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^-.