The basic question of how motoneurons transform synaptic inputs into spike train outputs remains unresolved, despite detailed knowledge of their morphology, electrophysiology, and synaptic connectivity. We have approached this problem by making measurements of a synaptic input under steady-state conditions and combining them with quantitative assessments of their effects on the discharge rates of cat spinal motoneurons. 2. We used a modified voltage-clamp technique to measure the steady-state effective synaptic currents (IN) produced by rubrospinal input to cat triceps surae motoneurons. In the same motoneurons we measured the slope of the firing rate-injected current (f-i) relation in the primary range. We then reactivated the rubrospinal input during steady, repetitive firing to assess its effect on motoneuron discharge rate. 3. We found that changes in the steady-state discharge rate of a motoneuron produced by this synaptic input could be described simply as the product of the net effective synaptic current measured at the soma and the slope of the motoneuron's f-i relation. This expression essentially redefines synaptic efficacy in terms of a cell's basic input-output function. Further, measurements of effective synaptic current simplify the task of estimating synaptic efficacy, because detailed knowledge of neither the electrotonic architecture of the postsynaptic cell nor of the locations of the presynaptic boutons is required.
