There are four key differences between the introvert and extrovert brain: the quantity of blood that flows to the brain, the path the blood takes through the brain, the chemicals needed to feel good and the type of nervous system most commonly activated. Introverts have a greater blood flow to the brain than extroverts. Blood flows to parts of the body that are stimulated, suggesting that introverted individuals tend to be more easily stimulated than extroverted induviduals. Extroverts must compensate for this by appealing to the outside environment for stimulation through social contact, new experiences and physical activity, which is why they tend to be more engaged with the outside world. What’s more, the path the blood takes within the introvert’s brain is longer and more complex than that of the extrovert. Perhaps unsurprisingly, blood in the introvert brain flows through areas that have a greater internal focus: like memory and planning. The extrovert blood pathway takes a more experiential route, focusing mostly on immediate sense experiences (excluding smell, for some reason). Thirdly, introverts and extroverts require different chemicals to feel good. The extrovert engine needs dopamine to run, and lots of it. Unfortunately, the extrovert neural pathway is not very sensitive to dopamine and it must make more using adrenaline. Adrenaline is produced during action, which is why extroverts feel good the more active they are: they produce more adrenaline which in turn produces more dopamine. Introverts, with their increased blood flow, are more sensitive to dopamine and can easily overindulge. So their dominant brain pathway avoids dopamine and instead uses the neurotransmitter acetylcholine (a-see-to-ko-leen). This chemical produces a good feeling when the person is engaged in thinking or feeling, so these activities are likely to be more rewarding than action for the introverted person.

Source: The Neuroscience of Introversion. | Eclectic.