The Reticular Activating System (RAS) is a network of neurons that extends in many different directions from the reticular formation, going to the different areas of the brain and spinal cord
Major functions of the RAS:->Regulate cortical arousal (alertness)
->Influencing whether we are awake, drowsy, asleep or in some state in between
->When our RAS is less active, we go to sleep.
Because the RAS is associated with alertness and arousal, we can say that many anaesthetics used in surgery work to reduce the activity and influence of the RAS, rendering a person unconscious. Furthermore,
damage to the RAS can severely disrupt our sleep-wake cycle and because damage to the RAS can also damage our alertness, often severe enough damage to the RAS can cause a person to go into a coma or a vegetative state. In the maintenance and regulation of cortical arousal, the RAS also influences attention and more so, influences what we CHOOSE TO ATTEND; that is;
the RAS influences selective attention.
Neurons of the RAS send out a steady stream of impulses that keep the cerebral cortex active and alert; taking account of the incoming flow of sensory and motor information. The RAS can highlight certain neural information that is of particular importance, and in this way
our RAS can direct our attention and consciousness towards potentially significant events.
For example; when a driver is tired and driving and suddenly sees a cow on the road, his RAS will bombard the cerebral cortex with stimulation so that specific cortical areas (relating to alertness) can be aroused and the driver won’t run over the cow.
Our RAS can also detect and filter out certain incoming weak sensory information, so that we are able to focus on a particular stimulus.
~~~
Another way in which sensory information can be routed to the cerebral cortex is through the
thalamus.
The thalamus and RAS don’t work independent of one another; the
ascending RAS tracts connect to central areas of the thalamus – both influencing arousal and attention through the thalamus.
REMEMBER: The thalamus receives all information from our senses,
EXCEPT smell.
The thalamus serves two main roles:
-> It
receives incoming sensory information and transmits this information to the cerebral cortex; where it is then sent to a relevant cortex for the sensory information to be processed.
For example: if you saw a cute colourful coffee mug, your eye’s sensory (visual) receptors would first detect the colour of it… this sensory information would then travel to the thalamus; and the thalamus would then transmit this information to the specific cortex (in this case, the occipital lobe and the visual cortex) and you'd then say to yourself “heyy that’s a really colourful mug!
”
->The
thalamus also plays an important role in attention. The thalamus actively filters large amounts of information; giving more importance to some information and less emphasis to others. Research using PET scanning has indicated that, when people pay attention to specific sensory information, certain parts of the reticular formation and the thalamus are active. This sensory information is then sent to the relevant part of the cortex; which then communicates back to another part of the thalamus; indicating which parts of the stimulus to attend to and which parts to ignore.
This may be the reason why people with damage to certain areas (associated with attention) in the thalamus may experience difficulty filtering stimuli; that is, attending to one task while ignoring another task.
Furthermore, because the thalamus is connected to the reticular formation and nerve pathways of the reticular formation; the
thalamus plays an important role in regulating arousal.
Damage to these areas (associated with arousal) in the thalamus, results in lowered arousal which can range from lethargy (tiredness) to coma.
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