INNERVATION TERRITORIES OF MECHANO-RESPONSIVE C NOCICEPTOR UNITS IN HUMAN SKIN ROLAND SCHMIDT, MARTIN SCHMELZ, MATTHIAS RINGKAMP, HERMANN O. HANDWERKER and H. ERIK TOREBJRK Departments of Clinical Neurophysiology, University Hospital, Uppsala, Sweden1 and Physiology and Experimental Pathophysiology, University of Erlangen/Nrnberg, Germany
APStracts 4:132N, 1997.
ABSTRACT
Innervation territories of single mechano-responsive C nociceptors in the skin of the leg and foot were explored in normal human subjects. Microneurographic recordings were obtained in the peroneal nerve from 70 mechano-heat responsive (CMH) and 7 mechano- but not heat responsive (CM) units. Units were identified by their constant long latency response to intracutaneous electrical stimulation of their terminals. Responsiveness to mechanical, heat or transcutaneous electrical stimuli was verified by transient slowing of conduction velocity after activation by such stimuli. We determined their thresholds to mechanical stimuli (mean 33.7 mN, median 30 mN, range 3 - 750 mN) and heat (mean 42.5C, median 42.5C, range 37- 49C). Most mechano-receptive fields (mRFs) were found on the foot dorsum (60 units), fewer on the lower leg (14 units) and toes (3 units). Most units had one continuous mRF, but 10 units had more complex fields. Areas of mRFs mapped with a von Frey filament (750 mN) ranged from 10 to 363 mm2 (mean 106 mm2). The mRFs were oval or irregularly shaped with greatest diameters ranging from 3 to 45 mm. Mean areas of mRFs were largest on the lower leg (198 mm2), smaller on the foot dorsum (88 mm2) and smallest on the toes (35 mm2). Forty-nine of the 77 units had identical mechano-receptive (mRFs) and electro- receptive fields (eRFs). Twenty-six units had larger eRFs than mRFs, while the opposite was found for 2 units only. Areas of eRFs ranged from 16 to 511 mm2 (mean 121 mm2). An estimate of the innervation density based on the present data and the presumed number of C fibers in cutaneous fascicles of the peroneal nerve suggests a considerable overlap of nociceptive endings in the skin. Such overlapping nociceptor innervation in the skin allows for substantial spatial summation in response to punctate noxious stimuli, which may be a prerequisite for a good accuracy in localizing painful events from a C-fiber input. The reduction in size of innervation territories distally allows for finer discrimination of spatial dimensions of noxious stimuli distally as compared to proximal regions of the extremities. Mean maximal diameters of the mechano- receptive fields of CMH and CM units on the lower leg (22.3 mm) and foot (15.3 mm) are of similar size as the radius of axon reflex flares evoked by noxious mechanical stimuli in these regions. 

Received 8 April 1997; accepted in final form 3 July 1997.
APS Manuscript Number J283-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 24 July 1997