Synchrony in the interconnected circuitry of the thalamus and cerebral cortex

The corticothalamic system is organized to play a key role in synchronizing the activities of thalamic and cortical neurons. Its synapses dominate the inputs to relay cells and to the GABAergic cells of the reticular nucleus; its organization in focused and diffuse projections promotes both coherent activity of relay neurons and the spread of activity across the cortex and thalamus. The capacity of relay neurons to operate in voltage-dependent tonic or burst mode permits corticothalamic inputs to directly excite the relay cells or indirectly inhibit them via the reticular nucleus. This enables the corticothalamic projection to synchronize high- or low-frequency oscillatory activity, respectively, in the thalamo-corticothalamic network. Differences in the subunit composition of AMPA receptors at synapses formed by branches of the same corticothalamic axon in the reticular nucleus and dorsal thalamus are an important element in the capacity of the cortex to synchronize low-frequency oscillations in the network. Intrinsic voltage-gated calcium channels of different kinds expressed in the relay neurons form a substrate for corticothalamic interactions with the relay cells that promote high- or low-frequency oscillations. Focused corticothalamic axons arising from layer VI cortical cells and diffuse corticothalamic axons arising from layer V cortical cells, in conjunction with the core and matrix cells of the dorsal thalamus, form a substrate for synchronization of widespread populations of cortical and thalamic cells during high-frequency oscillations that underlie discrete conscious events.