Neuroplasticity

by Moheb Costandi

Reward, Motivation, and Addiction Addictive narcotic and prescription drugs act on and modify brain systems involved in reward and motivation. The most important of these systems is the mesolimbic pathway, which begins in a small region of the midbrain called the ventral tegmentum. In the human brain, the ventral tegmentum contains approximately 400,000 neurons. These cells synthesize and release the neurotransmitter dopamine and project their long axonal fibers to the nucleus accumbens, part of a set of subcortical structures called the basal ganglia, which are involved in procedural learning, habit formation, and the control of voluntary movement. The nucleus accumbens in turn projects to numerous other brain regions, including parts of the cerebral cortex involved in memory and decision making and the amygdala, a small, almond-shaped structure involved in fear, anxiety, and assigning emotions to our experiences. Normally, these structures cooperate to translate motivation into goal-directed actions in order to obtain natural rewards such as food, water, and sex. The nucleus accumbens plays a central role in these processes. Everything that we find pleasurable causes ventral tegmentum neurons to fire and release dopamine into the nucleus accumbens, which then evaluates how rewarding it is, according to the amount of dopamine released. For this reason the nucleus accumbens is popularly referred to as the brain’s “reward center,” and dopamine as “the pleasure molecule,” al...