Final answer:
Parkinson's disease is a neurodegenerative disease that causes the loss of dopamine neurons in the substantia nigra, leading to motor symptoms. Excess dopamine in the brain is associated with schizophrenia.
Step-by-step explanation:
Parkinson's disease is a neurodegenerative disease characterized by the loss of dopamine neurons in the substantia nigra, a brain region that controls movement. The reduced dopamine levels in the brain affect the basal ganglia circuitry and result in motor symptoms such as tremors, slowed movement, and rigid muscles. On the other hand, excess dopamine in the brain has been linked to schizophrenia, a mental illness that affects a person's ability to differentiate between reality and imagination, regulate emotions, and think clearly.