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.
The Ventral Tegmental Area (VTA) is the second major dopamine production site in the brain (the other one here adjacent in blue - the SNpc - we talked about last week. The key difference is obvious - the SNpc projects to the midbrain whereas the VTA projects to the cortex - thus its role in reward learning, depression, obsession, and a myriad of other things!
The substantia nigra pars compacta (SNpc) control some of the dopamine production in the brain. As such - it plays a role in a range of activities from motor control to learning to emotion. When it is damaged, or its projections are damaged, it leads to clinical issues such as Parkinson's Disease.
The Primary Motor Cortex essentially controls the muscles of the body. When the region of the Primary Motor Cortex corresponding to the right bicep muscle fires, then a neural pathway fires that ends up with the alpha motor neuron of the right bicep muscle contracting (bear in mind there are literally tens of thousands of neurons dedicated to controlling this one muscle). Also - note that the left Primary Motor Cortex controls the right hand side of the body, and the right Primary Motor Cortex controls the left hand side of the body. Finally, note the image below. The number of neurons in the Primary Motor Cortex dedicated to a given body region corresponds to the amount of fine control needed. So, the face requires a lot of fine motor control - think of the array of facial expressions needed - whereas the hip does not to the same extent. So, fine motor control means more neurons means a larger portion of the Primary Motor Cortex dedicated to control.
Neuroscience Bite: Broca's Area
Broca's Area is the motor speech area of the brain. What does that mean? Your brain works with language in what is called "a verbal code" amongst other things - its basically just the way that your brain represents language. When you want to "speak" Broca's Area translates the verbal code into motor commands which then can be used to contracts the muscles needed to produce speech. People with Broca's Aphasia have damage to this area and are unable to comprehensible speech - but they can produce sounds that have no meaning. It is also important to realize that Broca's Area is also a part of the overall language process and works with areas like Wernicke's Area for speech comprehension and production.
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