The calcium component of the action potential in spinal motoneurones of the rat.

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Abstract
Intracellular recordings were made from motoneurones of the neonatal rat (1‐14 days old) spinal cord isolated and perfused in vitro. An increase in extracellular Ca2+ concentration from 2 to 20 mM produced an increase in the amplitude of the after‐depolarization (a.d.p.), while replacement of Ca2+ by Mn2+ virtually abolished the a.d.p. These changes in the a.d.p. occurred in parallel with those in the after‐hyperpolarization (a.h.p.). The amplitudes of the a.d.p. and the a.h.p. were dependent upon the membrane potential: hyperpolarization increased the a.d.p. and decreased the a.h.p.; the opposite effects were produced by depolarization. The presence of Ca2+ spikes was demonstrated either by suppression of the voltage‐dependent K+ conductance with tetraethylammonium (TEA) or after blocking the Na+ spike. The Ca2+ spike was all‐or‐none in nature and blocked by Mn2+ or Co2+. The a.h.p. amplitude was dependent upon the extracellular K+ concentration but also correlated with the amplitude of the Ca2+‐dependent response. It is concluded that the a.d.p. is a Ca2+‐dependent potential whose amplitude is under normal conditions markedly reduced by the voltage‐dependent K+ conductance; the a.h.p. seems to be produced by an increase in the Ca2+‐dependent K+ conductance.