Biomedical Research 



exercise in preventing the cardiovascular deconditioning associated with space 

 travel? 



A matter of terminology is significant. The word "adaptation" denotes a favorable 

 modification of structure or function in response to environmental stress. The 

 motion sickness of space adaptation syndrome often passes after the first day or 

 two. This disappearance of motion sickness may be an adaptation to unusual 

 sensory perceptions, much in the way a sailor adapts or gets sea legs after a few 

 days at sea. 



However, the word "deconditioning" signifies an unfavorable change. A well- 

 trained athlete put to bed for a month undergoes deconditioning; a deliberate 

 effort must be made to regain the effect of previous training. The cardiovascular 

 deconditioning (and the changes in muscle and bone described below) adversely 

 affects the astronaut upon return to Earth. We do not know if this deconditioning 

 impairs the individual's ability to perform in space. 



Neurophysiology and Behavioral Science 



Among the sensory systems most likely instrumental in the pathogenesis of space 

 adaptation syndrome, the vestibular system is the most probable candidate. The 

 vestibular apparatus consists of semicircular canals in the inner ear that sense 

 angular momentum and otoliths that sense rectilinear acceleration. The afferent 

 and efferent neural pathways and neurotransmitters to and from the labyrinth to 

 proprioceptive receptors, posterior columns, the cerebellum, and autonomic control 

 centers in the medulla are not well understood. Currently, investigations of 

 vestibular function in humans include measurement of Coriolis stress susceptibility 

 (measuring an individual's susceptibility to motion sickness following movement 

 out of the plane of rotation), and measurement of otolith function using 

 acceleration sleds, swings, and parabolic flight. NASA currently flies a KC 135 

 aircraft in a parabolic profile to simulate microgravity; however, this does not 

 provide a sustained environment of microgravity for more than 30 seconds, and 

 the change in gravitational force is not uniform. Obviously, this technique has 

 limited application, and short of actual flight time in space, a suitable mechanism 

 does not exist to study vestibular function in microgravity. 



Space motion sickness is one of the most serious concerns in short-duration space 

 flight. The mechanism(s) is unknown by which labyrinth function in microgravity 

 is altered. The sensory conflict theory, hypothesized to explain space motion 

 sickness, postulates that "motion sickness occurs when patterns of sensory input 

 to the brain from the vestibular system, other proprioceptors, and/or the visual 

 system are markedly rearranged, at variance with each other, or differ substantially 

 from expectations of stimulus relationships in a given environment" (9). In gravity, 

 head movement is associated with changes sensed in both the otoliths and 

 semicircular canals. In microgravity, there may be unexpected stimulation of only 

 the semicircular canals. Whatever the cause, a large percentage of astronauts 

 experience nausea, vomiting, and malaise. At present, the most effective 

 pharmacologic treatment of space motion sickness is some combination of 



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