Parkinson's disease (PD), or simply Parkinson's,[9] is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly and, as the disease worsens, non-motor symptoms become more common.[2][4] The most obvious early symptoms are tremor, rigidity, slowness of movement, and difficulty with walking.[1] Cognitive and behavioral problems may also occur with depression, anxiety, and apathy occurring in many people with PD.[4] Parkinson's disease dementia becomes common in the advanced stages of the disease. Those with Parkinson's can also have problems with their sleep and sensory systems.[1][2] The motor symptoms of the disease result from the death of cells in the substantia nigra, a region of the midbrain, leading to a dopamine deficit.[1] The cause of this cell death is poorly understood, but involves the build-up of misfolded proteins into Lewy bodies in the neurons.[6][4] Collectively, the main motor symptoms are also known as "parkinsonism" or a "parkinsonian syndrome".[4]