Potentiation of transmission at Ia-motoneuron connections induced by repeated short bursts of afferent activity.

Single medial gastrocnemius Ia-afferent fibers and motoneurons to which they projected were simultaneously impaled in anesthetized cats. Each Ia-afferent fiber was electrically stimulated once every 2 s with short high-frequency bursts (32 shocks at 167 Hz) followed by 1-11 test shocks. The resulting motoneuron excitatory postsynaptic potentials (EPSPs) were recorded and averaged in register. The interval between the end of one burst and the beginning of the next was 2 s; therefore, the amplitude of the first EPSP in the burst was considered to be a measure of efficacy of transmission 2 s after the burst. At most connections (23/29) the mean amplitude of the first EPSP in the burst was equal to or larger than the mean amplitude of control EPSPs produced by low-frequency (18-Hz) stimulation. Enhancement of transmission was maximum 50-100 ms after the burst, and the amplitude of the test EPSP delivered at this time was always greater than that of the control. The period of enhanced transmission appeared to decay more rapidly at connections with small EPSPs. The greatest amount of EPSP amplitude enhancement at 50 or 100 ms after the burst was observed at connections at which EPSP amplitude increased during the burst. The shape (rise time, half width) of potentiated EPSPs was the same as control EPSPs averaged during low-frequency (18-Hz) stimulation. Multiple shocks delivered at low frequency between bursts revealed that enhanced transmission following the high-frequency burst is very sensitive to the effects of low-frequency test stimulation. Furthermore, increasing the number of shocks during the interval between bursts reduced the enhancement of the first EPSP in the burst. We suggest that modulation of synaptic transmission after high-frequency bursts differs across Ia-motoneuron connections. These time-dependent changes associated with short bursts of firing (which are similar in frequency to those observed in Ia-fibers supplying hind-limb muscles during stepping) emphasize the necessity to consider the history of the discharge pattern of the group Ia fiber in assessing efficacy at individual Ia-motoneuron connections.