Electrophysiology of mammalian thalamic neurones in vitro

Although much is known about the morphology and physiology of thalamic neurones1, no information is available regarding the ionic basis for the electrical excitability of these cells. Furthermore, possible differences in the electrical properties of the principal nerve cells in the various thalamic groups have not been studied in sufficient detail to determine whether the thalamus is, electrophysiologically, a uniform set of neuronal elements. Here we present evidence that thalamic neurones have voltage-sensitive ionic conductances capable of generating two distinct functional states—a repetitive and a burst-firing mode. The neurones are switched from one state to the other by membrane potential changes, each state being dominated by different voltage-dependent ionic conductances. At membrane potentials more positive than −60 mV, the neurones respond to a depolarization with repetitive firing via Na+dependent action potentials, whereas at potentials more negative than −65 mV, depolarization of the cell results in short bursts of action potentials via an inactivating all-or-none Ca2+-dependent spike. This property, present in all the neurones comprising the different thalamic nuclei, serves as the basis for their oscillatory properties. Particularly, the inactivating Ca2+ conductance represents the ionic basis for the post-anodal exaltation, the mechanism most probably responsible for the alpha rhythm.