In recordings from the human supraorbital nerve with tungsten microelectrodes, eleven afferent units with unmyelinated (C) axons were identified on the basis of their conduction velocities (0.6-1.4 m/s). 2. Eight units had low mechanical thresholds (less than or equal to 0.23 g) and could be activated up to their maximal firing rates of about 100 impulses/s by weak tactile stimuli, whereas three units had higher thresholds (5.5 g) and responded vigorously to noxious stimuli only. 3. During a skin indentation the low-threshold units adapted to an irregular low-frequency discharge, and release of the stimulus elicited a prominent off-response often ending with an after-discharge. Slow stroking was a particularly effective stimulus, even when done with cotton wool, whereas rapid stroking reduced the response. All types of stroking stimuli were occasionally followed by after-discharges. Repeated mechanical stimulation at short intervals resulted in a decline of the response, indicating receptor fatigue. For two units a response to skin cooling was observed. 4. The above low-threshold C units have all the main characteristics of the C mechanoreceptors known from the cat and primates but not previously proven to exist in man. The high-threshold C units are similar to the polymodal nociceptors found in other human skin areas.
