Long-lasting neuronal activity in rat dorsal horn evoked by impulses in cutaneous C fibres during noxious mechanical stimulation

The responses of 44 nociceptive neurones in the lumbar dorsal horn evoked by controlled mechanical stimulation of the skin, with or without conduction block in myelinated afferent fibres, were studied in the halothane-anaesthetized rat, in order to evaluate the effects of impulses in cutaneous nociceptive C fibres on dorsal horn neurones. Continuous non-noxious pinch of the skin evoked a short-latency discharge (mean latency 15 ms) in all the 13 class 2 neurones (i.e. neurones responding to both non-noxious and noxious stimulation of the skin) tested. The short-latency discharge was followed by weak prolonged activity in 6 neurones. Following noxious pinch of the skin a prominent late discharge (peak latency 150 ms-2 s) was evoked, which in all but two class 2 neurones outlasted the stimulation period (5-10 s). The discharge evoked by noxious pinch in class 3 neurones (i.e. neurones responding to noxious stimulation only) did not usually outlast the stimulation period. In all but two nociceptive neurones tested (n = 26) the late activity evoked by noxious pinch remained, albeit at a lower frequency in some neurones, during a conduction block in A fibres2,3. Hence this late discharge is probably mainly generated by impulses in nociceptive C fibers. It is concluded that nociceptive C fibres have an important role in sustaining long-lasting activation of class 2 neurones during noxious stimulation of the skin and that long-lasting discharges in these neurones indicates tissue damage to their receptive fields.