Two Different Ionic Mechanisms Generating the Spike "Positive" Afterpotential in Molluscan Neurons

The ionic bases of the "positive" afterpotential (ap) have been examined in the so-called DInhi neurons of the central nervous system of Cryptomphallus aspersa. In these cells E (K) has been determined and its value compared with the equilibrium, potential of the ap (E (ap)). It has been found that in half of the studied cells the E (K) value is very close to E (ap) whereas in another half, the difference (E (K) - E (ap)) is large and amounts to circa -10 mv. The effects of changes in the concentration gradients of K(+), Cl(-), and Na(+) were assayed in both groups of cells. When the [K(i)/[K](o) ratio is reduced in both groups of neurons, the ap amplitude and the E (ap) diminished. In cells displaying a large (E (K) - E (ap)), Cl-free Ringer's solution diminished the ap amplitude and E (ap), but produced no effect in the neurons with a reduced (E (K) - E (ap)). A similar effect was observed if [Cl], was increased by intracellular injection of NaCl. Changes in both [Na](o) and [Na](i) were ineffective. It is concluded that K(+) is the only ion involved in the origin of the ap in the groups of cells with a low value for (E (K) - E (ap)). On the contrary, the ap of the neurons presenting large (E (K) - E (ap)) is produced by a simultaneous increase in the fluxes of both K(+) and Cl(-).