Significance of vestibular organs in problems of weightlessness.
Two important questions concerning the organs of equilibrium are posed by orbital and space flights. The first is whether exposure to weightlessness may evoke symptoms of vestibular origin, and the second is how to prevent symptoms of vestibular origin should it be decided to abolish weightlessness by causing the spacecraft to spin. It is important to determine whether vestibular symptoms are to be ascribed to the otolith apparatus or to the semicircular canals, or to both. During exposure to weightlessness the usual gravitational stimulus to the otolith apparatus is lost, and this reduction in afferent input might be expected to disturb the integrative patterns in the central nervous system. All such disturbances are special instances of a common cause for the appearance of functional syndromes such as seasickness. If orbiting vehicles are caused to rotate in order to abolish weightlessness, the angular velocity will be determined by the effective radius of rotation and the rate of spin, i.e., how nearly it is desired to simulate 1 g. At very short radii the complicating effects of exposure of the body to variations in centripetal force must be considered. With longer radii the principal stress will arise as a result of constant rotation, which, in combination with head movements, generates Coriolis forces. These forces cause bizarre patterns of stimulation of the semicircular canals which in turn may result in illusions and "canal sickness". This type of motion sickness has its origin in the semicircular canals and deserves the term "canal sickness". The angular velocities which represent a negligible stress or a tolerable stress are defined. A brief description is given of illusory phenomena and other manifestations together with the time-course of the adaptive process.
Graybiel
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U.S. Naval School of Aviation Medicine, Pensacola, Florida, USA.
D000054:Acceleration; D000222:Adaptation, Physiological; D000818:Animals; D002498:Centrifugation; D004198:Disease Susceptibility; D006801:Humans; D008297:Male; D009041:Motion Sickness / Q000209:etiology / Q000503:physiopathology*; D009760:Nystagmus, Physiologic; D010037:Otolithic Membrane / Q000502:physiology*; D012399:Rotation; D012665:Semicircular Canals / Q000502:physiology*; D012720:Severity of Illness Index; D013026:Space Flight; D018489:Space Motion Sickness / Q000209:etiology / Q000503:physiopathology*; D014893:Weightlessness / Q000009:adverse effects*