Limitation of sensitization to injured parts of receptive fields in human skin C-nociceptors

Unmyelinated cutaneous mechano-heat fibers (CMH) in the peroneal nerve of healthy human volunteers were studied by means of a 'marking' technique which allows stable recordings from identified single units over extended periods. Mechanoreceptive field sizes were 105 ± 13 mm² in 25 units. These large receptive fields indicate extensive terminal branching of C fibers in the skin of foot and lower leg. Sensitization of CMHs was tested by assessment of thresholds for mechanical (von Frey hair) and heat stimuli before and after topical application of mustard oil (allyl isothiocyanate) and capsaicin (8-methyl-N-vanillyl-6-noneamide). While in a group of 14 CMHs the entire receptive field was treated with these irritant substances, in another group of 11 CMH units only parts of the receptive field were treated to check for signs of spreading sensitization through axon collaterals. Mustard oil application did not change mechanical thresholds, regardless of whether parts of or complete receptive fields were treated. However, mean heat thresholds dropped by 5.6°C to 36.5 ± 1.5°C in completely treated receptive fields and by 5.7°C to 37.3 ± 3.4°C in treated parts of receptive fields ('primary sensitization'). In contrast, heat thresholds in the non-treated parts did not change significantly (42.1 ± 3.4°C VS 41.2 ± 3.9°C), i.e. 'secondary sensitization' to heat was lacking. The absence of primary sensitization to probing with von Frey hairs indicates that sensitization of insensitive C fibers and recruitment of insensitive axon collaterals may be more important for mechanical hyperalgesia than sensitization of conventional CMH units - apart from the contribution of central mechanisms. The lack of spread of sensitization to untreated parts of the receptive fields o CMHs ('secondary sensitization') indicates that this fiber group is probably not involved in any form of secondary hyperalgesia to heating.