The ascending reticular activating system (ARAS) is the neuroanatomic structure primarily responsible for arousal and cortical activation. It is located in the paramedian tegmental zone in the dorsal part of the brainstem. The input of somatic and sensory stimuli to the cerebral cortex is controlled by the ARAS and functions to initiate arousal from sleep. The brain’s cognition centers are located primarily in the cerebral cortex and serve to determine the content of consciousness.