Neural and neuroendocrine adaptations to microgravity and ground-based models of microgravity.

The functional properties of the motor system of humans and non-human primates are readily responsive to microgravity. There is a growing body of evidence that significant adaptations occur in the spinal cord and muscle in response to prolonged exposure to microgravity. Further, there is evidence that the processing of sensory information from the periphery, particularly that input associated with the function of muscle tendons and joints, is significantly altered as a result of prolonged microgravity. We present evidence that the fundamental neural mechanisms that control the relative activity of the motor pools of a slow and fast extensor muscle is changed such that a slow, postural muscle is less readily activated during locomotion following spaceflight. Another type of change observed in mammals exposed to spaceflight relates to the release of a growth factor, called bioassayable growth hormone, which is thought to be released from the pituitary. When an individual generates a series of isometric plantarflexor contractions, the plasma levels of bioassayable growth hormone increases significantly. This response is suppressed after several days of continuous bedrest or spaceflight. These results suggest a unique neuroendocrine control system and demonstrate its sensitivity to chronic patterns of proprioceptive input associated with load-bearing locomotion.