Differential responses of nociceptive vs. non-nociceptive spinal dorsal horn neurones to cutaneously applied vibration in the cat

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Abstract
Extracellular single-unit recordings were made from dorsal horn neurones in the lumbar spinal cord of cats which were anaesthetized or were anaemically decerebrated. Each neurone was classified functionally as wide dynamic range (WDR), non-nociceptive nociceptive specific or proprioceptive. Vibration was then applied to the hind limb using a feedback-controlled mechanical stimulator. WDR neurones had 3 distinct types of response to vibration (80 Hz; 0.3-1.0 mm): excitation, depression and a biphasic response consisting of excitation followed by depression [16]. The type of response depended upon the location of the stimulator probe. With the stimulator probe placed inside that part of the receptive field from which low intensity, non-vibrational cutaneous stimuli elicited excitation, 35 neurones were excited by the vibratory stimulation, none was depressed and 4 showed the biphasic response. On the other hand, when the probe was positioned outside the receptive field for low intensity stimuli, 7 WDR neurones were excited, 164 showed depression or the biphasic response and 7 were unaffected. On-going activity and activity evoked by iontophoretic application of glutamate were decreased during the depressant response and during the depressant phase of biphasic response. In terms .of non-nociceptive neurones, all (n = 30) were excited by vibration, depressant or biphasic responses were not observed. Excitation was elicited by placing the probe either inside or outside the receptive field for non-vibrational stimuli. All nociceptive specific neurones (n = 3) were depressed by vibration regardless of the position of the stimulus. All proprioceptive neurones (n = 12) were excited by vibration. The predominantly depressant effect of vibration on nociceptive neurones vs. the predominantly excitatory effect on non-nociceptive neurones prompts us to suggest that the increase in pain threshold and the clinical analgesia elicited by vibration may be mediated at the spinal level by a decrease in the rate of firing of nociceptive neurones and/or by excitation of non-nociceptive neurones.