Electrophysiology of the giant nerve cell bodies of Limnaea stagnalis (L.) (Gastropoda: Pulmonata)

A mean resting potential of -53.3 (S.D. ± 2.7) mV has been obtained for 23 neurones of the parietal and visceral ganglia of Limnaea stagnalis (L.). Changes in the resting potential of between 28 and 43 mV accompany tenfold changes in [K+(o)]. A modified constant field equation accounts for the behaviour of most cells over the range of external potassium concentrations from 0.5 to 10.0 mM/l. Mean values have been estimated for [K+(i)], 56.2 (S.D. ± 9.0) mM/l and P(Na)/P(K), 0.117 (S.D. ± 0.028). Investigations on the ionic basis of action potential generation have revealed two cell types which can be distinguished according to the behaviour of their action potentials in sodium free Ringer. Sodium sensitive cells are unable to support action potentials for more than 8-10 min in the absence of sodium. Sodium slopes of between 29 and 37 mV per decade change in [Na+(o)] have been found for these cells. Tetrodotoxin (5 x 10-5M) usually blocks action potentials in these neurones. Calcium free Ringer produces a marked reduction in the overshoot potential and calcium slopes of about 18 mV per decade change in [Ca2(o)] are found. Manganous chloride only partially reduces the action potential overshoot in these cells at concentrations of 10 mM/l. Sodium insensitive neurones maintain action potentials in the absence of external sodium. Stimulation only slightly reduces the amplitude of the action potential under these conditions and such cells are readily accessible to potassium ions in the bathing medium. A calcium slope of 29 mV per decade change in [Ca2(o)] has been observed in these cells in the absence of external sodium. It is concluded that both sodium and calcium ions can be involved in the generation of the action potential in neurones of Limnaea stagnalis, their relative contribution varying in different cells.