The dependence of pacemaker discharge of Aplysia neurons upon Na⁺ and Ca⁺⁺

In total absence of Na⁺ some identified neurons of Aplysia, after a period of silence, resume pacemaker discharge in the normal pattern with normal action potentials, while other identified neurons remain silent. In absence of Ca⁺⁺ all pacemaker neurons increase spontaneous discharge and develop abnormal bursting patterns. Those neurons which discharge spontaneously in Na⁺ free solutions show much less dependence on Na⁺ and much greater dependence on Ca⁺⁺ for action potentials initiated by electrical stimulation than do those neurons which do not fire spontaneously in absence of Na⁺. In absence of both Na⁺ and Ca⁺⁺ all neurons become inexcitable, but much more rapidly at higher temperatures.