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 motoneurons in the dorsomedial (DM) region of the ventral horn at the lower portion of the L5 and the L6 segmental levels of the rat spinal cord were determined following 14 days of spaceflight and after 9 days of recovery on Earth and compared with those in the retrodorsolateral (RDL) region of the ventral 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 microm(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 perineal muscles (associated with predominantly fast, low-oxidative muscles which are recruited for relatively brief periods at high activation levels and have no load-bearing function at 1G) were not affected by microgravity. Copyright 2000 John Wiley & Sons, Inc.