Post-tetanic potentiation of muscle contraction strength (PTP) occurs in cat soleus and gastrocnemius muscles. However, the mechanisms of potentiation are different in these two muscles. Soleus PTP is predominantly a neural event. The application of a high frequency stimulus to the soleus nerve regularly causes each subsequent response to a single stimulus to become repetitive. This post-tetanic repetitive activity (PTR) originates in the motor nerve terminal and is transmitted to the muscle. Consequently each potentiated soleus contraction is a brief tetanus. In gastrocnemius PTR occurs too infrequently to account for PTP. Furthermore, PTP occurs in curarized directly stimulated gastrocnemius muscles to the same extent as in the indirectly stimulated muscle. In this instance PTP is a muscle phenomenon.Post-tetanic potentiation (PTP) of muscle contraction seems to be a general phenomenon of neuromuscular systems. It has been observed in almost every muscle in which it has been sought (for review, see Hughes, 1958). Curiously, this ubiquity seems to be a major factor in the failure to provide a satisfactory explanation for the phenomenon. The fact that m a n y muscles respond similarly to a high frequency stimulus has tostered the belief that they share a common mechanism of potentiation, but to date no single mechanism has been found to reconcile the diverse observations.Neurally stimulated cat soleus and gastrocnemius muscles develop PTP of similar appearance. However, examination of the phenomenon in these two muscles revealed two distinct mechanisms of potentiation: one residing in the motor nerve terminal, the other in the muscle. Furthermore, it was found that both mechanisms operate in both muscles, but in different degree. Soleus PTP is predominantly neural, while gastrocnemius PTP is a l m o s t wholly muscular.

A B S TR A C T Subsequent to conditioning by a high frequency stimulus axons of the cat soleus nerve respond to single stimuli with brief trains of repetitive action potentials. This phenomenon, post-tetanic repetitive activity (PTR), was studied in individual axons and single motor units of an in situ cat soleus nerve-muscle preparation. The occurrence, intensity, and duration of PTR are principally dependent on the frequency and duration of the conditioning tetanus. PTR occurs synchronously in the axon and muscles of single motor units. An analysis of the temporal relationships of the repetitive nerve and muscle potentials showed that PTR is generated in the motor nerve terminal. It is postulated that PTR is produced by a generator potential which is developed in the post-tetanic period between the unmyelinated nerve terminal and the last node of Ranvier.The after-effects of high frequency stimulation have interested many workers (for a review, see Hughes 1958). In the neuromuscular system, attention has focused chiefly on the post-tetanic augmentation of contractile response, frequently termed post-tetanic potentiation (PTP). Mechanisms underlying P T P production were sought for almost a century before Morison (1937), andCannon (1940) discovered that repetitive muscle action potentials accompany the PTP. Feng, Li, and Ting (1939) confirmed these observations and added others. Of greatest importance was their discovery that the post-tetanic repetitive potentials arise in the terminal portions of the motor nerve. They also noted that in the cat posttetanic repetitive activity occurs less frequently in the gastrocnemius than 53

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