Do sympathetic preganglionic neurones have a recurrent inhibitory mechanism?

In anaesthetized and immobilized cats 32.5% of the intermediolateral sympathetic preganglionic neurones (B2-SPN) in T3, T8-9 and L2 spinal segments can be re-excited antidromically by repeated stimuli only at very long interstimulus intervals (16 ms and more) considerably exceeding the refractoriness of the somata and axons of these neurones. This was observed in the absence of any association with subnormality or other peculiarities B2-SPN axons. In this group of B2-SPNs with prolonged time interval between repeated antidromic discharges, orthodromic spikes effectively inhibited the generation of antidromic ones for a period which was substantially longer than the possible collision time. The preceding antidromic activation of a fraction of the B2-SPN segmental pool partly inhibited the orthodromic reactions of the others in this segmented elicited by stimulation of segmental afferent fibres or spinal descending pathways. These data definitely indicate that some of the B2-SPN group have a recurrent inhibitory mechanism.