Threshold temperatures of diencephalic neurons responding to scrotal warming.

Neurons in the rat's thalamus and hypothalamus abruptly change their firing rates from minimum to maximum, or vice versa, with increase in scrotal temperature of 1 degree C or less ("switching response"). The threshold temperature of the switching response was compared in pairs of neurons simultaneously recorded from the right and left thalamus or from the right thalamus and the left hypothalamus. In dynamic conditions in which scrotal skin was gradually warmed at rates of 1-7 degrees C/min, the threshold temperatures of each neuron pair differed by only 0.4 degree C or less. The threshold temperatures of static responses of each pair of neurons, which was determined with step changes in scrotal temperature, fell in a range less than 0.8 degree C. Scrotal warming produced desynchronization of cortical EEG which was monitored during unit recordings. The change in EEG occurred at about the same scrotal temperature at which the switching responses of the diencephalic neurons were elicited.