Reticular Activating System - History and Etymology

History and Etymology

Moruzzi and Magoun first investigated the neural components regulating the brain’s sleep-wake mechanisms in 1949. Physiologists had proposed that some structure deep within the brain controlled mental wakefulness and alertness. It had been thought that wakefulness depended only on the direct reception of afferent (sensory) stimuli at the cerebral cortex.

The direct electrical stimulation of the brain could simulate electrocortical relays. Magoun used this principle to demonstrate, on two separate areas of the brainstem of a cat, how to produce wakefulness from sleep. First the ascending somatic and auditory paths; second, a series of “ascending relays from the reticular formation of the lower brain stem through the mesencephalic tegmentum, subthalamus and hypothalamus to the internal capsule.” The latter was of particular interest, as this series of relays did not correspond to any known anatomical pathways for the wakefulness signal transduction and was coined the ascending reticular activating system (ARAS).

Next, the significance of this newly identified relay system was evaluated by placing lesions in the medial and lateral portions of the front of the midbrain. Cats with mesancephalic interruptions to the RAS entered into a deep sleep and displayed corresponding brain waves. In alternative fashion, cats with similarly placed interruptions to ascending auditory and somatic pathways exhibited normal sleeping and wakefulness, and could be awakened with somatic stimuli. Because these external stimuli would be blocked by the interruptions, this indicated that the ascending transmission must travel through the newly discovered RAS.

Finally, Magoun recorded potentials within the medial portion of the brain stem and discovered that auditory stimuli directly fired portions of the reticular activating system. Furthermore, single-shock stimulation of the sciatic nerve also activated the medial reticular formation, hypothalamus, and thalamus. Excitation of the RAS did not depend on further signal propagation through the cerebellar circuits, as the same results were obtained following decerebellation and decortication. The researchers proposed that a column of cells surrounding the midbrain reticular formation received input from all the ascending tracts of the brain stem and relayed these afferents to the cortex and therefore regulated wakefulness.