APA Dictionary of Psychology

an extensive network of nerve cell bodies and fibers within the brainstem, extending from the medulla oblongata to the upper part of the midbrain, that is widely connected to the spinal cord, cerebellum, thalamus, and cerebral cortex. It is most prominently involved in arousal, alertness, and sleep–wake cycles but also controls some aspects of action and posture. Also called brainstem reticular formation. See also reticular activating system.