Effect of noxious chemical stimulation of the receptive field on the spontaneous discharges and noxious pinch-evoked responses. A: Noxious pinch stimuli applied to the orofacial skin (blackened area) produced a barrage of excitatory postsynaptic potentials (EPSPs) accompanied by action potentials in SI neurons (NS-type). Blackened area indicates the location and size of the receptive field responding to noxious pinch stimulation. Arrow heads show resting membrane potential. B: Example of responses of spontaneous discharges and of noxious pinch stimulation after subcutaneous injection of mustard oil (MO) into the receptive field area. Note that after MO injection (5 min), spontaneous discharges of SI neurons increased, lasting for 10-15 min. Noxious pinch-evoked discharge rate was increased and the response was accompanied by augmented EPSP amplitudes. C: Change in the mean membrane potential of SI neurons after MO administration (10 min). *, P < 0.05. D: Change in the mean noxious pinch evoked discharge of SI neurons after MO administration (10 min). *, P < 0.05. E: Change in the mean noxious pinch evoked EPSP amplitude of SI neurons after MO administration (10 min). *, P < 0.05.