Yawn Reflex
The yawn reflex (pandiculation) is not a classical neural reflex arc, but rather a coordinated neural and muscular arousal reflex mediated by the brain stem. A yawn is a sequence of events that begins with a deep inspiration of air and ends with a forced and deep expiration of air concurrent with a general contraction of several muscle groups, especially those associated with mouth, throat, and face. Yawns last four to eight seconds and the yawn reflex is well established by the second trimester of fetal development. Male yawns tend to be more vigorous than female yawns. The exact mechanisms stimulating the yawn reflex remain scientifically contentious.
Anatomically, the yawn is a coordinated movement of thoracic muscles, diaphragm, larynx, and palate. The chest wall expands, the diaphragm lowers, the palate rises and there is a tendency to move the tongue downward and to the rear of the mouth. Groups of facial muscles contract, there is an abduction of the vocal cords during expiration. Yawning may also be associated with a general stretching of muscles in the neck, arms, and legs. During yawning, the eyes usually close and tearing (lacrimation) may occur.
At one time, physiologists argued that the yawn reflex was a brain stem reaction to high levels of carbon dioxide (CO2) in the blood stream. The deep inspiration and expiration was argued to allow the intake of extra oxygen during the deep inspiration and the explosive venting of built up carbon dioxide during the deep expiration. That yawning is often associated with boredom was explained by the fact that the slower metabolic rate in sedentary or bored individuals resulted in slower and shallower respiration that allowed the build-up of carbon dioxide. Recent research involving controlled inspiration of mixtures of air with varying amounts of oxygen failed to clearly indicate or support the inspiration-ventilation hypothesis. Moreover, researchers have also discovered that there is not necessarily a direct link between the level of alertness and yawning. In some studies, the number of yawns was not altered by exercise. Accordingly, some physiologists now argue that different physiological mechanisms control the respiration and yawning.
Yawning may be mediated or controlled by the hypothalamus. Increased levels of neurotransmitters, neuropeptide proteins, and certain hormones in the hypothalamus are associated with yawning.
The yawn reflex does have other important physiological and anatomical effects. During a yawn, there is often an increase in blood pressure and heart rate. A yawn reflex also serves as a mechanism to initiate the contraction of several muscles and muscle groups
Yawning occurs in many species, and there are social implications to the yawn reflex. Accordingly, there are sociological and evolutionary biology based explanations of some aspects of the yawn reflex, especially the "contagious" aspects where a yawn by one person in a group is followed by a chain of yawning in other members of the group. One hypothesis is that contagious yawning served to coordinate group behavior and acted as a signal initiated by external stimuli (or lack thereof). Some evolutionary biologists argue that the pronounced jaw movements associated with yawning are a remnant of primitive teeth display behavior.
Excessive yawning can also be a symptom of various neurological problems including encephalitis and tumor growth.
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