Innervation territories of mechanically activated C nociceptor units in human skin

Innervation territories of single mechanically activated 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 42.5°C, median 42.5°C, range 37-49°C). Most mechano-receptive fields (mRFs) were found on the foot dorsum (60 units) and some 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 mRFs and electro- receptive fields (eRFs). Twenty-six units had larger eRFs than mRFs, whereas the opposite was found for two 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 skiff. Such overlapping nociceptor innervation in the skin allows for substantial spatial summation in response to punctate noxious stimuli which may be a prerequisite for high 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 with 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.