Temperature-sensitive mechanoreceptors, thermoreceptors and heat nociceptors in the feathered skin of pigeons

Abstract

1. Single unit recordings from afferent fibres of a cutaneous branch of the radial nerve were done during thermal stimulation of the skin of the wing with a thermode or with radiant heat or cold. The response of all cutaneous receptors to this thermal stimulation was studied. 2. Rapidly-adapting mechanoreceptors were never excited by thermal stimulation. 3. Slowly-adapting mechanoreceptors had characteristics similar to the type I slowly-adapting mechanoreceptor in mammals. There was an excitatory overshoot during rapid cooling and a transient inhibition during warming when activated by a thermode. The phasic response depended on the rate of temperature change and on the temperature range. Radiant heating or cooling was ineffective in eliciting a response in most cases. The static curves showed a broad maximum between about 36 and 43 °C. At temperatures above about 45 °C there was always a reduced activity. 4. Two receptors with thermoreceptor characteristics were found. One cold receptor showed increased activity at low skin temperature and one warm receptor was excited by moderate heating in the temperature range 35–40 °C. 5. Heat nociceptors were excited both by radiant heating and thermode stimulation. During staircase-like increases in skin temperature the mean threshold of the response of eight heat nociceptors was 47.1 ± 1.4 °C. There was a steep increase in firing rate up to at least 52 °C. Some of the heat nociceptors were also excited by noxious mechanical stimulation.