EFFECTIVE SYNAPTIC CURRENT AND MOTONEURON FIRING RATE MODULATION

1. We used a modified voltage-clamp technique to measure the steady-st ate effective synaptic currents (IN) produced by activating four diffe rent input systems to cat hindlimb motoneurons: Ia afferent fibers, Ia -inhibitory interneurons, Renshaw interneurons, and contralateral rubr ospinal neurons. In the same motoneurons, we measured the slope of the firing rate-injected current (f-I) relation in the primary range. We then reactivated these synaptic inputs during steady, repetitive firin g to assess their effects on motoneuron discharge rate. 2. Our measure ments of IN were derived from recordings made near the resting membran e potential, whereas the effects of the synaptic inputs on repetitive discharge were evaluated at more depolarized membrane potentials. Thus we adjusted the IN values for these changes in driving force based on estimates of the synaptic reversal potential and the mean membrane po tential during repetitive discharge. 3. We found that changes in the s teady-state discharge rate of a motoneuron produced by these synaptic inputs could be reasonably well predicted by the product of the estima ted value of IN during repetitive firing and the slope of the motoneur on's f-l relation. Although there was a high correlation between predi cted and observed changes in firing rate for our entire sample of moto neurons (r = 0.93; P < 0.001), the slope of the relation between predi cted and observed firing rate modulation was significantly greater tha n 1. 4. The systematic difference between predicted and observed firin g rate modulation observed in the overall sample was primarily due to the fact that our predictions underestimated the changes in firing rat e produced by Ia excitation and Ia inhibition. The slope of the relati on between observed and predicted firing rate modulation in response t o the rubrospinal and recurrent inhibitory synaptic inputs did not dif fer significantly from 1. 5. The potential sources of error associated with our predictions of firing rate modulation are discussed and eval uated. The greater than predicted efficacy of Ia excitation may result from the significant transient component present in the excitatory sy naptic current. The greater than predicted decrease in firing rate pro duced by Ia inhibitory synaptic input may indicate that the effects of injected and synaptic currents on motoneuron discharge rate are not a lways equivalent.