In a motion as simple as picking up a pencil, thousands of neurotransmitters are sent from the brain through the spinal cord to the nerves in the arm and hand. The nerves stimulate the appropriate muscles in the hand to pick up the pencil. In Parkinson's Disease, some of the cells in the substantia nigra are damaged and therefore are not producing dopamine. This leads to an imbalance between the amount of dopamine and the amount of acetylcholine in the brain. This chemical imbalance is what causes the difficulty in smooth muscle movements which characterizes Parkinson's Disease.
Many patients experience tremor - a constant shaking.
Instead of a smooth, quick motion to pick up the pencil, the hand muscles are trembling, which makes the goal of picking up the pencil much more difficult. The development of Parkinson's Disease is slow and gradual as more and more cells in the brain that produce dopamine die. Patients do not just wake up one day with severe tremor. Early signs of Parkinson's Disease can be hard to recognize and are often written off as part of regular aging. Diagnosis is made by a neurological examination and evaluation of symptoms by a medical professional.