Endurance exercise alters the morphology of fast- and slow-twitch rat neuromuscular junctions.

The present investigation studied the effects of an endurance exercise program on the morphology of nerve terminals of phasic extensor digitorum longus (EDL) and tonic soleus (SOL) muscles from young mature female Sprague-Dawley rats. The animals were treadmill exercised for 30 consecutive days for 1 h/day at velocities up to 30 m/min. The area, perimeter, nerve longitudinal extent length, and fiber diameter of camera lucida drawings of zinc iodide osmium-stained nerve terminals were analyzed with a computerized morphometry system. Additionally, the number of nerve terminal branches and percentage of end-plates with sprouts were determined. Nerve terminals from exercised EDL and SOL had significantly greater areas than those from corresponding controls, with no changes in the perimeters. The magnitude of the changes were greater in the EDL. The exercised EDL nerve terminals were also significantly longer than controls and had more branches and sprouts. No training differences in fiber diameter were observed for either muscle. These data confirm previous suggestions that physical activity or exercise levels affect the morphology of the neuromuscular junction.