Moruzzi and Magoun40 in 1949 demonstrated that stimulation of the brain stem of cats desynchronizes and activates the EEG in a manner paralleling arousal from sleep or alerting to attention. This effect was obtained from the medial bulbar reticular formation, pontine and midbrain tegmentum, dorsal hypothalamus, and subthalamus. From their electrophysiologic observations the authors assumed the structural substrate of the "brain stem activating system" to be a series of ascending reticular relays, which are activated by collaterals from sensory paths.
The observations of Moruzzi and Magoun40 have subsequently been confirmed and extended by Magoun and his co-workers. The reticular activating system has been identified in the brain stem of the monkey,17 and continued experimental research * has lent support to the assumption that the reticular activating system of the brain stem is "made up of a series of ascending relays coursing forward from the reticular formation of the
BRODAL A, ROSSI GF. Ascending Fibers in Brain Stem Reticular Formation of Cat. AMA Arch NeurPsych. 1955;74(1):68-87. doi:10.1001/archneurpsyc.1955.02330130070009