SIMILAR AGE-RELATED-CHANGES IN 2 REGIONS OF MUSCLE WITH DIFFERENT PROPERTIES

Two regions of the medial gastrocnemius (MG) muscle, with different co ntractile properties and innervated by different nerve branches, were investigated in male Sprague-Dawley rats at three ages: 2-3 months, 6- 7 months and 24-25 months (i.e., the '50% survival age and beyond': a recommended definition of aged rodents derived from lifespan data on a given colony). At the 50% survival age, both regions of the MG showed decreased mass, slowed contraction times and a decreased number of fa st-twitch, but not slow-twitch, muscle fibres. The 40% loss of fast-tw itch muscle fibres was not reflected in the loss of motoneurones, sugg esting that muscle degeneration precedes motoneurone loss at the 50% s urvival age in the rat.