A peripheral "cold" fiber population responsive to innocuous and noxious thermal stimuli applied to monkey's face

The activity of 134 cold fibers innervating the hairy skin of the face was recorded from fine dissected strands of the infraorbital nerve in rhesus monkeys anesthetized with sodium pentobarbital. A precisely controlled contact thermode was used to produce rapid temperature shifts of approximately 10 degrees C/s in the cooling and warming directions with a 20-60 degrees C range. Cold fiber receptive fields usually were single spots less than 300 mum in diameter. The mean conduction velocity of 94 cold fibers determined by electrical stimulation of the receptive field was 9.0 m/s, with a range indicating an almost exclusive A-delta population. Rapid cooling shifts of 1-10 degrees C produced an initial transient period of high-frequency discharges, which decayed rapidly and was followed by a period of slow adaptation. Intensity functions were linear for cooling shifts up to 6-8 degrees C, and the slope of the intensity function was independent on the base-line temperature over a 30-40 degree C range. Rapid warming shifts produced a transient suppression of cold fiber activity. Previous cooling stimuli also influenced cold fiber responses, and these effects were dependent on the intensity of the previous stimulus as well as the stimulus interval. The average maximum discharge frequency of cold fibers to constant or steady-strate temperatures occurred at 30 degrees C, but varied over a 20-35 degrees C range for individual fibers. Periodic burst discharges separated by silent periods were present at steady-state temperatures of 20-35 degrees C.