NOCICEPTIVE C-FIBER INPUT TO THE PRIMARY SOMATOSENSORY CORTEX (SI) - A FIELD POTENTIAL STUDY IN THE RAT

In the present study, noxious thermal stimulation of the skin with sho rt pulses of CO2-laser radiation was used to identify a cutaneous noci ceptive C fibre input to SI and investigate the organization of this i nput in halothane-nitrous oxide anaesthetized rats. Noxious CO2-laser stimulation of the glabrous skin of the hindpaw consistently evoked la te surface positive field potentials in SI (average onset latency 226 ms, peak latency 296 ms). It was demonstrated that these late potentia ls were evoked by an input from nociceptive C fibres, using a combinat ion of latency measurements, anodal block of A fibre conduction and gr aded intensities of stimulation. Compared to the tactile evoked potent ials in SI, the nociceptive C fibre evoked potentials were more widesp read and exhibited a crude somatotopical organization. Intracortically , both tactile and nociceptive C fibre evoked potentials reversed pola rity and exhibited a peak negativity in laminae III-IV. The nociceptiv e C fibre evoked potentials exhibited an additional peak negativity in laminae V-VI. The latter potential had a different time course as com pared to the nociceptive C fibre potential evoked in laminae III-IV. I n conclusion, an input from cutaneous nociceptive C fibres to SI was d emonstrated for the first time in animal experiments. The input to SI from tactile receptors and cutaneous C nociceptors were differently or ganized.