Final answer:
Illegal drugs primarily affect the nucleus accumbens in the brain, which is a part of the limbic system responsible for emotions, motivation, and reward.
Step-by-step explanation:
Illegal drugs primarily affect the nucleus accumbens in the brain. The nucleus accumbens is a part of the limbic system, which is responsible for emotions, motivation, and reward. When drugs stimulate the nucleus accumbens, they can produce feelings of pleasure and reward, leading to their popularity. Illegal drugs gain their popularity because they primarily affect the nucleus accumbens, a central component of the limbic system known for its significant role in the reward circuitry of the brain. These drugs typically influence the level of neurotransmitters, especially dopamine, within the nucleus accumbens which is closely associated with pleasure, addiction, and reward. Many psychoactive substances function as either agonists or antagonists to neurotransmitters to exert their effects, thereby affecting emotions, motivation, pleasure, and reinforcement learning - key factors in the development of drug addiction.