Transmitter release from insect excitatory motor nerve terminals.

1. Intracellular and extracellular electrodes were used to study spontaneous and impulse-linked release of transmitter at locust retractor unguis nerve-muscle synapses.2. At most extracellular recording sites the amplitude distributions of the excitatory post-synaptic potentials (e.p.s.p.s) were apparently non-Poisson. However, interpretation of these amplitude distributions was complicated by the effect on the extracellular recordings of the complex structural organization of the retractor unguis nerve terminal with its spatially distinct transmitter release sites extending over distances of 15-30 mum.3. The spontaneous miniature excitatory post-synaptic potentials (min e.p.s.p.s) did not occur at random intervals, bursts of min e.p.s.p.s being frequently recorded. As a result the spontaneous release of transmitter rarely approximated a Poisson process.4. For a period of at least 390 msec following a conditioning nerve impulse a test e.p.s.p. was facilitated and the probability of spontaneous transmitter release was enhanced. A large primary phase of facilitation of impulse-linked and spontaneous release was invariably followed by one or more secondary phases of smaller magnitude.