Facilitation of the responses of primate spinothalamic cells to cold and to tactile stimuli by noxious heating of the skin

Spinothalamic tract cells in anesthetized monkeys were found to respond to noxious cold stimuli ($\text{18}\text{19}$ cells tested), as well as to noxious heat and noxious mechanical stimuli. Responses to repetition of the noxious cold stimuli after a series of noxious heat stimuli were enhanced. However, subtraction of the enhanced background activity that resulted from damage of the skin revealed that the enhanced responses to noxious cold stimuli were due to superposition of the original responses upon an enhanced background activity, rather than to sensitization of the responses to noxious cold stimuli per se. Furthermore, the responses to innocuous mechanical stimuli applied either within the area that was damaged or outside this area were enhanced, provided the noxious heat was applied for a long enough time. Thus, damage to a region of skin can result in enhanced responsiveness of spinothalamic cells to stimuli applied in an undamaged region of the receptive field. The possible relationship between these observations and cutaneous hyperalgesia is discussed.