Characteristics of C-fibre receptors in the cat's foot responding to stepwise increase of skin temperature ot noxious levels.

1. Response characteristics of unmyelinated cutaneous fibres with receptors sensitive to heating of the skin were determined by controlled radiant heat stimulation applied to the foot sole of anaesthetized cats. To evaluate response characteristics without repetivie stimulation the skin surface temperature was raised stepwise ("staircase") up to noxious levels in each trial. 2. A monotonically increasing discharge rate on increasing temperature (up to the highest temperatures tested, 52-55 degrees C) was shown by 29 out of 47 heat sensitive C-fibres, i.e. by about 60% of the receptors, whereas 13 fibres, about 30%, had discharges which were found to be presumably unsuitable for discriminating temperature levels above 50 degrees C. The threshold temperatures of both fibre groups were found to be usually near 40 degrees C. Thus our results indicate different temperature ranges among heat receptors with potential nociceptive functions. 3. Five fibres of our sample, i.e. about 10%, could be identified as "warm fibres". They had tresholds near 30 degrees C and had their discharge maxima at 44 degrees C. In contrast to the "heat-fibres" mentioned above they were inactivated during tonic noxious heat stimulation and in the time following. Though warm fibres respond with a burst of high frequency when the skin temperature is raised suddenly from normal to noxious levels and though they start to discharge again irregularly at noxious temperatures (e.g. 50 degrees C) after a silent period, when stimulated tonically, warm fibres seem to transmit little quantitative information about noxious temperature levels. Therefore they can presumably be discarded as "nociceptors".