Effects of Exposure to Microgravity on Neuromuscular Systems: A Review

Parabolic flight lasting 20–30 s and short-term exposure to microgravity for several weeks have been used to examine changes in the neuromuscular system. Previous studies have shown that short-term exposure to microgravity induces an atrophy of fibers, shifting of fiber type from slow-twitch to fast-twitch with synthesis of new fiber types, decreased oxidative enzyme activity, and down-regulation of the mRNA expression of heat shock proteins in the antigravity skeletal muscles of rats. These changes are similar to those observed after decreased muscular activity and loading levels on Earth. Recent studies using long-term exposure to microgravity for several months have also revealed a decreased oxidative capacity of gamma and alpha motoneurons in the spinal cord of mice, as well as degenerative changes in the skeletal muscles. In this article, we reviewed changes in the neuromuscular system in response to parabolic flight of an airplane and short- and longterm exposures to microgravity on orbit around Earth. We especially focused on the oxidative capacity of spinal motoneurons that innervate muscle fibers.