Parkinson's disease

Parkinson's disease is dopamine deficiency in the circuit that permits movement

The basal ganglia normally act as a brake on unwanted movement, releasing the brake via dopaminergic input from the substantia nigra to the striatum to permit intended movement smoothly. In Parkinson's disease, progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta removes this facilitating signal, so the basal ganglia's inhibitory output to the thalamus and motor cortex becomes relatively excessive and movement is constantly over-suppressed. This is why the disease produces too little movement, bradykinesia, rather than abnormal extra movement.