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Addiction is marked by compulsive substance use, in spite of psychological, social, and health problems, that is coupled with episodes of substance craving. However, the key concept in substance abuse is that, followed over the long-term, it has a chronic course marked by symptoms that wax and wane. Addiction can be best thought of and treated as a brain disease. For example, if one has a family history of diabetes, one then has a greater chance to become diabetic. One did not decide this, it is just a susceptibility passed on from one's parents. If one is diabetic and does not receive treatment, the diabetes will run its course, and one will have complications that will affect almost every organ system in the body. Analogously, if one has a family history of substance abuse, one then has a greater chance to become an addict. One did not decide this, it is just a susceptibility passed on from one's parents. If one uses substances of abuse, the disease will run its course, and one will have complications that will affect almost every organ system in the body.
To understand the motivation that addicts endure in their addiction, it is necessary to outline the specific areas of the brain that characterize the "reward" and "craving" that are involved in the disease. Once an individual has been sensitized to a substance of abuse the brain areas involved are powerful. To appreciate the power of the brain it is best to conceptualize brain function as following a sequential order of importance. The brain keeps important functions simple, so they are not easily disrupted. (This is in contrast to relatively non-important functions, such as deciding which shirt to wear in the morning, in which higher areas of the brain that are very complicated are utilized.) An example of something simple for the brain to orchestrate is breathing. It is a simple act that is very important, and it needs to be done constantly. The brain centers that control simple, important acts tend to be primitive, i.e., they are found in most animals and have persisted through evolution, and the primitive structure that controls breathing is the brain stem.
At the rostral end (closest to the nose) of the brain stem is an area called the midbrain. Housed in the midbrain are the origins of brain cells that utilize the neurotransmitter dopamine. These cells project throughout the brain, and the role of dopamine has been implicated in many brain functions and diseases. For example, Parkinson's disease is due to loss of dopaminergic brain cells housed in the midbrain. Stimulation of these dopaminergic neurons in the midbrain can cause a release of dopamine in a certain small area, approximately one centimeter, of the brain called the nucleus accumbens. The nucleus accumbens can be thought of as the "pleasure center" of the brain and is used to reward behaviors. Natural behaviors such as sex and eating utilize this reward pathway. How this pathway relates to addiction is that most, if not all, substances that are abused by individuals tap into this primitive pathway. This pathway reinforces the drug-seeking behaviors and craving that characterize addiction. Since this pathway is primitive and relatively simple, it is difficult to disrupt. Hence, the nature of addiction, which is marked by continued substance abuse in the face of self-destructive behavior.
As the addiction progresses, increasingly more of the substance of abuse is needed to produce the reward in the brain. This occurrence is called tolerance. Tolerance drives addicts to use more, and contributes greatly to the toxicity of abuse. Even though the addicted brain is a powerful force, there are other aspects to the phenomena of addiction. The mind and environment play an important role in the course of the disease.