Parkinson's disease is a chronic, progressive and incurable disease of the central nervous system. It affects walking, talking, writing and swallowing, characterised by involuntary movements, such as shaking, rigidity and difficulty walking. It can also include non-motor symptoms such as pain, sensory changes, gastrointestinal system changes, depression and problems with memory, thought and sleep. The condition is linked to an imbalance between two brain chemicals, or neurotransmitters: dopamine and acetylcholine. The destruction of cells in a tiny region of the brain called the substantia nigra leads to a lack of dopamine, which is important in the control of movement.
An Australian study in 2011 estimated that about one in 350 Australians had the condition. In 2018, it is closer to one in 300, or approximately 80,000 people, making it one of the most common neurological conditions, second only to dementia. The condition can strike in early adulthood, but most of the new cases detected afflict people who are older than 65.
Damaged brain cells emit chemical noise, rather than a series of appropriate chemical signals. The brain has to sift through the unclear message to identify genuine movement signals, which slows reaction time from the normal 80 milliseconds (ms) to more than 250 ms, characteristic of Parkinson's patients, and also results in false movement commands. Slow reaction time is one basis for diagnosing the severity of Parkinson's disease.