Abstract

Acetylcholine (ACh) produces a burst of low-amplitude spike discharges in mammalian and frog gangila, as recorded extracellularly. This study examines the question whether or not the percentage of discharging cells is a fractor in the low amplitude. For each of 69 cells ACh (10 µl of 5.5 mM) was injected into the Ringer's solution flowing by the ganglion during intracellular recording. The dose depolarized all 69, reduced the the amplitude of the transmitted population response by 64% but evoked spikes in only eight cells Spikes induced by ACh were repetitive and similar to antidromically evoked but not to orthodromically evoked, spikes, confirming the work of Ginsborg and Guerrero (1964) In eight cells exposed to lower doses of ACh no spike responses were produced, whereas injection of 10 µl of 16.5 mM ACh initiated antidromic-like spike responses in 5 of 12 cells and also caused maximal transmission block. Thus within the dose range between thershold and maximal transmission block, ACh injection induces only a fraction of the cells in the population to spike. This, together with imperfect synchronization accounts for the low-amplitude discharge recorded extracellularly. The nonorthodromic character of the ACh-induced spikes, as well as other findings in this study do not support the view that injected ACh mimies transmitter action at the ganglion.