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| − | The yawn is one of the most underappreciated of behaviors. It is a stereotyped and often repetitive motor act characterized by gaping of the mouth accompanied by a long inspiration, a brief acme followed by a short expiration.
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| − | Stretching and yawning (the association has gone identified as pandiculation) are neglected features among categories of maintenance behavior. Ethologists agree that almost vertebrates yawn. Yawning is morphologically similar in reptiles, birds, mammals and fishs. They may be ancestral vestiges surviving throughout evolution with little variation (phylogenetic old origins). Systematic and coordinated pandiculations occur in a compound pattern of almost identical general form among them, during the transition’s behaviors occuring with different cyclic life’s rhythms: sleep-arousal, feeding and reproduction. This kinesis appears as one undirected response to an inner stimulation, underlying the homeostasis of these three behaviors. In the human embryo, yawning occurs as early as 12 weeks after conception and remains relatively unchanged throughout life (precociouness ontogeny) see below.
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| − | Neurophysiology.
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| − | A good number of clinical and pharmacological arguments indicate that yawning involves the hypothalamus (particularly the paraventricular nucleus, PVN), the bulbus and pontic regions, with frontal region connections in primates and to the cervical medulla. The PVN is an integration centre between the central and peripheral autonomic nervous systems. It is involved in numerous functions ranging from feeding, metabolic balance, blood pressure and heart rate, to sexual behaviour and yawning. In particular, a group of oxytocinergic neurons originating in this nucleus and projecting to extra-hypothalamic brain areas (e.g., hippocampus, medulla oblongata and spinal cord) controls yawning and penile erection. Activation of these neurons by dopamine and its agonists, excitatory amino acids (N-methyl-D-aspartic acid) or oxytocin itself, or by electrical stimulation leads to yawning, while their inhibition by gamma-amino-butyric acid (GABA) and its agonists or by opioid peptides and opiate-like drugs inhibits yawning and sexual response. The activation of these neurons is secondary to the activation of nitric oxide synthase, which produces nitric oxide. Nitric oxide in turn causes, by a mechanism that is as yet unidentified, the release of oxytocin in extra-hypothalamic brain areas. Other compounds modulate yawning by activating central oxytocinergic neurons: sexual hormones, serotonin, hypocretine and endogenous peptides (adrenocorticotropin-melanocyte-stimulating hormone). Oxytocin activates cholinergic neurotransmission in the hippocampus and the reticular formation of the brainstem. Acetylcholine induces yawning via the muscarinic receptors of effectors from which the respiratory neurons in the medulla, the motor nuclei of the Vth,VIIth, IXth, Xth, and XIIth cranial nerves, the phrenic nerves (C1-C4) and the motor supply to the intercostal muscles. An arousal response accompanied by yawning behavior can be evoked by electrical and chemical stimulation of the hypothalamic paraventricular nucleus (PVN) in rats, although the mechanism responsible for the arousal response accompanied by yawning evoked by PVN stimulation is still unknown.
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| − | Ontogeny
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| − | 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 which coordinates the respiratory, cardiac and digestive regulations. These circuits that generate organized and repetitive motor patterns, such as those underlying feeding, locomotion and respiration belong to the Central Pattern Generators in the medulla (CPG) which are genetically determined, subserving innate motor behaviours essential for survival. As an example, yawning occurs as early as 12 weeks after conception and remains relatively unchanged throughout life. Its survival without evolutionary variations postulates a particular importance in terms of developmental needs. The ability to initiate motor behavior generated centrally and linked to arousal and respiratory function is a property of the brainstem reticular formation, which has been remarkably conserved during the phylogeny of vertebrates including agnathans, fishes, amphibians, reptiles, and birds. Thus, yawning and stretching have the traits of related phylogenetic old origins.
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| − | Contagiousiness of yawning
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| − | Although socially offensive to many, yawns often bring pleasure to the yawner. Hominids have the unique capacity to be receptive to the contagiousiness of yawning (echokinesis appears a better word). Yawning appear to trigger a sort of social coordination function (arousal synchrony) and reflect the capacity to unconsciously, automatically be influenced by the behaviour of others, supporting the hypothesis that contagious yawning share the neural networks involved in empathy.
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| − | Why and how yawning’s echokinesis?
