Inhibitory effects of nucleus raphe magnus on neuronal responses in the spinal trigeminal nucleus to nociceptive compared with non-nociceptive inputs

The effect of stimulation in nucleus raphe magnus on neurones in the spinal trigeminal nucleus was studied in decerebrate cats. Short trains of raphe stimulation reduced or abolished the responses of neurones in nucleus oralis and nucleus caudalis to nociceptor stimulation (tooth pulp or cornea) but had a much weaker or insignificant effect on responses to non-noxious inputs (hair movement or low intensity stimulation of the infraorbital nerve). Maximum inhibition was seen 10--50 msec following the raphe stimulation and persisted up to 300 msec. The depression of neuronal responses was paralleled by an increase in the threshold of the jaw-opening reflex evoked by tooth pulp stimulation. The relation to stimulus-produced analgesia is discussed.