The effects of denervation on contractile properties or rat skeletal muscle.

1. Isometric contractions of fast and slow twitch muscles of rats were recorded 1-42 days after denervation. 2. The major changes occurred over the period from 2 to 6 days after denervation. These changes were qualitatively similar in the two types of muscle. The most important effects were on the twitch: times to peak and half relaxation were prolonged, active tension and peak rate of rise of tension were increased. Tetanic tension per unit cross-sectional area and the maximum rate of rise of tension decreased during this period but, in the second week, the tension recovered substantially and the maximum rate recovered completely. 3. Very small differences were seen between muscles denervated with short and long nerve stumps at day 4. 4. In the late stage of denervation (7-42 days) twitch and tetanic tension fell more than cross-sectional area, but this may have been due to greater atrophy of fibres compared with other muscle tissue. 5. Apart from this tension fall, there were only small changes in the fast muscle in the late stage of denervation. These were a fall in twitch-tetanus ratio and a prolongation of relaxation. 6. In more than half of the soleus muscles there was a late reversal of some of the denervation changes, and these muscles showed a greater degree of atrophy. The less atrophied soleus muscles maintained a prolonged twitch and a low rate of development of tension. 7. It is concluded that denervation affects the contractile properties of muscle as early and as abruptly as it does the membrane properties, and that most of the contractile changes are a direct consequence of changes in excitation-contraction coupling alone.