title: The effects of aging on physiological properties of fast and slow twitch motor units in the rat gastrocnemius.
abstract: The mechanical properties of individual motor units were studied in the medial gastrocnemius of old (26-30 months) rats anesthetized with urethane and chloralose. The data were compared with results from motor units in the same muscle of middle-aged (11-14 months) rats. In the old rats there was a significant increase in mean tetanic tension for type S (slow twitch) units, whereas there was a slight decrease for type F (fast twitch) units. Twitch contraction time after maximum post-tetanic potentiation was shortened in type F units, but not in type S units. Conduction velocity of motor axons decreased in both unit types, but more marked change was found in type F units. These findings suggest the differential effects of aging on type F and type S motoneurons.
mesh_terms: D000375:Aging; D000818:Animals; D004576:Electromyography; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction; D009132:Muscles / Q000294:innervation / Q000502:physiology*; D009929:Organ Size; D051381:Rats; D011919:Rats, Inbred Strains; D011930:Reaction Time / Q000502:physiology; D009435:Synaptic Transmission