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 Yawning: an evolutionary perspective Smith EO







mise à jour du
27 mai 2006
Sleep Res
Yawning: a vigilance-enhancing factor?
HJ Aubin et L Garma
centre d'étude et de traitement des troubles du sommeil
Hôpital La Salpêtrière, 75013 Paris
en français
Yawning is a common behavior among animals and is attested all through man's life; paradoxicelly medicine has shown scant Interest in it. Even within the field of sleep-wake disorders, it is hardly considered as a useful sign. However thanks to present developments in neurobiological research, new ways of observing the specific yawning behavior are now available.
Why yawning ?
Until now the most common hypothesis was that yawning augmented vigilance owing to the increasing arterial oxygenation following ventilatory act. This hypothesis has been recently infirmed by Provine et al (1), when they showed that inhalation of a C02-enriched gas increased respiratory frequency, but had no effect on yawning occurrence frequency.
We submit that yawning could stimulate, through a powerful contraction of masseteric muscles, structures in charged of cortical activation (2).
- When yawning occurs, a ventilatory aspect and a tonic temporo-mandibuler aspect are associated. For yawning is not simply opening the mouth: It is the simultaneous contraction of antagonistic muscles: masticatory muscles, jaw closing, and digastric muscles, jaw opening. During this usually very powerful contraction, masticatory muscle spindles, being receptors sensitive to the elongation of muscles fibers, discharge through the afferent la categary fibers, located in the mesencephelic trlgaminal root. These fibers establish a monosynaptic connection with the motoneurons of the same muscles (3). The masseteric reflex is based on it.
In the case of "encéphale Isolé" cat, the cranial nerves remain intact, while the spinal cord afferences are interrupted; the sleep waking cycle is retained in this condition. When severing successively all the cranial afferences, the cutting up of the trigeminal afferences is the only one which triggers immediate sleep with spindles (4). This fact emphasizes the role of the trigeminal afferences activity in wakefulness.
It may be inferred that these fibers project, throuph the mesencephalic trigeminal nucleus, to the reticular formation and to the locus coeruleus, whose role in wakefulness mechanisms is well-known. The fact that masseteric reflex amplitude very in a concomitant way with levels of vigilance could reinforce this point of view (5). Yawning would then appear to be a vigilance reflex through the stimulation of reticular formation and locus coeruleus activity.
How does one yawn ?
Another way of considering the physiological aspects of yawning is throuph the study of the central mechanisms which lead to it, investigated mainly by pharmacologists. Recent studies of drug induced yawning in rats have put to light some neurobiological mechanisms.
Inhibitory dopaminergic pathways are at the onset of a number of neuronal sequences:
- Mesolimbic dopaminergic neurons (A 10) having synaptic connections with septohippocampic cholinergic neurons (6);
- Nigrostriatal dopaminergic neurons (Ag) having synaptic connections with cholinergic neurons of the striatum, modulated by the dorsal raphe (B7), through presynaptically serotoninergic link (7);
- Hypothalamic dopaminergic neurons, being oxytocin secretion inhibitors; oxytocin, activating pituitary ACTH release, which increases the hippocampic cholinergic turn-over (8).
These three neuronal sequences lead to a common cholinergic final pathway, perhaps in the hippocampus, that activates a Yawning Central Pattern Generator (6). However such drug induced yawning in rats may differ from spontaneous yawning In humans.
Yawning is often thought to "reactivate" vigilance. Commonly, however, an increase in the level of vigilance co-occurs with an increase of anxiety. In the present case it all happens differently. As is well known, when yawning, one experiences a particular psychic state: a dimming of awareness followed by a particular state of relaxation, while worries, recade temporarily.
Could yawning bean anxiolytic, or "antistress", behavior, which not only would not decrease, but actually enhance, the level of vigilance?
Contrary to a commonly received notions, yawning does not lead to sleep but to wkaefulness.
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