Synaptic action on Clarke's column neurones in relation to afferent terminal size

1. Excitatory post-synaptic potentials (e.p.s.p.s) were recorded intracellularly from Clarke's column neurones (DSCT neurones) of the cat in response to adequate stimuli applied to a variety of sensory receptors.2. The amplitude of e.p.s.p.s so produced varied from less than 0.2 mV to more than 2-3 mV. The amplitude distribution of e.p.s.p.s suggested that the mean number of ;quanta' of transmitter released by one impulse varied widely from one fibre to another arising from a given type of sensory receptor.3. The average amplitude of e.p.s.p.s evoked by single afferent impulses was significantly smaller for cutaneous inputs than for muscle or joint inputs. However, synaptic action on DSCT neurones produced by different sensory inputs was equally greater, on the average, than that on spinal motoneurones.4. Both small and large e.p.s.p.s in DSCT neurones failed to increase in amplitude during post-synaptic hyperpolarization applied through the cell body. This failure could not be attributed to possible anomalous rectification in the post-synaptic membrane.5. Small and large e.p.s.p.s were comparable in half-decay time, but there was a positive correlation between amplitude and time-to-peak of e.p.s.p.s. It is suggested that the locations of synapses responsible for small and large e.p.s.p.s are intermingled on the dendrites and that large e.p.s.p.s are associated with a longer duration of transmitter action than small e.p.s.p.s.6. Degenerating terminals of primary afferent fibres on DSCT neurones and motoneurones were examined with the electron microscope after chronic section of the dorsal roots.7. Dendritic degenerating terminals showed no significant difference in size between motoneurones and DSCT neurones. Degenerating ;giant' terminals were found on DSCT neurones, but they were located only on or very close to the cell body.8. It is concluded that the major factor responsible for a large number of ;quanta' of transmitter released at synapses on DSCT neurones is the number of multiple synaptic contacts formed by one afferent fibre rather than the size of individual synapses.