Thin fibre territories of nerves innervating hairs in the human forearm estimated from axon reflex vasodilatations
B. Gunnar Wallin, Leif Hultin, Göran Pegenius and Anne-Lene Krogstad
Institute of Clinical Neuroscience, Unit of Clinical Neurophysiology, Sahlgren Hospital, University of Göteborg, Sweden
To study the territories of thin nerve fibres innervating hair follicles, we extracted single hairs from forearm skin. Scanning laser Doppler methodology was used to measure the evoked local increase of skin perfusion, the underlying assumption being that axon reflex vasodilatation would be evoked within the territory of extraction-activated thin nerve fibres. Ninety-two single hairs were extracted in 14 healthy males.
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 extraction-evoked pain correlated with the peak area of the response. In addition, after pre-extraction local anaesthesia, response components were seen in only 50 % of the cases and when they occurred they were very small.
The response had two components which could occur independently of each other. An early short-lasting component consisted of one or several separate areas with a peak total extension of 176 ± 176 mm2 (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 consisted of a single area of 51 ± 107 mm2, an intensity of 342 ± 301 % and a duration of up to approximately 60 min. Perfusion could be influenced from a single hair in an asymmetrical skin area with diameters at right angles of 23 ± 9 and 16 ± 9 mm, respectively.
We suggest that the responses were evoked by two sets of thin nerve fibres, one at a superficial level with fairly large innervation territories, and the other located more deeply close to the hair follicle and with smaller innervation territories.
