The effect of denervation on the mechanical and electrical responses of fast and slow mammalian twitch muscle

The soleus (slow twitch), flexors digitorum and hallucis longus muscles (fast twitch) of the cat were denervated. Isometric contractions and electrical responses were examined 2-160 days after the operation.2. In the first week the time course of the twitch and the ratio of tetanus tension to twitch tension were normal in both muscles. The maximum rate of rise of tension in the tetanus was reduced in fast muscles.3. In the second week all the twitches showed a normal contraction phase but relaxations were interrupted by a repetitive after-contraction. This became less marked after longer periods of denervation.4. During the third and subsequent weeks, the contraction and relaxation phases of the twitches in all muscles became slower than normal. These changes were greater in fast muscles which, nevertheless, remained quite distinct from soleus. The ratio of tetanus tension to twitch tension fell below normal. It is suggested that these changes are brought about by more complete activation of the contractile proteins in a twitch. In flexor hallucis longus the rate of rise of tension in isometric tetani was found to be further reduced. No change was found in soleus.5. In extracellular and intracellular records the initial response was a single action potential. An after-discharge occurred in a proportion of fibres during the relaxation phase of the twitch.6. The intracellularly recorded action potential was smaller and had a longer duration than that of normal muscle. Refractory period increased. Conduction velocity decreased. These changes were greater in fast muscle and differences between fast and slow twitch muscle were less marked than in normal muscle. Fibrillation potentials occurred at the same rate in fast and slow muscle.7. The mechanisms of the mechanical changes are discussed and the possible relevance to the question of motor nerve influence on muscle is indicated.