[The mechanisms behind the generation of the slow oscillations found in EEG recordings during sleep]

Electroencephalogram (EEG) recordings reflect different oscillatory activities during slow wave sleep (stages 2 and 3-4 of the sleep-waking cycle), namely d oscillations (< 4 Hz), sleep spindles and K complexes. These activities are essentially generated by the activity of the thalamo-cortical relay neurons, the neurons of the thalamic reticular nucleus and by the neurons in the cerebral cortex.

The combination of the intrinsic electrophysiological properties of the thalamic and cortical neurons, together with their synaptic connections, are responsible for the generation of these oscillations. Extra or intracellular recordings of these neurons during spontaneous or anaesthetic-induced sleep show how these neurons change their electrical activity during slow sleep due to the hyperpolarization of their membrane potential. Thus, the thalamic neurons lower their response to sensory stimuli and filter this information towards the cerebral cortex. Glial cells also contribute to the generation of the d waves seen in the EEG and oscillate synchronously with the cortical neurons. In addition, the oscillations of this neuronal and glial network is linked to important changes in the concentration of certain ions in the extracellular space; for instance, the K+ and Ca2+ concentration oscillates synchronously with the neuronal and the glial activity.

The oscillations in the EEG, which are slower than those observed in the waking state or in paradoxical sleep, play a fundamental role in processing the information handled by the central nervous system and filter the irrelevant information towards the cerebral cortex.