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

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.