Parkinson's disease is a progressive neurodegenerative disorder characterized by four main symptoms: bradykinesia, muscular rigidity, resting tremors, and loss of postural reflexes. It results from the loss of dopaminergic neurons in the substantia nigra, disrupting the balance between inhibitory dopaminergic and excitatory cholinergic neurons. Treatment involves drugs that influence the dopaminergic or cholinergic systems in the brain. Levodopa is often used but must be combined with carbidopa to reduce peripheral side effects. Long term levodopa therapy can cause abnormal movements or behavioral issues.