EFFECTIVE SYNAPTIC CURRENT AND MOTONEURON FIRING RATE MODULATION

1. We used a modified voltage-clamp technique to measure the steady-state effective synaptic currents (IN) produced by activating four different input systems to cat hindlimb motoneurons: Ia afferent fibers, Ia-inhibitory interneurons, Renshaw interneurons, and contralateral rubrospinal 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 firing to assess their effects on motoneuron discharge rate. 2. Our measurements of IN were derived from recordings made near the resting membrane 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 potential during repetitive discharge. 3. We found that changes in the steady-state discharge rate of a motoneuron produced by these synaptic inputs could be reasonably well predicted by the product of the estimated value of IN during repetitive firing and the slope of the motoneuron's f-l relation. Although there was a high correlation between predicted and observed changes in firing rate for our entire sample of motoneurons (r = 0.93; P < 0.001), the slope of the relation between predicted and observed firing rate modulation was significantly greater than 1. 4. The systematic difference between predicted and observed firing rate modulation observed in the overall sample was primarily due to the fact that our predictions underestimated the changes in firing rate produced by Ia excitation and Ia inhibition. The slope of the relation between observed and predicted firing rate modulation in response to the rubrospinal and recurrent inhibitory synaptic inputs did not differ significantly from 1. 5. The potential sources of error associated with our predictions of firing rate modulation are discussed and evaluated. The greater than predicted efficacy of Ia excitation may result from the significant transient component present in the excitatory synaptic current. The greater than predicted decrease in firing rate produced by Ia inhibitory synaptic input may indicate that the effects of injected and synaptic currents on motoneuron discharge rate are not always equivalent.