Neurones in the somatosensory cortex of the rat responding to scrotal skin temperature changes

1. In rats the scrotal temperature was raised or lowered with a water-perfused thermode while micro-electrode recordings were made of unit activity in the somatosensory (SI) cortex. The electrodes were inserted in the area where the largest evoked potentials had been found from electrical stimulation of the scrotum.2. Changes in firing rate of cortical neurones were found only in the scrotal temperature range of 32-41 degrees C. Within this range 40% of all the cells tested were excited or suppressed by skin warming. At temperatures above or below this range, activity was not affected. Most of the cells responded just to temperature and only 14% were also excited by touch.3. Raising temperature in the range 33-41 degrees C caused 83% of the thermally responding cells to decrease their firing rate and 17% to increase their rate. Individual neurones showed a sudden and maintained change in their activity for scrotal temperature increases of only 2, 1 or even 0.5 degrees C. Mean firing rates changed several-fold with these temperature increases and further warming did not change the rate. These step-like changes in firing rate were found at different points over the whole skin temperature range of 33-41 degrees C but most were between 35 and 39 degrees C.4. For a given neurone the step-like change in activity occurred once its critical temperature was reached, irrespective of whether this was achieved by a step increase of skin temperature over 1-2 sec, or by a slow ramp increase lasting several minutes.5. The results are very similar to those found in the thalamus (preceding paper), but the proportions of cortical cells which were excited or suppressed on skin warming were the reverse of the proportions seen in the thalamus.