The reticular formation is a diffuse network of neurons running through the brainstem. It plays a central role in regulating arousal, sleep-wake transitions, and attention.

Ascending fibers project to the thalamus and cortex, helping to maintain wakefulness and alertness. Descending fibers influence muscle tone and autonomic functions. Damage to the reticular formation can produce coma; stimulation can arouse a sleeping animal. The network also filters incoming sensory information, allowing relevant signals to reach higher centers while dampening irrelevant ones.

Different regions within the reticular formation contribute to specific aspects of arousal and motor control. The locus coeruleus and other monoaminergic nuclei are embedded in or adjacent to the reticular core and modulate global brain states. The structure is phylogenetically old and essential for the most basic regulation of consciousness.

When the brain shifts from sleep to wakefulness or maintains vigilance, the reticular formation is heavily involved.