productivity and the safety of the mission. The use of anti-motion sickness drugs has sometimes been beneficial: experiments to refine their individual differences and side effects are continuing [5, 7]. The usual motion sickness drugs act either by blocking the transmission of the information from the sensory organ to the brain, or by reducing the symptoms of motion sickness. Scopolamine, which is often used in ear patches, antagonizes a neurotransmitter, acetylcholine, involved in the labyrinthic reflexes. Anti-histamine drugs, such as Dramamine, on the other hand, block the vomiting reflex before it reaches the digestive system. Several theories have been proposed to explain space sickness. The sensory conflict theory involves a discrepancy between visual and vestibular inputs. The notion of a ‘vertical' normally results from the integration of information from vision and from the inner ear, which is sensitive to gravity, linear and circular acceleration. In the absence of any gravitational signal from the inner ear, the otolithic organ cannot provide any reliable and consistent information about the position of the body. The astronauts can only use visual information to determine the position of their body [1, 3, 9]. Another theory links space motion sickness to the cardiovascular adaptation to weightlessness. Fluid shifts towards the upper body and could thus affect the vestibular system through changes in the venous and interstitial pressures. Other investigators have suggested that the changes in cerebro-spinal fluid's pressure could result in motion sickness symptoms [2, 3]. These last theories, however, have not yet been supported by evidence and the most likely explanation seems to be that space motion sickness is created by similar sensory conflicts to those which cause terrestrial motion sickness. The adaptation that takes place after a few days probably involves a reinterpretation of the signals sent by the gravireceptors. Maintenance of this readaptation during the postflight period is maladaptative, resulting in postural instability with eyes closed and increased reliance on visual information for orientation [1]. Astronauts and cosmonauts who repeat space missions do not usually suffer from space sickness to the same extent, which could mean that their brain has ‘learned' to deal with the sensory conflicts. 1.2 The Cardiovascular System in Weightlessness The cardiovascular system guarantees the flow of metabolites, heat and hormonal substances to and from the cells of the body and is also involved in the exchange of materials and energy between the body and the external environment. In normal conditions on earth, the cardiovascular system has to fight against a gradient of hydrostatic pressure in order to assure the distribution of blood and oxygen to the upper parts of the body. It also has to be able to increase its output in response to the increased demand induced by any muscular exercise. The ability to perform these tasks reflects the level of cardiac conditioning. Cardiovascular deconditioning has been a universal finding both during and after space flight, amd manifests as a decreased ability to maintain heart rate and blood pressure either at rest or during provocative tests. Such changes have not been a major problem during flight, but have been significant during reentry and on return to normal gravity [1, 3, 4, 9]. Various features of the deconditioning process have persisted for weeks following weightlessness. Although ground-based simulations have provided information on man's response to zero gravity, the mechanisms underlying the observed changes are still unclear. In weightless conditions, the absence of hydrostatic pressure results in a redistribution of fluids towards the upper regions of the body, the thorax and the head, instead of their
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