Response characteristics of specific thermoreceptive afferents innervating monkey facial skin and their relationship to human thermal sensitivity

Abstract

The responses of thermoreceptive afferents to steady-state constant skin temperatures and temperature shifts were studied in rhesus monkeys by recording single fiber activity from fine dissected strands of the infraorbital nerve. Eighty-five warm and 134 cold fibers were identified by their specific sensitivity to warming and cooling of their receptive fields, which usually were restricted to single spots less than 1 mm in diameter. Warm fibers increased their firing rates with warming and decreased or ceased firing with cooling, while cold fibers showed an opposite behavior. Mean conduction velocity was 3.4 ± 1.7 m/s in warm fibers, and 9.0 ± 3.4 m/s in cold fibers. In response to constant skin temperatures in the innocuous temperature range below 43°C, warm fibers were active over the range of 30–42°C and showed maximum firing rates at 42°C. On the other hand, cold fibers were responsive to constant temperatures over the range of 20–42°C and their firing frequencies were maximal at 30°C and decreased as steady-state temperatures were raised or lowered. In addition, cold fibers exhibited periodic bursting activity when the temperature was below 35°C. The number of intraburst spikes in each burst increased as the constant temperature was