Rat somatosensory (SmI) cortex: I. Characteristics of neuronal responses to noxious stimulation and comparison with responses to non-noxious stimulation.

Single unit responses to noxious and non-noxious somatic stimulation were investigated in the somatosensory (SmI) cortex of rats under halothane-nitrous oxide anaesthesia. Four categories of neurons were observed: (1) neurons driven by non-noxious cutaneous stimulation, (2) neurons driven by non-noxious deep stimulation, (3) neurons driven by noxious stimulation only (nociceptive specific neurons), (4) neurons driven by noxious as well as non-noxious stimulation (convergent or nociceptive non-specific neurons). The receptive fields of the neurons driven by contralateral cutaneous non-noxious stimulation were small. These neurons responded phasically to cutaneous stimulation in the majority of cases. Neurons driven by stimulation of deep receptors (e.g. joint movement) could also be recorded in the same part of SmI cortex. Neurons driven by noxious stimulation had large receptive fields and were often tonically driven by noxious stimulation. Convergent (or nociceptive non-specific) neurons could often be inhibited from body parts not included in their excitatory receptive field. Some neurons driven by noxious stimulation were able to encode stimulus parameters such as temperature of a hot water bath or surface of the skin area stimulated. The different categories of neurons defined above could be successively recorded during a given electrode penetration. Evidence for the somatotopic organization of the different categories of inputs was obtained. These results strongly suggest that the first somatosensory (SmI) neocortex is involved in nociception.