1. Experiments were carried out on giant neurones in the abdominal ganglia of Helix aspersa in an attempt to determine (a) the relationship between the ionic composition of the extra-neuronal environment and that of the bathing medium, and (b) the identity of the ions responsible for carrying the inward current of the action potential.

2. Potassium ions diffuse readily between the bathing medium and the extraneuronal fluid, the mean half-time for equilibration being 19.2±1.6 sec (S.E.). With the inner connective tissue sheath intact, the mean half-time is increased to 32.8 ± 2.2 sec. Both half-times can be accounted for purely by the geometry of the extra cellular system. 3. Although their effects on the nerve membrane are not quantifiable, it seems likely from the rapidity of their action that other ions, such as sodium, calcium and manganese, are able to move at similar rates between the bathing medium and the surface of the neurones.

4. Repetitive stimulation at frequencies above 2/sec causes a reversible decline in action potential overshoot. The time course of this effect and of the recovery suggests that it is due to depletion of ions in the restricted extracellular space, with subsequent replacement by active transport, and by diffusion from the bathing medium. Some cells also show a hyperpolarization, which is probably due to a change in ionic permeability of the resting cell membrane.

5. Although many cells can give action potentials in sodium-free solution, the overshoot declines gradually over a period of some hours. This decline is reversibly accelerated by 2 mM/l cyanide, but is unaffected by 10-1 M ouabain. But if ouabain is applied in normal Ringer the overshoot declines slowly, and after a time the ability to give action potentials in sodium-free solution is irreversibly lost.

6. Experiments with 10 mM/1 manganous ions gave variable results, some cells being made inexcitable, others little affected; correlation with ability to give action potentials in sodium-free solution was not good. Tetrodotoxin at 10-5 M caused only small reductions in overshoot.

7. It was concluded that the extra-neuronal environment in Helix aspersa is not subject to ionic regulation. The behaviour of the action potential can best be explained by assuming that both sodium and calcium ions contribute to the inward current. The neuronal calcium pump probably resembles the calcium/sodium exchange pump found in squid axons. Under low-sodium conditions, this pump is unable to operate fully, so that the overshoot declines slowly due to loading of the cells with calcium. Poisoning with cyanide accentuates this effect. Ouabain affects only the sodium pump; in sodium-free solution there is no passive sodium influx, so the overshoot is unaffected. In normal Ringer the cells become loaded with sodium, so that subsequent exposure to sodium-free solution reverses the sodium gradient across the cell membrane, counteracting the inward calcium current, and making the cells inexcitable. Restoration of a favourable sodium gradient causes rapid recovery of the action potential in all cases.

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