Recurrent synaptic activation of the bullfrog sympathetic ganglion cells by direct intracellular stimulation

In a certain group (type 2 cells) of bullfrog sympathetic ganglion cells, an action potential exhibiting a triphasic after-potential was produced when the cells were activated by direct intracellular stimulation. This triphasic after-potential consisted of two different potential components, namely, a depolarizing response (DR) and an after-hyperpolarization. The amplitude of DR was increased by increasing the interval between stimuli. Large DRs exceeded the threshold of the cell membrane and produced repetitive firings of spike potentials. The DR was selectively depressed and eventually eliminated in a low-Ca solution. Eserine (10(-5) M) reversibly increased both the amplitude and duration of the DR, and d-tubocurarine reversibly depressed it. These results indicate that the DR of type 2 cells is the EPSP mediated by the nicotinic action of acetylcholine released from preganglionic nerve terminals. Preganglionic nerve fibers innervating type 2 cells are activated through some kind of recurrent pathway formed between them. Recurrent activation of type 2 cells seems to be thus induced when the cells are activated by direct intracellular stimulation.