Other systems are affected by weightlessness, but their relative importance seems less and has not been investigated to a great extent. These systems are: red blood cells and their aggregation; immune response, both cellular and humoral; and the digestive system. We shall discuss these systems in the order of chronological appearance of changes in their functioning. 1.1 Sensory Motricity in Space Sensory motricity is the ability of the human body to move in an oriented fashion within its environment. On earth, a normal man is able to know the position of his body in relation to the vertical, and to execute a variety of movements while controlling the position of his centre of gravity. One of the first effects of weightlessness is a sensation of imbalance, of flotation. It results from perturbation of the sensory organs such as the vestibular system of the inner ear, vision, and tactile sensitivity. This sensation is often associated with dizziness, nausea, and vomiting. The form and duration of these manifestations vary from one individual to another. They usually disappear within hours or days and reappear when the astronauts return to earth [1, 7, 8, 9]. The analysis of space motion sickness uses the results of studies in sensory motricity physiology. Sensory motricity physiology studies the functioning of sensorial captors, the central treatment of the information they provide, and the use of this information for perception and control of movement. It also studies the adaptation process in case of a conflict between different sensory organs, or of a change in the external environment. One major sensorial captor is the otolithic organ of the inner ear: the sensory (nervous) cells are covered at one extremity with hair that come into contact with calcareous crystals embedded in a gelatinous matrix. In normal gravity, the hair is pulled downwards by the gelatinous mass. During rotation of the head, the hair is parallel to the rotational vector. The non-hairy extremity of the cells transmits the information to the brain; it gives information about the linear and rotatory acceleration of the head, the changes in the velocity of motion, and about the position of the head relative to the gravity field. Reflexes served by this organ (which are often called labyrinthic reflexes) compensate for changes in the direction of acceleration. The brain also receives information about the relative position of the body in space from captors in the muscles, the joints and the skin (tactile information), in addition to visual information. Under normal conditions, the nervous system uses all of this information to determine the vertical, which for all terrestrial animals has been closely linked to the presence of gravity for a quarter of a billion years. Space motion sickness has been experienced by astronauts and cosmonauts, and it may be an important limiting factor on man's ability to work in space. On average, one out of every two people flying for the first time suffers from severe space sickness [7]. This generally consists of nausea, pallor and disorientation during the first three to seven days of weightlessness, and is increased by large movements of the head. There does not seem to be any correlation between susceptibility to terrestrial motion sickness (on ships or aircraft) and the susceptibility to space sickness [7], Adaptation may take a week and motion sickness is followed by a prolonged drowsiness or lethargy. These symptoms may threaten the crew's well-being, its
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