Cutaneous thermoreceptors in primates and sub-primates.

1. Cutaneous thermoreceptors were examined electrophysiologically in primates (monkey, baboon) and in sub-primates (dog and rat) by recording from single units dissected from peripheral nerves.2. Thermal stimuli were delivered from thermodes in contact with the skin.3. Primate ;cold' receptors had spot-like receptive fields and were found in both hairy and glabrous skin. The conduction velocities of the axons ranged from 0.6 to 15.3 m/sec.4. The discharge from the primate receptors characteristically appeared in bursts with intervals of silence within the range temperatures of 18-40 degrees C. Static and dynamic sensitivity curves were established, with maxima about 30 degrees C.5. Cold receptors in the lip of the dog had maximal sensitivity at 31-37 degrees C. The axons were myelinated with conduction velocities less than 20 m/sec.6. ;Warm' receptors, with maximal sensitivity at 40 degrees C and non-myelinated axons, were abundant in the scrotal nerve of the rat. The ;cold' receptors had maximal responses at 23-28 degrees C.7. The ;spurious' thermoreceptor behaviour of slowly adapting mechanoreceptors is described and the way in which they may distort integrated potential records from whole nerves is analysed.