[Mechanisms and physiologic role of yawning]

As well as being present in all mammals, yawning occurs, at least in its mandibular component, in all vertebrates. The existence of pathological and pharmacologically induced yawning justifies study of this everyday ENT reflex. Its mechanism remains uncertain. The most likely hypothesis would seem to be stimulation of the reticular system of the brain-stem by signals originating in masseteric neuromuscular spindles sensitive to stretching. Serotoninergic inhibition from the dorsal raphe, a fall in hypothalamic dopaminergic inhibitory tone, followed by ocytocinergic hippocampal activation and then more diffuse cholinergic activation of cranial nerve motor nuclei seems likely. Various hormonal or socio-environmental influences can modify the activity of these different systems. The physiological consequences and the communicative value of yawning become more diversified with the phylogenetic evolution of the subject. Yawning appears to correspond to an alertness reflex which has acquired a paralinguistic value with evolution and may have a role in protection or social cohesion.