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MICROGRAVITY-INDUCED MALFORMATIONS OF THE BODY CORRELATE WITH THE DEPRESSION OF THE STATIC VESTIBULOOCULAR REFLEX.    E. Horn.  Gravitational Physiology, Department of Neurobiology, University of Ulm, Ulm, Germany.

Studies in an amphibian (Xenopus laevis) have demonstrated that the development of the roll-induced static vestibuloocular reflex (rVOR) was significantly depressed by a 9- to 10-day microgravity exposure but that it recovered completely during 1g-readaptation within 5 weeks. This decrease was explained by a developmental retardation of the network underlying the vestibuloocular reflex. It was also found that microgravity induced malformations of  the body; affected tadpoles developed tails which curved upwards. These lordotic malformations might be an unspecific effect of microgravity on the developing animals. Alternatively, it might be caused by a depressed sensitivity of the vestibular system because the vestibular system influences the base activity level of trunk muscles via the vestibulospinal pathway. If this hypothesis is right a correlation between the extent of malformation and the extent of rVOR depression is likely. A respective correlation analysis was performed with data recorded in tadpoles which flew on the space missions STS-55 and STS-84. The rVOR amplitudes (= angle between the maximal up and down movement of the eye during a complete 360° lateral roll) of lordotic tadpoles with 0g-experience were compared with those recorded from both normally developed tadpoles with microgravity experience and normal tadpoles from the 1g-ground and 1g-inflight controls. The comparison showed a significant lower rVOR amplitude in lordotic tadpoles than in normal ones. No difference was found in normally developed tadpoles with µg-experience and the respective 1g-controls. Based on the correlations between anatomical, physiological and behavioral development in the time domain and the organization of vestibuloocular and vestibulospinal projections, microgravity induced malformations of the body are probably an expression (1) of the tonic influence of the vestibular nuclei on the motor system and (2) of a non-synchronous development of trunk and body muscles.