Abstract
1. The effects of a high frequency indirect tetanus on the responses to subsequent infrequently applied nerve shocks have been compared in the tibialis anterior and soleus muscles of cats and rabbits.2. Post-tetanic augmentation of twitches in the cat soleus muscle was shown to be partly due to repetitive firing and partly due to increased synchronization of the muscle fibre response. Post-tetanic repetitive firing was not evident in the responses of the other three muscles studied.3. Post-tetanic repetitive responses in the cat soleus muscle and nerve did not originate in the nerve trunk and were not produced by direct muscle stimulation; they were abolished by doses of tubocurarine smaller than those necessary to reduce the twitch tension below the pre-tetanic level. These findings support the conclusion of others that the repetitive firing originates at the neuromuscular junction.4. The repetitive firing could not be explained by an increase in the sensitivity of the motor endplates to acetylcholine, suggesting that an increase in and/or a prolongation of the output of transmitter from the motor nerve contributes to it.5. The cat soleus muscle was shown to be more sensitive to neostigmine than were the other three muscles studied, and acetylcholinesterase determinations showed that this muscle possesses less enzyme activity.6. It is concluded that an increase in transmitter output, coupled with a weaker cholinesterase activity, probably accounts for the post-tetanic repetitive activity in the cat soleus muscle.7. Post-tetanic repetitive firing was absent in cat soleus muscles which had been cross-innervated with the nerve formerly innervating a fast-contracting muscle.