1. Steady-state inhibitory postsynaptic potentials (IPSPs) were evoked in tibialis anterior and extensor digitorum longus motoneurons of the cat by using tendon vibration to activate Ia-afferent fibers from the antagonist medial gastrocnemius muscle. 2. The effective synaptic currents (I(N)) underlying the steady-state IPSPs were measured by the use of a modified voltage-clamp technique. The amplitudes of the effective synaptic currents (1.62 ± 0.66 nA, mean ± SD; n = 20) extended over a fivefold range (0.5-2.7 nA) but were not correlated with the intrinsic properties of the motoneurons or with putative motor unit type. 3. We calculated the synaptic conductance (G(S)) underlying the steady-state Ia IPSPs from measurements of motoneuron input conductance during the activation of the Ia synaptic input. As was expected from Ohm's law, the Ia-inhibitory G(S) and I(N) were correlated (r = 0.49; P < 0.05). Like I(N), G(S) (175 ± 202 nS, mean ± SD; n = 20) was not correlated with the intrinsic properties of the motoneurons. 4. As has been reported previously for transient Ia IPSPs, the amplitudes of the steady- state IPSPs were correlated with motoneuron input resistance (r = 0.74; P < 0.001) and homonymous Ia excitatory postsynaptic synaptic potential (EPSP) amplitude (r = 0.72; P < 0.001). 5. The amplitudes of the steady-state Ia IPSPs and the homonymous Ia EPSPs were plotted on logarithmic axes. The slope (0.59) was significantly <1, which indicates that the gradient of Ia inhibition across the motoneuron pool is less steep than that of Ia excitation. This difference can be ascribed to the distributions of the effective synaptic currents generated by these two input systems: the I(N) underlying homonymous Ia EPSPs covaries with the intrinsic properties of the motoneurons, whereas the I(N) underlying reciprocal Ia IPSPs does not.
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