1. We have developed a technique to measure the total amount of current from a synaptic input system that reaches the soma of a motoneuron under steady-state conditions. We refer to this quantity as the effective synaptic current (I(N)) because only that fraction of the synaptic current that actually reaches the soma and initial segment of the cell affects its recruitment threshold and firing frequency. 2. The advantage of this technique for analysis of synaptic inputs in comparison to the standard measurements of synaptic potentials in apparent from Ohm's law. Steady-state synaptic potentials recorded at the soma of a cell are the product of I(N) and input resistance (R(N)), which is determined by intrinsic cellular properties such as cell size and membrane resistivity. Measuring I(N) avoids the confounding effect of R(N) on the amplitudes of synaptic potentials and thus provides a more direct assessment of the magnitude of a synaptic input. 3. Steady-state synaptic inputs were generated in cat medial gastrocnemius (MG) motoneurons by using tendon vibration to activate homonymous Ia afferents. We found that the magnitude of the Ia effective synaptic current (Ia I(N)) was not the same in all MG cells. Instead, Ia I(N) covaried with R(N) (r=0.64; P<0.001), being about twice as large on average in motoneurons with high R(N) values as in those with low R(N) values. Ia I(N) was also correlated with motoneuron rheobase, afterhyperpolarization duration, and axonal conduction velocity. 4. A comparison of transient Ia EPSPs with steady-state Ia EPSPs (Ia EPSP(SS)) evoked in the same cells suggested that the effective synaptic current that produces the transient Ia EPSP was also greater in motoneurons with high R(N) values than in those with low R(N) values. 5. The factors responsible for the Ia I(N)-R(N) covariance are uncertain. However, our finding greater values of Ia I(N) in high R(N) motoneurons is consistent with other evidence suggesting that Ia boutons on these motoneurons have a higher probability for neurotransmitter release than those on low R(N) motoneurons. 6. The neural mechanisms underlying orderly recruitment are discussed. The effect of the Ia input is to produce an approximately twofold expansion of the differences in motoneuron recruitment thresholds that are generated by intrinsic cellular properties. It is suggested that the higher efficacy of Ia input in low-threshold motoneurons confers particular importance on this input system in the control of vernier movements.
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