Intracellular citrate or externaly applied tetraethylammonium ions produce calcium-dependent action potentials in an insect motoneurone cell body.

1. Electrophysiological observations have been made upon the cell body of an identified motoneurone of the cockroach, Periplaneta americana. Normal responses were compared with those observed after intracellular injection of citrate anions or when the preparation was bathed in solutions containing tetraethylammonium ions (TEA+).2. Normally when depolarized, the motoneurone soma gave a series of damped oscillations; the amplitude of these responses increased with increase in the applied current.3. After citrate ions had been injected into the neurone soma, all-or-none action potentials were evoked by depolarization; such responses appeared about 5-10 min after the onset of citrate injection. Injection of EGTA produced similar effects. Citrate and EGTA probably produce their effect through a reduction in the intracellular free calcium concentration.4. When preparations were bathed in saline solution containing 50 mM-TEA+, soma depolarization produced prolonged all-or-none action potentials (up to approximately 100 msec duration).5. The action potentials produced by citrate injection or externally applied TEA+ appeared to have a similar ionic mechanism; they were not depressed by sodium-free solutions or by tetrodotoxin (4 x 10(-6)M) but were reversibly blocked in saline solution containing 40 mM-manganous chloride.6. The overshoot amplitude of action potentials recorded after injection of citrate anions or in solutions containing TEA+ showed a 22.5 mV change for a ten-fold change in the external calcium concentration.7. Both intracellular citrate and external TEA+ caused a significant increase in the input resistance and membrane time constant of the motoneurone.8. It is concluded that action potentials generated under various experimental conditions in the soma of this insect motoneurone map have differing ionic mechanisms.