Multiple dynamical modes of thalamic relay neurons: rhythmic bursting and intermittent phase-locking.

A model of thalamocortical relay neuron is studied to assess whether a 7-14 Hz (spindle) oscillation and a 0.5-4 Hz (delta) oscillation may result from the interplay between a T-type calcium current and a non-specific cation sag current. With moderate change of membrane parameter values, the model neuron can exhibit both the spindle and delta rhythms, at different levels of hyperpolarization; only the slower (delta) one or none. In the case when the model neuron is not intrinsically oscillatory, its response to rhythmic hyperpolarization is complex, and displays the "intermittent phase-locking" phenomenon where bursts of Na+ action potentials occur infrequently but their occurrence is phase-locked to the rhythmic input. The rhythmic bursting, whenever possible, is shown to emerge (bifurcate) from a subthreshold oscillation. Near the bifurcation chaotic discharge patterns are observed, where spikes occur intermittently at randomly chosen cycles of a mostly subthreshold slow oscillation. Furthermore, when both the spindle and delta modes can be realized, the transition between the two appears as a sudden drop of the rhythmic frequency with increased hyperpolarization. The T-type calcium current and the sag current may explain the "intermittent phase-locking" phenomenon that is characteristic to thalamic relay neurons during spindle oscillation and provide a cellular basis for the 7-14 Hz rhythm and the slower 0.5-4 Hz rhythm.