The electrophysiological properties of the high- and low-threshold Ca spikes described in inferior olivary neurones were analysed in detail. 1. During hyperpolarization the low- and high-threshold Ca action potentials can coexist as two distinct spikes, demonstrating non-mutual exclusion. 2. The high-threshold Ca spike shows a lack of refractoriness, is generated remotely from the site of recording and is composed of several all-or-none components, the last two properties suggesting a dendritic origin. 3. Hyperpolarization of the neurones allows the activation of the low-threshold Ca spike, which has activation properties resembling those of the early K conductance described in invertebrates. This low-threshold Ca spike shows refractoriness. 4. The relation between membrane polarization and low-threshold Ca spike is S-shaped. Low-threshold Ca spikes become apparent at -70 mV and have a maximum rate of rise (saturation) at polarization levels more negative than -85 mV. Thus, hyperpolarization removes a voltage-dependent Ca inactivation which is present at normal resting membrane potential (-65 mV). 5. Replacement of extracellular Ca by Ba or addition of tetraethylammonium to the bath corroborates the lack of fast inactivation for the high-threshold Ca spike and the inactivation properties of the low-threshold Ca conductance. It also demonstrates that the duration of the after-depolarization is determined by an interplay between inward Ca current and both voltage-dependent and Ca-dependent K currents. 6. Extracellular recordings from single cells indicate that the Na-dependent spike and the low-threshold Ca action potential are somatic in origin, while the high-threshold Ca spike (after-depolarization) and the hyperpolarization that follows are apparently located in the dendrites. 7. The ionic conductances comprise the main components of the oscillatory behaviour of these cells. The sequence of events leading to oscillation entails initially a low-threshold Ca spike or Na spike, followed by an after-depolarization/after-hyperpolarization sequence and then a post-anodal exaltation product by a rebound low-threshold Ca spike.
