Thin fibre territories of nerves innervating hairs in the human forearm estimated from axon reflex vasodilatations

1. To study the territories of thin nerve fibres innervating hair follicles , we extracted single hairs from forearm skin. Scanning laser Doppler metho dology was used to measure the evoked local increase of skin perfusion, the underlying assumption being that axon reflex vasodilatation would be evoke d within the territory of extraction-activated thin nerve fibres. Ninety-tw o single hairs were extracted in 14 healthy males. 2. In 93% of the cases perfusion increased transiently near the site of the extracted hair. No responses occurred when arm blood flow was occluded. In support of an underlying axon reflex mechanism the intensity of hair extra ction-evoked pain correlated with the peak area of the response. In additio n, after pre-extraction local anaesthesia, response components were seen in only 50% of the cases and when they occurred they were very small. 3. The response had two components which could occur independently of each other. An early short-lasting component consisted of one or several separat e areas with a peak total extension of 176 +/- 176 mm(2) (mean +/- S.D.), a peak maximal intensity (in percentage of pre-extraction perfusion) of 484 +/- 272%, and a duration of 6-8 min. A later long-lasting component consist ed of a single area of 51 +/- 107 mm(2), an intensity of 342 +/- 301% and a duration of lip to approximately 60 min. Perfusion could be, influenced fr om a single hair in an asymmetrical skin area with diameters at right angle s of 23 +/- 9 and 16 +/- 9 mm, respectively. 4. We suggest that the responses were evoked by two sets of thin nerve fibr es, one at a superficial level with fairly large innervation territories, a nd the other located more deeply close to the hair follicle and with smalle r innervation territories.