The thermal sensitivity of the polymodal nociceptors in the monkey.

1. The static and dynamic sensitivities to thermal and mechanical stimuli of polymodal nociceptors in hairy skin of the anaesthetized monkey have been investigated by recording activity in their primary nerve fibres. 2. Polymodal nociceptors responded to skin pricking, pinching and heating to temperatures higher than 40 degrees C. They did not respond to touch, stretch or cold. The conduction velocity of their axons was from 0.6 to 1.1 m/sec. 3. Three types of cutaneous receptive fields have been observed: single spot-like areas of 1-2 mm2; multiple spot-like areas of 1-2 mm2; and larger areas up to 25 mm2 with heterogeneous sensitivity. 4. Polymodal nociceptors were subjected to heat stimuli that commenced from a 33 or 37 degrees C adapting temperature. A series consisted of heating their receptive fields to 43, 45, 47 and 50 degrees C from one or the other adapting temperatures at a constant rate of 0.2 degrees C/sec. Each heat stimulus intensity was maintained for 4 min after which the skin was returned to the adapting temperature. Immediately after the first series the identical series was repeated in order to determine the effect of prior heating upon the dynamic responses to re-heating. The dynamic responses were characterized by three phases: an initiation of a discharge at a threshold level of skin temperature; a dynamic discharge during the suprathreshold change, that reached a peak frequency when the temperature reached its maximum; and an adaptation phase while the temperature remained at the high intensity. Adaptation was rapid initially, and then slowed during the final minutes at the high intensity. 5. Adapting the receptive field to either 33 degrees C or to 37 degrees C before the heat stimuli did not affect the sensitivity and the discharge pattern of the polymodal nocicpetors. 6. During the first series of stimulations, the threshold at which the individual polymodal nociceptors began to discharge to heat stimuli varied from 40 to 46.5 degrees C. The mean threshold of the population was 42.5 degrees C. 7. No change in the threshold was observed when responses to 0.2 and 1.5 degrees C/sec rates of heating were compared...