Parkinson's disease (also known as Parkinson disease or PD) is a degenerative disease of the brain (central nervous system) that often impairs motor skills, speech, and other functions. Parkinson's disease belongs to a group of conditions called movement disorders. It is characterized by muscle rigidity, tremor, a slowing of physical movement (bradykinesia) and, in extreme cases, a loss of physical movement (akinesia). The primary symptoms are the results of decreased stimulation of the motor cortex by the basal ganglia, normally caused by the insufficient formation and action of dopamine, which is produced in the dopaminergic neurons of the brain. Secondary symptoms may include high level cognitive dysfunction and subtle language problems. PD is both chronic and progressive. PD is the most common cause of chronic progressive Parkinsonism, a term which refers to the syndrome of tremor, rigidity, bradykinesia and postural instability. PD is also called primary parkinsonism or idiopathic PD (classically meaning having no known cause although many genetic mutations associated with PD have since been discovered). While many forms of parkinsonism are idiopathic, secondary cases may result from toxicity most notably of drugs, head trauma, or other medical disorders.