Recurrent Synaptic Activation of the Bullfrog Sympathetic Ganglion Cells by Direct Intracellular Stimulation

Abstract

In a group (type 2 cells) of bullfrog [Rana catesbeiana] 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 2 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 DR 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. The DR of type 2 cells is the EPSP [excitatory post-synaptic potential] 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 induced when the cells are activated by direct intracellular stimulation.