Parkinson's disease (characterized by tremors and progressive rigidity of limbs) is caused by degeneration of brain neurons that are involved in movement control and make use of neurotransmitter:

Parkinson's disease is a degenerative, progressive disorder that affects nerve cells in deep parts of the brain called the basal ganglia and the substantia nigra. Nerve cells in the substantia nigra produce the neurotransmitter dopamine and are responsible for relaying messages that plan and control body movement. Related Theory Dopamine plays important roles in executive function, motor control, motivation, arousal, reinforcement, and reward through signaling cascades that are exerted via binding to dopaminergic receptors. Caution Dopamine system is linked to motivation. Serotonin is involved in emotions, which can affect your overall mood.