The reticular activating system is a collection of nuclei and thus neurons in the brain stem that have a critical purpose - it helps to "gate" or "filter" information passed through the brainstem to the cortex and from the eyes and ears (see the diagram). The reticular activating system also helps gate outgoing motor output. Deficits in neurotransmitters in ADHD cause differences in the function of the reticular activating system which underlies some of the differences in behaviour we see in people with ADHD.
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