The brain's dopaminergic system is a series of pathways that moderate control of some behaviors and of voluntary movement. It depends on the neurotransmitter dopamine, which is produced in the midbrain. The dopaminergic system activates reward sensations during various, usually pleasant, activities, and its malfunction is linked to drug and alcohol addiction by this particular response. Parkinson's disease symptoms result from the destruction of dopamine-producing neurons, which are treated in part by the administration of dopamine to patients to restore the system's normal function.
The dopaminergic system originates in the midbrain, where dopamine is produced in the neurons of the substantia nigra from a molecule called L-DOPA, for leva-dopa. From there, the axons of those nerves synapse on locations across the brain. One set of axons influences cognition in the frontal lobes, particularly judgment and similar control mechanisms, such as decisions about whether or not to act in a certain way. Another branch of nerves reaches the temporal lobe's limbic system, where dopamine modulates habit formation by enhancing the neural correlation between pleasure and a given behavior.
The reward system of the brain involves pathways of neuron groups in the midbrain and cerebral cortex, especially in the mesolimbic system. When pleasurable experiences like food, drink, sex, and intake of various drugs occur, dopamine is released. This causes a feeling of reward, a high, which then leads to psychological reinforcement of the initial pleasurable behavior, and gradually increases the amount of the behavior or substance required to produce the reward sensation. For this reason, many researchers think that altered or overactive dopamine pathways may be the ultimate cause of addictive behaviors.
An important role of the dopaminergic system lies in the control of voluntary movement, a process moderated by release of dopamine. Parkinson's disease is caused by the degeneration of the dopamine-producing nerve cells in the brain stem and midbrain, particularly the important regions of the substantia nigra and the locus ceruleus. When the dopaminergic system is disrupted by the depletion of dopamine supplies, and cells no longer manufacture more of the neurotransmitter, the motor control regions of the brain, like the cerebellum, cease to function normally. This results in jerky movements, and impaired walking and grasping.
Dopamine is injected intravenously to increase the activity of the sympathetic nervous system, stimulating higher blood pressure and general awareness. The treatment of movement disorders caused by Parkinson’s disease is a major pharmacological intervention into the dopaminergic system's function. Since dopamine cannot cross the blood-brain barrier and thus reach the areas affected by Parkinson’s disease, the precursor substance L-DOPA is injected into patients instead. It can cross into the brain and be metabolized there into its active form by a series of chemical reactions.