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.