Calcium and sodium ions as charge carriers in the action potential of an identified snail neurone.

1. The soma of cell A in Helix aspersa produced action potentials in sodium-free or calcium-free saline, but not in saline with neither sodium nor calcium. 2. The axon had a sodium-dependent action potential. 3. Tetrodotoxin (5 x 10(-6) M) had no effect on the overshoot except at low external divalent ion concentrations. 4. The action potential in sodium-free saline was blocked by cobalt. 5. The slope of action potential overshoot against sodium concentration in the presence of 10 mM calcium was 10.5 mV/tenfold change. That of overshoot against calcium concentration in the presence of 75 mM sodium was 22 mV/tenfold change. 6. In sodium-free saline the slope of overshoot versus calcium concentration was 27 mV/tenfold change. 7. It is concluded that calcium is an important charge carrier in the action potential of cell A.