Changes in mechanical properties of the inferior oblique muscle of the rabbit after denervation

Abstract

The inferior oblique muscle (IO) of the rabbit was denervated. The mechanical properties of the muscles were determined at 35 degrees C in vitro 2--76 days after the operation. After denervation the muscles exhibited a considerable hypertrophy. The cross-sectional area of muscles denervated 30 days or longer grew to about twice as innervated controls. The length-tension relationship of passive or activated long-term denervated IO differed from normal in a higher stiffness. After denervation the time parameters of single twitches and tetanic contractions evoked by massive stimulation were prolonged, the fusion frequency was decreased, and the fatigue resistance was increased. During the first week after cutting the motor nerve both twitch and tetanic tensions decreased drastically. A minimum was reached at the end of the second weak. In the subsequent weeks the tension development was increased again, but per unit cross-sectional area it was always smaller than in innervated muscles. After denervation the twitch:tetanus ratio was increased. Cooling to 25 degrees C was followed by an increase in time parameters of single twitches and tetanic contractions and by a depression of twitch and tetanic tensions. Following a repetitive stimulation denervated IO showed a posttetanic depression of the single twitch.