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How is a yawn triggered?
O. Walusinski & B. Deputte
How is a yawn triggered?
Neurophysiology of yawning
The curious phenomenon of contagious yawning
Echokinetic yawning, theory of mind, and empathy
Video of a typical Yawn
Flavio Aloe Yawning
Daquin G, J Micallef, O Blin Yawning
Smith EO Yawning: an evolutionary perspective
Why do people yawn ? 
Neural basis of drug induced yawning Cooper SJ, Dourish CT
Cerebral structures controlling yawning among vertebrates
The facial bone structure and the brain become distinct starting from a common embryonic structure, the ectoblast. The cephalic pole comprises an original embryological encephalo-facial and encephalo-cervical segmentation with a strict topographical correspondence: the naso-frontal and premaxillary structures are joined to the anterior brain; the maxillo-mandibular and anterior cervical structures are joined to the brainstem and its nerves. At the beginning of the third month, the embryo becomes a fetus with the occurrence of the first oral and pharyngal motor sequences under the control of the neurological development of the brainstem: development of the suction-deglutition and yawning activity. Therefore, suction and yawning have the same embryological origin, which shows the importance of the brain stem in the neurophysiological development of the oropharyngeal activity coordinated with the respiratory, cardiac and digestive regulations which have the same neuroanatomical localisation. Starting at the 12th week of pregnancy, echography reveals yawning and suction activities, at a developmental stage when the brain stem is already individualised and the pituitary gland has become functional, whereas the extension of the temporal and frontal neocortex takes up to 22-24 weeks to reach completion.

Until now, no specific cerebral structure has been identified as a yawning centre. A good number of clinical and pharmacological arguments indicate that yawning involves the hypothalamus (particularly the paraventricular nucleus), the bulbus and pontic regions, with frontal region connections in primates and to the cervical medulla. Muscles which contract during a yawn are controlled by cranial nerves 5,7,9,10,11,12, cervical nerves C1-C4 (phrenic nerve) and dorsal nerves innervating the intercostals, or accessory breathing muscles. During the few hours of life of anencephalous babies, it has been noted that they yawn and stretch, a sign of the mammalian syndrome of awakening activity or « Rekel Syndrom ». Patients afflicted with the locked-in syndrome, still yawn, despite being paralysed. This shows that yawning originates in the brain's archaic structures common to all vertebrates.

The central nervous system is based on a common overall organisational plan and reveals, from the most ancient to the most recent vertebrates, a gradual increase in complexity corresponding to life levels that are increasingly independent and functionally developed. Yawning helps understand the phylogenesis of the encephalon by inferring a functional organisational pattern of the nervous system similar to that advanced by Paul MacLean with the superposition of :

- an ancestral "reptilian" brain (brain stem and central gray nuclei), where yawning originates,

- a "paleomammalian" brain (limbic system) common to all mammifers, functioning as a synaptic and humoral interface, in fact the seat of the monkeys' emotivity yawn,

- a "neomammalian" brain characterised by human's cortical development, particularly the frontal lobes, seat of the "contagious" yawn.

Vigilance and yawning

Let's forget about the old theories disproved by contemporary explorations. Yawning does not oxygenate the brain (a marathoner would have to yawn at each stride!), does not alter the thyroid's activity. Ample inspiration hinders the venous return to the heart, thus increasing the peripheral venous pressure; this contributes indirectly to the flow of cephalo-rachidian fluid, but without any identifiable consequence (5,6).

Sleep and awakening regulation is controlled by some 15 different and redundant circuits, mainly located in the pons (adrenergic), the peduncle (dopaminergic), the hypothalamus (histaminergic), the Meynert basifrontal region (cholinergic). There are two permissive networks controlling awakening that must be inhibited for sleep to occur. Experiences realised on rodents and non-human primates confirm the role of each of these neurotransmitters in triggering yawning.

How is a yawn triggered?

At the moment there is no definite and irrefutable data on this. Here are some of the proposed mecanisms.

Vegetative data: the controls exercised by the autonomous nervous system (or vegetative) on the major vital functions are modulated by states of vigilance. The balance between sympathetic and parasympathetic activity is modified depending on the state of vigilance at play. There is a tendency to a progressive increase in vagal (parasympathetic) activity from waking to slow sleep with a peak during tonic paradoxical sleep, while sympathetic activity decreases almost proportionally. During sleep, this leads to a clear reduction of muscle activity in the upper respiratory airways. It reaches its maximum during paradoxical sleep, which is of longer duration at the end of the night, and combines overall peripheral muscular hypotonia with a tendency to collapsus of the upper respiratory airways. At awakening, the yawning and stretching open the pharyngo-larynx to its maximum and activate the return of muscle tone, which increases cardiac frequency, blood pressure, muscle metabolism, all associated with a loosening up of articulations. Repeated stress experiments trigger the suppression of paradoxical sleep in association with the disappearance of yawning. This seems to confirm the close link between paradoxical sleep and yawning, which does not mean that the effect of stress does not count.

Reflex data: fatigue, boredom, lack of sleep, etc, have an impact on muscle tone, which is perceived by the nervous system (deep sensibility). The control of muscle tone in the nape of neck (trapezius) and of the masseters is one of the elements contributing to the triggering of our awakening. The modification of this tone would be the triggering event of the yawning reflex. During the powerful contraction caused by yawning, the spindles of the masticatory muscles (masseters, temporal, inner and outer pterygoid), which have receptors that respond to stretching, send influxes via afferent fibres of the Ia category, which are located in the mesencephalic root of the trifacial nerve. With the motoneurons of the same muscles these fibres form a monosynaptic link. This is the basis of the masseteric reflex. These fibres have projections on the reticular formation and the locus cœruleus (two structures involved in the awakening mechanisms) which are anatomically close to the nucleus of the trifacial nerve (motor nucleus of the mastication muscles). Through the massive contraction of the masseteric muscles, yawning stimulates those structures responsible for cortical activation. The fact that the amplitude of the masseteric reflex varies in parallel with the level of vigilance constitutes another argument.

Under this perspective, yawning is triggered by the stimulation of reticular activity and of the locus cœruleus, thus acting as a vigilance reflex, as confirmed by the nature of neurotransmitting secretions.

Abstract in English ..Why do people yawn ?
Yawning : an evolutionary perspective
The neural basis of contagious yawning
Contagious yawning: the role of self-awareness and mental state attribution
Empathy and contagion of yawning