Membrane impedance changes during synaptic transmission in cat spinal motoneurons.

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Abstract
The location of synapses that participate in the generation of postsynaptic potentials [PSPs] recorded intracellularly at the soma of cat spinal mononeurons was studied. To test the hypotheses of Eccles and Rall, attempts were made to measure the postulated permeability (impedance) change accompanying PSPs. Inhibitory postsynaptic potentials evoked by orthodromic or antidromic stimulation were invariably accompanied by impedance changes. Thus, the results of these and prior experiments were consistent with the Eccles hypothesis that IPSP [inhibitory postsynaptic potential] synapses are chemically mediated and located near the soma. Less than half the EPSPs [excitatory postsynaptic potential] studied were associated with detectable impedance changes. Given the sensitivity of the experimental technique, the results are incompatible with the Eccles hypothesis, but are consistent with the Rall hypothesis. Further support for the Rail hypothesis was derived from a correlation between the detectability of impedance changes and the waveform parameters of the EPSPs investigated. Experimental results were most consistent with the Rall hypothesis, viz., that dendritically located synapses contribute significantly to the PSP activity recordable at the motoneuron cell soma.