Elsevier

Brain Research

Volume 622, Issues 1–2, 17 September 1993, Pages 262-270
Brain Research

Nociceptive C fibre input to the primary somatosensory cortex (SI). A field potential study in the rat

https://doi.org/10.1016/0006-8993(93)90827-AGet rights and content

Abstract

In the present study, noxious thermal stimulation of the skin with short pulses of CO2-laser radiation was used to identify a cutaneousnociceptive C fibre input to SI and investigate the organization of this input in halothane-nitrous oxide anaesthetized rats. Noxious CO2-laser stimulation of the glabrous skin of the hindpaw consistently evoked late surface positive field potentials in SI (average onset latency 226 ms, peak latency 296 ms). It was demonstrated that these late potentials were evoked by an input from nociceptive C fibres, using a combination of latency measurements, anodal block of A fibre conduction and graded intensities of stimulation. Compared to the tactile evoked potentials in SI, the nociceptive C fibre evoked potentials were more widespread and exhibited a crude somatotopical organization. Intracortically, both tactile and nociceptive C fibre evoked potentials reversed polarity and exhibited a peak negativity in laminae III–IV. The nociceptive C fibre evoked potentials exhibited an additional peak negativity in laminae V–VI. The latter potential had a different time course as compared to the nociceptive C fibre potential evoked in laminae III–IV. In conclusion, an input from cutaneous nociceptive C fibres to SI was demonstrated for the first time in animal experiments. The input to SI from tactile receptors and cutaneous C nociceptors were differently organized.

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      Fig. 1b shows the distribution of the recording sites used. To elicit C fibre evoked potentials, the glabrous skin of the right hind paw/forepaw areas were stimulated using a CO2 laser with a pulse duration of 21–33 ms. These stimulation energies have been shown to evoke late cortical field potentials reliably in the rat SI cortex through the activation of cutaneous nociceptive C fibres (Kalliomaki et al., 1993a,b). Based on previous latency measurements (Kalliomaki et al., 1993a) we here classify evoked potentials with an onset latency exceeding 120 ms and 180 ms for forepaw and hind paw, respectively, as C fibre evoked.

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