High-frequency stimulation of single group Ia-fibers results in modulation of excitatory postsynaptic potential (EPSP) amplitude recorded in target motoneurons. This can be either positive (EPSP amplitude increases in response to successive stimuli in the high-frequency burst) or negative (decrease in EPSP amplitude). We have investigated whether the magnitude of modulation is associated with the stimulated afferent, the responding motoneuron, or the amplitude of the EPSP. In agreement with previous findings, we found that positive modulation tends to occur at connections generating small EPSPs and negative modulation, at those producing large EPSPs. Because large EPSPs generally are evoked in motoneurons with low values of rheobase, we found, as anticipated, that connections on low rheobase motoneurons are prone to negative modulation during high-frequency stimulation, whereas those on high rheobase motoneurons (which tend to generate small EPSPs) are prone to positive modulation. In experiments where the projection of multiple afferents to a single motoneuron was studied, we found that amplitude modulation was similar despite differences in EPSP amplitude. Thus in a given motoneuron there is no relationship between modulation and amplitude, in contrast to the existence of such a relationship in the population of connections as a whole. In the converse experiments where the projection of single afferents to multiple motoneurons was studied, we found marked variability in the modulation patterns with clear indications that amplitude and modulation are correlated as in the entire population of Ia/motoneuron connections. We tested the constancy of modulation patterns evoked in a given motoneuron by comparing the modulation patterns evoked in motoneurons by single fibers, and by stimulation of the heteronymous nerve. A similar dependence of modulation on motoneuron rheobase was found for both types of EPSPs, suggesting that modulation of EPSPs generated at each of the connections activated by heteronymous nerve stimulation is similar to that produced by single homonymous afferent fibers terminating on the same motoneuron. These results indicate a key role of the postsynaptic cell in determining the modulation behavior of these connections. They are discussed in terms of potential mechanisms responsible for this behavior and in terms of their role in the function of the motoneuron pool.
