Anti-parkinson medications are used to treat Parkinson’s disease, which is a movement disorder where the dopamine-producing neurons in the substantia nigra of the brain undergo degeneration. It’s a progressive, adult-onset disease, and is more common with age, affecting about 1% of people over 60. The substantia nigra is a part of the basal ganglia, a collection of nuclei in the brain that control movement through their connections with the motor cortex. The substantia nigra actually can be split into two sub-regions. First, there’s the pars reticulata, which receives signals from another part of the basal ganglia called the striatum, and relays messages to the thalamus via neurons rich in the neurotransmitter GABA, also known as gamma-aminobutyric acid. Second, there’s the pars compacta, and this is the part of the substantia nigra affected in Parkinson’s.
The pars compacta sends messages to the striatum via neurons rich in the neurotransmitter dopamine, forming the nigrostriatal pathway, which helps to stimulate the cerebral cortex and initiate movement. In Parkinson’s disease, the neurons in the substantia die, so the individual may experience hypokinesia, which is difficulty initiating movements, and bradykinesia, or slowed movements. The substantia nigra also helps to calibrate and fine tune a person’s movements, which leads to the other clinical features of Parkinson’s, like “pill-rolling” tremor, which is the repetitive, involuntary rubbing of the thumb and index finger, as well as rigidity, stooped posture, and an expressionless, mask-like face. As the neurodegeneration progresses, non-motor dysfunction can appear, including depression, dementia, sleep disturbances, and difficulty smelling.