Pain
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(1) Sixty-eight convergent dorsal horn neurones have been recorded at the lumbar level in anaesthetized intact rats. All cells received prominent A alpha and C fibre afferents and correspondingly could be activated by high and low threshold stimuli applied to the peripheral excitatory receptive field. (2) The activity of 67/68 of these neurones was powerfully inhibited by noxious stimuli applied to various parts of the body. Since non-noxious stimuli were ineffective in this respect, the term "diffuse noxious inhibitory controls" (DNIC) is proposed. (3) DNIC could be evoked by noxious pinch applied to the tail, the contralateral hind paw, the forepaws, the ears and the muzzle; the most effective areas were the tail and muzzle. Noxious heat applied to and transcutaneous electrical stimulation of the tail were extemely effective in eliciting DNIC as was the intraperitoneal injection of bradykinin. (4) DNIC strongly depressed by 60-100% both the C fibre response following suprathreshold transcutaneous electrical stimulation and the responses to noxious radiant heat. (5) The spontaneous activity and the responses to low threshold afferents induced either by A alpha threshold electrical or natural stimulation were also powerfully inhibited. (6) In the majority of cases, long lasting post-effects directly related to the duration of conditioning painful stimulus were observed.
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Electrical potentials evoked by 5 intensities of painful dental stimulation were recorded at the scalp. During testing, volunteers indicated subjective painfulness by verbal pain ratings and visual analogue scales. Evoked potentials (EPs) to each intensity, observed between 50 and 400 msec, were characterized by 4 waveform components. ⋯ In contrast, the amplitudes of the two later components were associated with subjective painfulness but not with stimulus intensity. A strong linear relationship was observed between subjective painfulness and peak-to-peak amplitude for the EP component observed between 175 and 260 msec. The data suggest that the earlier EP components may reflect sensory transmission processes while the later components indicate brain activity when pain is perceived.
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Comparative Study
Diffuse noxious inhibitory controls (DNIC). II. Lack of effect on non-convergent neurones, supraspinal involvement and theoretical implications.
(1) Diffuse noxious inhibitory controls (DNIC) were tested for their effect on noxious only, non-noxious and proprioceptive cells in the dorsal horn of the intact anaesthetized rat. Unlike convergent neurones, as described in the previous paper, there was no effect of DNIC on these neurones. ⋯ Thus the neuronal substrate for DNIC must involve supraspinal structures. (3) Because of the level of firing in convergent neurones induced by hair and touch receptors, presumably constantly and randomly activated in the freely moving animal, a noxious message arriving at higher centres may be partly masked by this background noise. On the basis of the known role of convergent neurones in nociception, we propose the following mechanism which may interpret this paradoxical convergence: two pools of convergent neurones are influenced by a painful peripheral stimulation, one segmental pool being activated whilst the remaining population of cells is inhibited; the "contrast" between the messages from these two pools may well produce a significant pain signalling output from the convergent dorsal horn cells. (4) These results and their theoretical implications are discussed with regard to the concept of the "analgesic system", certain clinical observations and the paradoxical pain relieving effects of counterirritation and some forms of acupuncture.