Utilization of Parabolic Flight for the Investigation of Neuromuscular Responses to Altered Gravity Levels in Rats

Abstract

The microgravity-induced deterioration of skeletal muscle properties is one of the health problems, which are the serious medical concerns for the manned long-duration spaceflight mission. Responses of the activities of electromyogram (EMG) in hindlimb muscles and afferent and efferent neurograms at the L5 segmental level of spinal cord in conscious rats to altered levels of gravity were studied using parabolic flight of jet airplane. In soleus, EMG was increased in response to elevation of G during the ascending phase, but was eliminated in microgravity (micro-G) environment. Similar response of the level of afferent, not efferent, neurogram was also noted. These phenomena were related to the shortening of muscle due to plantarflexion of ankle joints. As for adductor longus, region-specific responses were observed. The EMG activity in the caudal, not rostral, region decreased and activity patterns were changed from tonic to phasic during micro-G exposure. These phenomena were related to shortening of fibers, caused by abduction of hip joints and extension toward backward. These results clearly indicated that unloading-related undesirable adaptation of antigravity muscles is closely related to inhibition of mechanical and/or neural activities.