mise à jour du
25 avril 2010
Yawning and the reticular formation
Jean Askenasy and Enosh Askenasy


The act of yawning begins during fetal life. The complex biochemistry involved in this action includes many enzymes and neurotransmitters, including: dopamine, acetylcholine, muscarine, histamine, adenosine, serotonin, nitric oxid, adrenocorticotropic hormine, oxytocin, alpha-melanocyte hormone, opioids and gammaaminobutyric acid.
An early research into the physiology of yawning was conducted by Charcot. His famous case was a woman with relentless yawning who was hospitalized for months in Salpetriere hospital. This patient presented with a yawning frequency of 8 yawns per minute, and Charcot noted that although her breathing pattern was severely disturbed her ventilation was not reduced.
Charcot then realized that the patient's yawning was functioning as deep inspirations replacing normal breathing movements (breathing through yawning). A single yawn can therefore be viewed as an isolated act of hyperventilation in which the oxygen saturation is increased and the PCO2 reduced.
K. P. van de Woestijne and D. Trop have demonstrated in their research on dogs that alveoli collapse to 60% of their initial volume after 2 hours of anaesthesia. A similar finding has been observed in humans. The alveolar collapse suggests the existence of shunting of venous blood causing a decrease in blood oxygenation. A single deep inspiration or yawn can restore the alveoli to their initial capacity. This finding has led Forrester to suggest that the function of yawning is maintenance of pulmonary alveolar patency, and that yawning serves as a defensive mechanism against alveolar collapse.
Observations in rats have shown an association between yawning and erection. Additional experiments found that castrating animals causes a decrease in yawning frequency and that a later course of testosterone replacement has the reverse effect of increasing yawning frequency. Since erection is function of autonomically mediated vasodilation the connection of yawning to autonomic stimulation became an area of experimental interest.
The deep inspiration produced by yawning causes the dilatation of lung bronchial muscles, stimulating a vagal response with discharge of acetylcholine, and inhibition of sympathetic&endash;adrenergic activity. Inhibition of sympathetic activity causes arterial dilatation which diminishes arterial resistance and accelerates the arterial circulation (Friedell 1974, Lehman 1979, Twiest 1974). The decreased sympathetic activity during yawning was demonstrated by direct recording with the microneurographic technique by Askenasy and Askenasy (1996). Shown in the picture is a decrease in sympathetic activity following every yawn:
During the yawning movement additional muscular contraction occurs in addition to diaphragmatic contraction. This movements are termed gasping (the wide opening of the mouth) and pandiculation (limb stretch). Gasping and pandiculation serve a vascular effect by discharging the venous plexuses located within the pterygoid and soleus muscles respectively into the circulation increasing the blood volume available for oxygenation. This mechanism is named "the peripheral hearts" or "the sural and tricipital pump". (Bhangoo 1974,Last 1963).
Arrows point to venous plexus location:
Pterygoid muscles and Soleus muscle
Yawning is considered by Bell and Suganami as being provoked by boredom, a consequence of diminished interest and stimulation by a source of information. The difficulty in maintaining cortical attention becomes a neural stimulus named "down-attention" which announces a change in the cerebral state.
The harmonic inter-connection between yawning, breathing and even coughing implies to the existence of a perfect synchronization mechanism of the respiratory, cardiovascular, and muscular systems. Such intricate control is naturally found in the central nervous system. The brain areas involved in the elaboration of such a harmony are: the neocortex, the límbic system, the hypothalamic-hypophyseal system and the reticular system.
Neocortical involvement in the regulation of yawning is demonstrated by the contagiosity of yawning. Simply viewing another person yawn may be a sufficient trigger for a yawn by the observer (Provine 2005). Involvement in yawn regulation of the parvocellular oxytocinergic neurons in the paraventricular nucleus of the hypothalamic system was proved experimentally by Kita I, and the involvement of the autonomic system was proved by microneurography (Askenasy & Askenasy). Reticular involvement is the subject of the present presentation.
The generally accepted causes of yawning are boredom, fatigue and drowsiness. The yawning center was hypothesized to be localised in the reticular formation (Askenasy 1989). The reticular formation is located in the brainstem. The brain stem is located between the thalamus and the spinal cord. This structure, with a width of only an inch has a critical function in the brain.
The brain relates to the environment through sensory information reaching the thalamus and to the body through ascending neurons from the spinal cord. The Corpus callosum connects the two hemispheres of the brain, and the pons (latin for bridge) connects the two sides of the cerebellum, connecting each side of the cerebellum with the opposite side and with the cerebral hemispheres. The Pons also connects the upper part of the nervous system with the medulla oblongata. The Medulla oblongata is the caudal part of the brain stem and sits above the top end of the spinal cord Both types of inputs supply the reticular formation. In the time it takes you to say these two words, the medulla oblongata will have regulated your breathing, blood pressure, heart rate and all cathartic reactions in the states of sleep and wakefulness.
Salzarulo intuition about the signaling role of yawning was stipulated in his theory about the stabilization effect on transitory periods of the sleep/wake cycle.
Karasawa and his team have studied the consequences of serial yawning on the electroencephalography and O2 saturation in patients with cerebral vascular accidents of the thrombotic type (1982). They found a significant decrease in electroencephalographic activity and yawning associated with a decrease in the partial pressure of sanguine oxygen, as it occurs during boredom, somnolence and fatigue. The authors stipulated that this effect is caused or provoked by a decreased activity of the ascending reticular system.
Monatagu (1962) consider that the reticular system stimulation is the result of the accelerated blood circulation and hyperoxygenation.
Robert Provine, suggested several times yawning as a result of a disturbed harmony of the nervous system.
Yawning entails a perfect match of the rhythmic vital functions of breathing and the cardiovascular circulation under a harmonic regulation of the nervous system. The proximity of the centers of these functions and the yawning center in the reticular formation explains the perfect harmony.
Yawning is a signal initiated by the reticular system and brough to our consciousness, that a change in the functional state of the nervous system occurs and need help.
Any change in the steady state as perceived by the brain provokes a reaction by the reticular formation. Therefore a yawn may be triggered by a diverse array of states which represent a change in situation: boredom, sadness, surprise, suffering, fatigue, stress, somnolence...
The reticular system's anatomy of intense proximity (under 2 inches) of both activating and inhibitory neuronal networks may explain why yawning appears in situation which are seemingley paradoxal such as boredom and excitation.
Inaccuracies in the harmonic regulation function occurs on pathologic basis, at different levels of the central nervous system, such as the cortical, limbic, hypothalamic and autonomic nervous system, but always involving the essential regulator "the reticular network".
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