Also known as the mesencephalon, the midbrain is the home of addiction. Usually, the midbrain (specifically a structure within it called the substantia nigra) kicks out carefully regulated levels of dopamine, but many mind-altering substances artificially jack the fun juice. We become habituated to higher dopamine levels and/or the midbrain loses its ability to produce natural dopamine levels and voilà: We’re addicted.
Dopamine disorder with roots in the midbrain also causes Parkinson’s disease. We use the collection of dopamine neurons known as the direct pathway to move, and the indirect pathway tamps down unwanted movement. In Parkinson’s disease, dopamine neurons in both pathways die. And so Parkinson’s sufferers lose the direct pathway’s ability to move voluntarily, while also losing the mellowing effects of the indirect pathway, resulting in involuntary movements such as shaking.