AN ELECTRICAL INVESTIGATION OF EFFECTS OF REPETITIVE STIMULATION ON MAMMALIAN NEUROMUSCULAR JUNCTION

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Abstract
Using a curarized phrenic nerve/ diaphragm strip prepn. from a rat, end plate potentials (epp.) were recorded before and after repetitive activity. There was seen to be an increase in size of epps. recorded after tetanic activity. This post-tetanic potentiation (PTP) began shortly after cessation of the tetanus, rising to a max. and then declining slowly. The time course of PTP varied directly with duration of the conditioning tetanus and the magnitude with its frequency and duration. The time course of individual eeps. was unchanged. In lightly curarized prepns. muscle spikes were recorded post-tetanically although only epps. were seen in pretetanic controls. The site of PTP was in the fine motor nerve terminals. When short test tetani were employed in place of single testing volleys, previous activity brought about changes in the pattern of successive epps. in the test tetanus series-the earlier epps. were larger than corresponding controls but the later ones,smaller. A mathematical analysis of this change indicated that after activity there was an increase in the fraction of acetylcholine released per impulse. (At rest, 0.45 of the free acetylcholine present in the nerve terminals was released per impulse). These effects could be mimicked by raising the K concn. in the medium. It is suggested that the cause of PTP is a redistribution of K ions at the neuromuscular junction.