![]() As they had expected, the mouse immediately attracted the cat’s attention. Then they placed a mouse in front of the cat. They sounded a click at regular intervals and measured the electrical activity of the cat’s auditory nerve.Įach time the click was sounded the activity of the nerve increased, producing a sharp increase in the recording as shown in Fig.5.5. They implanted electrodes in the auditory nerve of a cat, to find out what happens in the brain when more than one set of sensory organs are stimulated at the same time and how sensory gating takes place. This is shown very clearly by the study of (Hernandez Pion and his associates on cats). It can tune into one and only one channel at any particular moment. ![]() The brain, it seems, is able to attend to only one set of messages at a time. For example, sensory messages about a sudden loud noise will pass through, temporarily diverting out attention from the visual sensation involved in reading a book. The type of information passing through the sensory gates determines where our attention will be directed. It appears to tell them not to send in any conflicting messages (at least for the moment) and to hold all calls until the brain can get around to handling them. In addition, the system seems to provide some sort of feedback to the sense receptors. This means that strong sensations from one set of sense organs are allowed to pass, while information from the other sense organs is temporarily held back. The filtering process that goes on in the RAS is known as sensory gating. The messages from the eye go through the RAS and alert the brain that information is on the way and tell the brain how important the messages are going to be. These messages carry the content of the sensory information’s like particular patterns of light waves received by the eye. The eye, for example, sends messages to the brain through the optic nerve. Thus, the primary function of RAS is to alert the higher brain centres when important messages are received and to filter incoming messages. Anesthetic drugs that produce unconsciousness appear to act by depressing the RAS. If the RAS is destroyed, a profound and enduring coma results and for all practical purposes, the animal is reduced to a sleeping vegetable. The alerting function of the RAS has been inferred from the fact that direct electrical stimulation of the RAS will awaken a sleeping cat and produce EEG brain waves characteristic of natural alertness and excitement. The alerted cortex is then better able to deal with or process the specific information arriving over the specific sensory input channel to the cortex. This diffuse stimulation alerts the cortex, essentially telling it that some kind of news is arriving. Research has indicated that the probable operation of this system is as follows: new sensory information stimulates the RAS, which relays the presence of some kind of stimulation to various sensory receiving areas of the cortex. The RAS sends its unspecific messages to broad, diffuse areas of the cerebral cortex. Within the RAS collaterals from the various sensory channels are intermingled and lack specificity. On their way, however, these input cables send off collateral branches into the RAS. This is done through specific relay nuclei in the thalamus. ![]() The sensory input cables send their information to specific areas in the cerebral cortex. However, selected cells of the RAS are aroused or alerted when signals are being transmitted through sensory input cables from the skin, ear, nose etc. Anatomically this system in man is about the size of the little finger and is located at the core of the brain stem just above the spinal cord and below the thalamus and hypothalamus. This is made up of a net-like bundle of neurons that run through the hind-brain, mid-brain and a part of the fore-brain called the hypothalamus. The Reticular Activating System (RAS) of the brain stem is considered as one of the most important systems which facilitates the functioning of sensation and attention. After reading this article you will learn about the functions of Reticular Activating System (RAS).
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