An analysis of a thermal afferent pathway in the rat

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Abstract
Single unit activity was recorded in the thalamic, hypothalamic and raphe magnus nuclei of rats anesthetized with urethane. Neurons were sought which responded to changes in scrotal skin temperature applied with a water-perfused brass thermode. All 69 neurons in the thalamus and hypothalamus responded with abrupt changes in activity as the scrotum was warmed (switching response). The majority responded with an increase in activity from minimal to maximal firing rate as the scrotum was warmed over a range of less than 0.5.degree. C; in about 20% of the neurons the converse was observed. To determine whether the switching response of the thalamic and hypothalamic neurons depended upon a cortico-thalamic feed-back loop, the cortical surface was cooled to 18-20.degree. C to reversibly abolish cortical post-synaptic activity. Cortical cooling abolished the positive switching response of nearly all (15/19) ventrobasal thalamic neurons to scrotal warming. All 8 ventrobasal thalamic neurons with negative switching responses, and all 22 scrotal temperature-responsive neurons in other thalamic and hypothalamic nuclei were unaffected. Recordings (20) were also made from scrotal temperature-responsive neurons in the nucleus raphe magnus. All possessed switching responses similar to those observed in the thalamus and hypothalamus. None of the scrotal temperature-responsive neurons in the nucleus raphe magnus was affected by cortical cooling. Six neurons were observed in decerebrate rats with properties apparently identical to those in intact rats. The switching response of thalamic and hypothalamic scrotal temperature-responsive neurons is probably generated in the nucleus raphe magnus and passed in parallel to the thalamus and hypothalamus. Thalamic neurons depend on an intact link with the cerebral cortex for the generation of their switching responses.