Neuronal responses of periaqueductal gray to peripheral noxious stimulation.

Central pathways transmit pain from peripheral regions to one of the most important area of the descending pain modulatory system, the Periaqueductal gray (PAG). Independent discoveries in the past suggest that the PAG contains afferent input, output neurons and intrinsic interneurons. An attempt was made in the present study to find out the effects of more than one kind of noxious stimulus on the PAG neuronal activity. Experiments were conducted in rhesus monkeys and the effects of noxious mechanical, thermal and tooth-pulp stimulation on the activity of 14 neurons were studied. The neurons responded to more than one kind of noxious stimuli by increasing or decreasing its firing rate. No limb specificity could be identified and homogeneous distribution of the excitatory and inhibitory neurons was found.