The synaptic current evoked in cat spinal motoneurones by impulses in single group 1a axons.

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Abstract
Excitatory post-synaptic potentials (epsp) were evoked in motoneurons of anesthetized cats by impulses in single group 1a axons. Epsp with a time course that indicated a somatic site of origin were voltage-clamped using a single micro-electrode clamp. Excitatory post-synaptic currents (epsc) were found to peak in less than 0.2 ms, and to decay with an exponential time course. The time constant of decay was usually in the range 0.3-0.4 ms (at 37.degree. C). At the resting membrane potential, an epsp with a peak of 100 .mu.V was generated by an average peak epsc of 330 pA. This corresponded to an average peak conductance increase of 5 nS. The epsc decreased with membrane depolarization, and reversed to become an outward current at a null potential of +4.6 .+-. 2 mV (.+-. SE of mean; n = 7). Membrane hyperpolarization caused the peak epsc to increase and the time constant of decay of the epsc to decrease. The total charge in the synaptic current did not increase with hyperpolarization. This observation can explain earlier observations which showed that the peak amplitude of the epsp did not increase with hyperpolarization. The number of ion channels opened by transmitter release at a single somatic bouton was estimated to be in the range 40-240.