The reticular activating system (RAS) is a network of cells located in the reticular formation of the medulla oblongata. RAS cells receive information via collateral connections from neurons located in ascending sensory neural tracts and relay that information directly to higher cerebral structures. The function of the RAS is to regulate arousal reactions within the central nervous system (CNS). Proper RAS function is critical to brain or CNS alertness, as measured by an individual's responsiveness to external stimuli and electrical brain activity (as measured by an electroencephalogram or EEG).

Within the medullary brainstem, the reticular formation includes the areas of gray matter in the pons and mesencephalon, but does not include cranial nerves or cranial nerve nuclei. The RAS is continuous with the spinal reticular formation and with the thalamic system. The RAS also receives input from the auditory and visual tracts...