The reticular activating system (RAS) is part of the reticular formation(RF). It comprises a set of neurons that spread through the brain stem and sends projections to the cerebral cortex.It constantly receives stimuli from the ascending somatosensory tracts and transmits these stimuli to the cortex . Some of these stimuli are from pain, touch , temperature and pressure receptors . Stimuli from limb proprioceptors are pass from RAS to cortex. We are normally unconscious of proprioceptive stimuli. Because it is so diffuse, the RAS or the ascending reticular system (ARAS), is better thought of as a functional system. It has several important functions: 1, It determines the general normal level of consciousness or state of arousal . As such, it also monitors the transitions from wakefulness to non-REM sleep, and from resting sleep to dreaming or REM sleep 2. The RAS filters out excessive stimuli that would overload the consciousness and result in mental dysfunction.
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The RAS has neurons in the midbrain, the pons, and the medulla; it is sometimes considered a brain stem structure. However, it is simpler to think of it as a functional system. Neurons from the somatosensory tracts synapse with neurons in the RAS and keep it active: a. the anterior and lateral spino-thalamic tracts transmit pain , touch pressure and temperature stimuli. b. the spinocerebellar tracts convey proprioceptive stimuli from the limbs.. The RAS sends a constant stream of impulses to the cortex, but it does not send everything it receives from the ascending tracts. Both the RAS and the cerebral cortex filter out or disregard most of the environmental stimuli that impinge on the nervous system receptors. This filtering process prevents the conscious mind from becoming overloaded and dysfunctional. While the primary function of the RAS is to keep the cortex alert and the mind conscious, it also serves a cognate function as a sensory filter that facilitates habituation; it filters weak and familiar inputs and lets strong and novel ones through to the cortex. The RAS also plays a vital role in monitoring the levels of consciousness from wakefulness through slow wave restful sleep (NREM stages 3 and 4), to dreaming REM sleep. This is a very important role because slow wave sleep is restorative, and REM sleep affects emotional stability.