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Motion sickness or kinetosis, also known as travel sickness, is a condition in which a disagreement exists between visually perceived movement and the vestibular system's sense of movement. Depending on the cause, it can also be referred to as seasickness, car sickness, simulation sickness or airsickness. Dizziness, fatigue, and nausea are the most common symptoms of motion sickness.〔(Motion Sickness Prevention and Treatment )〕 Sopite syndrome, in which a person feels fatigue or tiredness, is also associated with motion sickness. "Nausea" in Greek means seasickness (''naus'' means ship).〔(Woodhouse's English-Greek Dictionary Page 745 )〕〔(Woodhouse's English-Greek Dictionary Page 766 )〕 If the motion causing nausea is not resolved, the sufferer will usually vomit. Vomiting often will not relieve the feeling of weakness and nausea, which means the person could vomit until the cause of the nausea is treated. ==Cause== The most common hypothesis for the cause of motion sickness is that it functions as a psychological defense mechanism against neurotoxins.〔(Motion sickness: an evolutionary hypothesis )〕 The area postrema in the brain is responsible for inducing vomiting when poisons are detected, and for resolving conflicts between vision and balance. When feeling motion but not seeing it (for example, in a ship with no windows), the inner ear transmits to the brain that it senses motion, but the eyes tell the brain that everything is still. As a result of the discordance, the brain will come to the conclusion that the individual is hallucinating and further conclude that the hallucination is due to poison ingestion. The brain responds by inducing vomiting, to clear the supposed toxin. An alternative theory, also known as the Nystagmus Hypothesis,〔Ebenholtz,S.M., Cohen,M.M.,& Linder,B.J. The Possible Role of Nystagmus in Motion Sickness:a Hypothesis. Aviation, Space, and Environmental Medicine,1994,65,1032-1035〕 has been proposed based on stimulation of the Vagus nerves resulting from the stretching or traction of extra-ocular muscles () co-occurring with eye movements caused by vestibular stimulation. There are three critical aspects to the theory: first is the close linkage between activity in the vestibular system, i.e., semicircular canals and otolith organs, and a change in tonus among various of each eye's six extraocular muscles. Thus, with the exception of voluntary eye movements, the vestibular and oculomotor systems are thoroughly linked. Second is the operation of Sherrington's Law〔Sherrington, C.S. Further experimental note on the correlation of action of antagonistic muscles. Proceedings of the Royal Society,B53,1893,407-420.〕 describing reciprocal inhibition between agonist-antagonist muscle pairs, and by implication the stretching of extraocular muscle that must occur whenever Sherrington's Law is made to fail, thereby causing an unrelaxed (contracted) muscle to be stretched. Finally is the critical presence of afferent output to the Vagus nerves as a direct result of eye muscle stretch or traction.〔Milot,L.A.,Jacob,J.L.,Blanc,V.F.,Hardy,J.F. The Oculocardiac reflex in strabismus surgery. Canadian Journal of Ophthalmology, 1983, 18, 314-317〕 Thus, 10th nerve stimulation resulting from eye muscle stretch is proposed as the cause of motion sickness. The theory explains why labyrinthine defective individuals are immune to motion sickness;〔Kennedy,R.S.,Graybiel,A.,McDonough,R.C.,Beckwith,F.D. Symptomatology under storm conditions in the North Atlantic in control subjects and in persons with bilateral labyrinthine defects. Acta Otolaryngology, 1968, 66, 533-540.〕〔Cheung, B.S., Howard I.P.,Money, K.E. Visually-induced sickness in normal and bilaterally labyrinthine-defective subjects. Aviation, Space, and Environmental Medicine, 1991,62, 527-531.〕 why symptoms emerge when undergoing various body-head accelerations; why combinations of voluntary and reflexive eye movements may challenge the proper operation of Sherrington's Law; and why many drugs that suppress eye movements also serve to suppress motion sickness symptoms.〔Ebenholtz,S.M.Oculomotor Systems and Perception. Cambridge University Press, 2005,148-153〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「motion sickness」の詳細全文を読む スポンサード リンク
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