Cholinergic and noradrenergic modulation of thalamocortical processing.

During periods of drowsiness and synchronized sleep, thalamocortical neuronal activity is dominated by rhythmic oscillations. The shift to waking and attentiveness is associated with an abolition of these rhythms and a marked increase in neuronal responsiveness to synaptic inputs. These shifts in thalamocortical processing are controlled by ascending modulatory neurotransmitter systems of which the cholinergic and noradrenergic components play a key role. By altering the amplitude of specialized potassium currents in thalamic and cortical neurons, acetylcholine and norepinephrine can block the generation of thalamocortical rhythms and promote a state of excitability that is consistent with cognition.