Comparison of the response of motoneurons innervating perineal and hind limb muscles to spaceflight and recovery

The succinate dehydrogenase (SDH) activities and cell body sizes of motoneu rons in the dorsomedial (DM) region of the ventral horn at the lower portio n of the L5 and the LS segmental levels of the rat spinal cord were determi ned following 14 days of spaceflight and after 9 days of recovery on Earth and compared with those in the retrodorsolateral (RDL) region of the ventra l horn at the same segmental levels. No changes in the mean SDH activity of motoneurons in the DM region were observed following spaceflight or after recovery. However, a decrease in the mean SDH activity of motoneurons with cell body sizes between 500 and 900 mu m(2) in the RDL region was observed following spaceflight and after recovery. These data indicate that moderate -sized motoneurons in the RDL region, which are most likely associated with the hind limb musculature, were responsive to the microgravity environment ; In contrast, the motoneurons in the DM region associated with the perinea l muscles (associated with predominantly fast, low-oxidative muscles which are recruited for relatively brief periods at high activation levels and ha ve no load-bearing function at 1G) were not affected by microgravity. (C) 2 000 John Wiley & Sons. Inc.