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. In addition to stimulating cerebral cortex activity by stimulating the RAS it is possible to stimulate the RAS via the cerebral cortex.
The reticular activating system serves as a monitoring and switching system through which a wide number of signals, originating from stimuli in both the external and internal environment pass. RAS activity directly correlates with electrical activity of the cerebral cortex, and with electrical activity in the efferent nerves regulating both motor and vegetative processes.
When asleep, there are decreased levels of electrical activity in the RAS. Excitation of neural cells in the RAS, however, causes individuals to wake from sleep and corresponds to changes in both metabolic rate and general muscle tone. Electrical stimulation of the RAS in animal test subjects shows a corresponding increase in cortical activity corresponding to an arousal reaction as measured by an EEG. The input signals to the RAS to that initiate the arousal reaction may come from a variety of sensory inputs (e.g., touching sensations, light, noises, etc). The type and strength of the input signals to the RAS causes varying degrees of reaction. Both pain from external stimuli and protective (proprioceptive) signals from the body are the most potent signals in terms of eliciting RAS activity.
In addition to stimulation to the RAS that results in a generalized increase in alertness, specific stimulation of thalamic connected regions result in more specific stimulation of certain areas of the cerebral cortex and thus may provide a mechanism to increase or heighten certain types of thinking or awareness.
Recent research has indicated that there may be separate areas of the brain responsible for maintaining alertness from those responsible for initial arousal.
Barbiturates depress brain stem activity, especially electrical activity in the RAS, and contribute to a general anesthesia of drowsiness and sleep.
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