Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Parkinson disease ?
    ### Response:
    What causes Parkinson disease? Parkinson disease occurs when the nerve cells in the brain that make dopamine, a chemical messenger which transmits signals within the brain to produce smooth physical movements, are slowly destroyed. Without dopamine, the nerve cells in the part of the brain known as the substantia nigra cannot properly send messages.
    