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Neural Research Laboratory, School of Anatomy, University of New South Wales, Kensington, Australia.

1. Excitatory postsynaptic potentials (EPSPs) evoked by impulses in single group I muscle afferents were recorded intracellularly in dorsal spinocerebellar tract (DSCT) neurones in the spinal cords of anaesthetized cats. 2. In the same experiments, electrotonic membrane properties of DSCT neurones were measured using the voltage response of each cell to a brief intracellular current pulse. 3. Single group I fibre EPSPs were found to exhibit a large range of amplitudes, from 210 microV to 3.4 mV. All of these EPSPs exhibited uniformly rapid rise times, in contrast to the wide range of time courses exhibited by group I a EPSPs recorded in motoneurones. 4. Electrotonic analysis of DSCT neurones indicated that the time constants of these cells ranged from 5.9 to 18.2 ms, with an average value of 10.9 ms. 5. Current pulse responses of the majority (approximately three-quarters) of DSCT neurones were well described by a simple cable model. Equivalent dendritic cable lengths were calculated for DSCT neurones and found to have an average value of 1.0 space constants, which is considerably less than that calculated for motoneurones. 6. Application of the simple cable model of DSCT neurones demonstrated that the rapid rise-times of single group I EPSPs can be explained by a substantial somatic input to these cells. However, in addition to this strong somatic component, there may also be a contribution from dendritic synapses which prolong the initial decay phase of these EPSPs. The final decay of single fibre EPSPs in DSCT neurones is explained simply by the passive membrane time constant of these cells.

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