Spatial and temporal transformations of input to spinothalamic tract neurons and their relation to somatic sensations

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Abstract
A total of 46 spinothalamic tract neurons and over 20 presumed interneurons of lumbar L6-L7 spinal cord dorsal horn were studied in rhesus monkeys whose carotid arteries were bilaterally ligated and who were maintained on 66% N2O and 34% O2. This analysis was undertaken to determine whether spinothalamic tract neurons contribute to spatial radiation of painful sensations, temporal summation of second pain, and touch-evoked aftersensations. Each cell was characterized in terms of its antidromic responses to stimulation of the ventral posterior lateral thalamic nucleus and its orthodromic responses to controlled thermal, mechanical, and electrical stimulation of receptive fields. This analysis yielded mainly 4 classes of units distinguished by the range of their responses to stimuli applied to their receptive fields. Class 1 and 2 units responded maximally to innocuous tactile stimuli. Class 3 neurons had receptive fields that contained a central zone, wherein touch activated the neuron, and peripheral zones, wherein only pressure or noxious pinch caused impulse discharge. Maximal frequencies were elicited by noxious stimuli applied to central zones. Class 4 neurons responded to firm pressure and maximally to pinch with serrated forceps. Both class 3 and 4 spinothalamic tract neurons responded when receptive-field skin temperatures were increased into the noxious range (> 45.degree. C). Brief repetitive noxious stimuli evoked responses analogous to human sensations. Each single electrical or heat pulse reliably evoked 1st and 2nd pain in human subjects and short- and long-latency neuronal responses in individual spinothalamic neurons. Prolonged temporal summation of heat-induced 2nd pain may be the result of spinal cord facilitative mechanisms activated by C polymodal nociceptive afferents. Movement of a camel''s hair brush 5 cm in 5 s along the major axis of receptive fields evoked long-lasting afterdischarges (20-56 s) in the majority of class 3 neurons tested. The same stimulus reliably evoked aftersensations when applied to the feet of human subjects. Output from class 3 neurons may play a role in touch evoked aftersensations in primates.