The spread of sensitization of polymodal nociceptors in the rabbit from nearby injury and by antidromic nerve stimulation.

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Abstract
Polymodal nociceptor units (93) with unmyelinated axons were isolated from rabbit sural nerves and 25 were used for control data. These showed normal sensitization on repeated heating of their receptive fields, measured here as a drop in mean heat threshold. Small injuries were made 5 (n = 15) or 10 (n = 12) mm outside the receptive fields of some polymodal nociceptors. This resulted in the development of spontaneous firing and lowered thresholds to heating of the receptive field. Local anesthetic previously injected into the site of injury blocked this spread of heat sensitization. Previous injection of saline had no effect. Antidromic stimulation of the sural nerve, proximal to the recording site, also resulted in heat sensitization of polymodal nociceptors (n = 10). Possible mechanisms for the spread of sensitization of polymodal nociceptors from nearby injury are discussed. Analogies are drawn between these results and those of Lewis (1935-36) on the spread of cutaneous hyperalgesia around a skin injury in man.