Otolith functions in weightlessness

The role of the vestibular organ in the exploration of space has been studied extensively during the past two decades. Many investigators have shown that some persons experience ill effects during the transition from the normal gravity to subgravity or weightlessness. Such adverse reactions can be related to a variety of sensory and somatic changes within the body systems; but it appears that the two major components of the unusual force field--namely, the absence of gravitational stimulation of the otolith organs and the occasional stimulation of the semicircular canals by head and body movements--bring about the motion sickness type reactions. Experiments in parabolic flights and in spacecraft revealed that the statolith organs respond to changes of acceleration during zero G. After an initial period of increased activity during the transition from 1 G to zero G, the number of nerve impulses from the otoliths is drastically decreased and later becomes steady on a somewhat lower than normal level of the discharge rate. The various theories concerning otolith responses in weightlessness are discussed and validated against the actual findings on astronauts and cosmonauts during spaceflight experiments and missions.