The electrogenic action of cations on cat spinal motoneurons

Potassium, rubidium, caesium, sodium, lithium, ammonium, tetramethylammonium, tetraethylammonium and choline ions were injected into cat spinal motoneurons electrophoretically through intracellular microelectrodes, and their effects on the resting potential were compared. Potassium and rubidium injections did not alter the resting potential to any appreciable extent, while depolarization to different degrees was produced by injections of caesium, sodium and lithium. It is suggested that the resting motoneuronal membrane is permeable to the alkaline cations in the order K ≑ Rb > Cs > Na ≑ Li. However, the depolarizing action of sodium and lithium injections varied considerably according to the preinjection level of the resting potential, and eventually was minimal when the membrane potential was very high, about —80 mV. Reasons for such insensitivity of the high resting potential to the ionic changes produced by the injections are discussed. After the injections of the alkaline cations there was recovery from the depressed membrane potential within 30 sec to 14 min; but ammonium and quaternary ammonium ions produced an irreversible depolarization which tended to become progressive with successive injections of these cations.