Modulation of human jaw reflexes: heterotopic stimuli and stress

The inhibitory reflexes in jaw elevator muscles, which are the predominant muscle responses to stimuli in or around the human mouth, are subject to modulation by nociceptive stimulation of remote parts of the body. The evidence for, and nature of, these modulatory effects are reviewed with particular emphasis on the reflex inhibition of masseteric activity evoked by electrical stimulation of the upper lip. This reflex is markedly reduced in magnitude by noxious stimulation of remote parts of the body surface or deeper tissues. Qualitatively similar effects on this reflex have been evoked by experimental stress and other psychological manipulations. However, recent research has eliminated the possibility that the modulatory effects of remote noxious stimuli act by inducing stress; it is more likely that they are a manifestation of the phenomena known as diffuse noxious inhibitory controls which act via inhibitory pathways originating in the medulla and ultimately producing post-synaptic inhibition of interneurones in the trigeminal nuclei.