Effects of periaqueductal gray and raphe magnus stimulation on the responses of spinocervical and other ascending projection neurons to non-noxious inputs

Experiments were performed in cats anesthetized with α-chloralose to examine the effects of stimulating in the periaqueductal gray (PAG) and nucleus raphe magnus (NRM) on the responses of spinocervical cells and unidentified ascending projection neurons to non-noxious peripheral stimuli. Peripheral stimuli consisted of low amplitude sinusoidal displacements applied to either the glabrous skin or the hairy skin of the neuron's receptive field. Stimulating in either the periaqueductal gray or nucleus raphe magnus reduced the impulse activity of most neurons in both groups. By applying brainstem stimuli at various phases of the sinusoidal peripheral stimulus, it was demonstrated that the effects of stimulating either the PAG or NRM on the responses of both types of neurons was dependent on the timing of the electrical stimuli relative to the peripheral input. The effects of stimulating in the PAG and NRM on the responses of these cells to non-noxious stimuli were reversibly blocked by naloxone. It was concluded that stimulating in the nucleus raphe magnus and in the periaqueductal gray can produce dramatic modifications in the responses of spinocervical cells and unidentified ascending projection neurons to non-noxious peripheral stimuli, suggesting a role for these descending systems in non-noxious information processing.