The sensitization of high threshold mechanoreceptors with myelinated axons by repeated heating.

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Abstract
High threshold mechanoreceptor units (HTM) (70) with myelinated axons were isolated from the sural nerves of cats and rabbits. Thirteen cat and 42 rabbit HTM were tested by controlled, repeated heating of the skin of the foot or lower leg to noxious levels. Many of the units (77% in the cat and 40% in the rabbit) fired to heating. Only 6 (11%) of these fired to the 1st brief heating to 50-55.degree. C. The rest required 2-6 heat trials before responding. Heat responding units always became more sensitive with repeated heat stimulation but their mechanical sensitivity showed no comparable changes when heat sensitization occurred. If these results are applicable to man, they suggest that HTM play little role in generating the 1st pain that follows skin heating but that they may be involved in the increased sensitivity to heat pain (hyperalgesia) shown by skin previously injured by heating.