normal motor behaviour when spindle afferents fire in patterns similar in duration and frequency to those used in the present experiments.