The effects of preoptic and hypothalamic thermal stimulation on tail skin temperature were observed at different scrotal temperatures. The threshold hypothalamic temperature for tail vasodilation at a scrotal temperature of 40°C was significantly lower than that at a scrotal temperature of either 25°C or 33°C. The effects of scrotal thermal stimulation on tail skin vasodilated by higher hypothalamic temperatures were observed. Cooling the scrotum from 42 to 30°C invariably caused a rapid fall in tail temperature, whereas scrotal cooling from 30 to 25°C did not cause any significant change. Cooling of either the left or right half of the scrotum caused a similar fall in tail temperature. The temperature characteristics of the preoptic hypothalamic thermo-sensitive neurons were determined at scrotal temperatures of 32, 36 and 26°C. The firing rate of warm-sensitive neurons at a given hypothalamic temperature was highest at a scrotal temperature of 36°C, while that of cold-sensitive neurons was lowest at that temperature. The scrotal temperature range over which the number of neurons activated by scrotal warming increased rapidly was between 36 and 39°C when hypothalamic temperature was held at 36-37°C.
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