Firing modes and membrane properties in lemniscal auditory thalamus.

In neurons of the auditory thalamus, patterned sequences of action potentials encode the features of sound stimuli. The patterns vary with the membrane potential, characterizing states of wakefulness and sleep. We studied the dependence of the patterns on the membrane potential and specific voltage-gated conductances, using whole-cell patch-clamp recordings from neurons in the ventral medial geniculate body (MGBv) of in vitro slices. Thalamocortical neurons, identified with neurobiotin, exhibited different firing patterns to an excitatory input, depending on the initial membrane potential. From depolarized potentials, the neurons fired in a tonic mode. The delay to firing in this mode was regulated by a balance of persistent Na+ and A-type K+ conductances. When transiently depolarized from hyperpolarized holding potentials, the neurons fired brief phasic responses (burst mode). Phasic responses were induced by low threshold Ca2+ spikes (LTSs); the LTS-amplitude was controlled by Na+ and K+ conductances. Under favourable conditions, an LTS triggered more than one action potential and one or more high threshold Ca2+ spikes (HTSs). Consciously perceived sound signals are transmitted in the tonic mode. During sleep, alerting stimuli may interact with membrane non-linearities, converting hyperpolarized bursting MGBv neurons to the tonic mode.