Injections of calcium ions into spinal motoneurones

1. In cats under Dial anaesthesia, Ca 2+ was injected inside lumbosacral motoneurones, by passing currents between CaCl 2 ‐ and KCl‐containing barrels of compound micropipettes. 2. There was a reduction in excitability and a fall in membrane resistance, both rapid in onset and quickly reversible. 3. The minimum effective injection current was ≈ 10 nA, and the effect reached a maximum with currents of ≈ 30 nA. The mean slope of resistance change against injection current was −1·7%/nA ( S.E. 0·35). 4. The most common change in membrane potential was a hyperpolarization; but in nearly half the cases, there was no clear change or a small depolarization. A reversal level for the effect of Ca 2+ could be measured in five cells: on the average, it was 10 mV more negative than the resting potential. 5. Observations on i.p.s.p.s showed that Ca 2+ probably does not alter g Cl : it was concluded that the fall in membrane resistance caused by intracellular Ca 2+ is mainly due to an increase in g K . 6. These results confirm previous suggestions that a steep transmembrane gradient of Ca 2+ is essential for the maintenance of a low membrane conductivity, and that a rise in internal free Ca 2+ — whether due to influx or release from internal stores — may play an important role in regulating neuronal activity.