Nervous outflow from the cat's foot during noxious radiant heat stimulation

Abstract

(1) Radiant heat pulses (10 sec in duration) at noxious levels were applied to the hind foot of the cat in order to establish quantitatively the response characteristics of single afferent units, and to evaluate the total afferent outflow. (2) In the plantar nerves, 16% of the C-fibers were activated by noxious heat (50 °C). The thresholds of the majority of these ‘C-heat receptors’ were between 40–45 °C. The discharges were related linearly to the level of skin temperature, the high correlation coefficient suggesting these receptors to be able to signal the intensity of heat stimuli. (3) The ‘C-heat receptors’ showed a steady discharge to long-lasting heat stimuli ( e.g. 5 min). About half of them could be excited in addition by mechanical stimuli, mostly in a noxious range (⩽ 300g/0.8sq.mm). Sensitization was observed in some of the ‘C-heat receptors’ by repeated exposition to noxious heat. (4) Among myelinated fibers about half of the high threshold mechanoreceptors with group III afferents exhibited a response to heating above 45°C, there being however a low correlation between temperature and discharge frequency. In the low threshold mechanoreceptors of the pad with group II afferents (RA and SA) a suppression of the response to adequate mechanical stimulation occurred during heating of the skin.