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| − | Echokinesis only occurs in situations of minimal mental stimulation (public transport, for example); during prolonged intellectual effort, people are not susceptible to this phenomenon. Using functional MRI (fMRI), Schürmann et al. confirmed that during echokinetic yawning, there is no activation of mirror neurons in motor areas of the human brain (left posterior inferior frontal cortex), whereas these neurons are activated during observation of other types of facial gestures (decoding of intentionality). These ethological and neurophysiological elements demonstrate that, strictly speaking, echokinetic yawning is not motor imitation. Recognition of human faces involves specific dedicated neurons in the temporal area. The inferior temporal region (IT) allows immediate overall recognition of faces, both their identity and their expression, apparently through its own autonomous, non-hippocampal memory. As for the superior temporal sulcus (STS), it is specifically activated during perception of eye and mouth movements, which suggests its implication in the visual perception of emotions, once again by the activation of mirror neurons. These neurons mime the expression perceived, helping the observer to understand it. Schürmann et al. demonstrated that the STS is activated during echokinetic yawning. This activation, automatic and involuntarily, is transmitted to the left amygdala, the posterior cingulate cortex and the precuneus. These structures are thought to play a role in differentiating emotions expressed by the human face and, especially, in evaluating the sincerity of the sentiment expressed. Using fMRI, Platek et al. found a correlation between personality traits and the activation of neuronal circuits beyond the STS. « In contrast to those that were unaffected by seeing someone yawn, people who showed contagions yawing identified their own faces faster, did better at making inferences about memal states, and exhibited fewer schizotypal personality characteristics. These results suggest that contagious yawning might be related to selfawareness and empathic processing ». Subjects considered empathetic, who were very susceptible to echokinetic yawning, activated the amygdala and the cingulate cortex, whereas schizotypal subjects, who were not susceptible to this type of yawning, did not activate these structures. Neurophysiological studies of empathy show similar zones of activation (STS, insula, amygdala, cingulate cortex). These data imply that contagious yawning may reside in brain substrates which have been implicated in self-recognition and mental state attribulion, namely the right prefrontal cortex.
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| − | Pathology
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| − | On Tuesday, Oct.23, 1888, Jean-Martin Charcot presented, during one of his celebrated Tuesday gatherings at La Salpêtrière, the observation of a young woman inconvenienced by 8 yawns a minute, that is 480 per hour! He qualified this as a form of hysteria, despite his examination revealing binasal hemianopsia, right-side cheirobrachial skin insensitivity to all stimuli and, loss of smell. Given our contemporary knowledge, this points to a pituitary adenoma. The disappearance of yawning may be due to an extrapyramidal syndrome, to the use of opioid drugs or high doses of caffeine, but is rarely a cause for complaint. The family-medicine practice shows that excessive yawning is a source of embarrassment in social circles. There are multiple causes of excessive yawning, that is, a cluster of 10 to 30 yawns, many times a day. Of short duration, they may predict a vasovagal reaction or neurovegetative disorders (dyspepsia, migraine-like syndromes). All insults to the intra-cranial central nervous system or the hypothalamo-hypophyseal region may be involved: tumors with intracranial hypertension, infections, temporal epilepsy, strokes, etc. For example, we coined the term “parakinesia brachialis oscitans”to describe cases of hemiplegia where the onset of yawning coincides with involuntary raising of the paralysed arm. We argued that a lesion in the internal capsule affecting an inhibitory pathway liberates certain subcortical structures that coordinate the massive inspiration of yawning and the motor control associated with quadrupedal locomotion. The development of psychotropic drugs has given rise to a rich iatrogenic pathology: serotoninergic agents, apomorphine, acetylcholinesterase inhibitors, sismotherapy and, opiate withdrawal are triggers of yawn clusters. Excessive sleepiness with excess of yawns needs to search for an obstructive sleep apnea syndrom. Finally, after After ruling out the other causes, it seems possible to individualise a particular type of chronic motor tic disorder, associated with yawn clusters, and treated with haloperidol.
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| − | Conclusion
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| − | Yawning and pandiculation are a universal behaviour amongst vertebrates, closer to an emotional stereotypy than a reflex. Phylogenetically ancient and ontogenetically primitive, they exteriorise homeostatic processes of systems controlling wakefulness, satiety and sexuality in the diencephalon. An arousal response accompanied by yawning behavior can be evoked.
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