Spinal cord coding of graded nonnoxious and noxious temperature increases

Abstract

Neurons in layers I and IV-VII of L7 and S1 were studied in unanesthetized spinal cats. Each cell was characterized in terms of its responses to electrical stimulation of cutaneous A and C fibers, graded intensities of radiant heat applied to blackened skin, and touch, pressure and pinching. This analysis yielded five classes of units distinguished by the range of mechanical stimuli over which they responded: (I) touch (32 units), (II) touch-pressure (27 units), (III) touch-pressure-pinch (64 units), (IV) pressure-pinch (22 units), and (V) pinch only (9 units). Class I and II cells were concentrated in lamina IV and class III-V cells were distributed in laminae I, IV-VII but mainly in V and VI. In addition to the mechanical classification, some of the class II-V units also responded to radiant heat ( 66 122) and were divided into three functional groups. Warming units (10) had threshold skin temperatures between 35 and 42 C and responded maximally below 43 C. Warming-noxious units (14) had threshold responses between 35 and 42 C and maximum responses above 43 C. Noxious heat units (42) had thermal thresholds between 43-50 C and usually had maximum responses at 50 C or higher. Warming units tended to be in class II, warming-noxious units were usually class III units, and noxious heat units were in classes III-V. The responses of warming units indicated that they were adapted for coding the rate of change of skin temperature as well as the skin temperature level within the warming range. Similar to heat-induced pain thresholds and flexion reflexes, noxious heat units responded at the same temperature level (m = 44.5 C) irrespective of the rate of skin temperature change. The response of the population of noxious heat units was linearly related to skin temperature from 43 to 49 C, similar to the psychophysical curve of pain intensity over the same temperature range. Response thresholds of all thermally sensitive units were distributed continuously over a 35-50 C range. Therefore, recruitment of higher threshold units may be an important factor in coding nociceptive information. Representative types of mechanically and thermally sensitive units projected in the ipsilateral dorsolateral columns and their physiological characteristics did not differ from nonprojecting neurons. © 1975.