Parkinson disease (PD) is a progressive neurodegenerative disorder characterized mainly by physical and psychological disabilities. This disorder was named after James Parkinson, an English physician who first described it as shaking palsy in 1817 (Goetz, Factr, and Weiner, 2002). Jean- Martin Charcot, who was a French neurologist, then progressed and further refined the description of the disease and identified other clinical features of PD (Goetz, Factr, and Weiner, 2002). PD involves the loss of cells that produce the neurotransmitter dopamine in a part of the brain stem called the substansia nigra, which results in several signs and symptoms (Byrd, Marks, and Starr, 2000). It is manifested clinically by tremor, rigidity, bradykinesia, hypokinesia, and postural instability. PD is a common disorder, affecting at least 1% of people by age 70 with it being 1.5 times more common in men than in women (Hauser, et al. 2010). The incidence has been estimated to be 4.5-21 cases per 100,000 population per year, with most studies estimating a prevalence of approximately 120 per 100,00 (Hauser, et al. 2010).
PD is known to be idiopathic, however, many studies have found that a combination of genetic and environmental factors are associated with the development of PD. Environmental risk factors include the use of pesticides, living in a rural environment, consumption of well water, and proximity to industrial plants or quarries. The genetic causes also have been studied and it was found that a genetic cause for the development of PD accounts for 5% of all cases (Hauser et al. 2010). Medications that block dopamine receptors also are considered to be a cause for the development of Parkinson disease. Interestingly, it was also found that people who have suffered a head injury are four times more likely to develop PD than those who have never had a head injury (Adams and Victor, 1985). Although many theories and hypotheses were raised as to why this happens, there is no solid information supporting the physiological cause of this, but it is surely one of various factors that may lead to the development of PD.
Neuroanatomy of the Basal Ganglia
It has been known that PD is a disorder affecting the basal ganglia. The term basal ganglia is generally applied to a collection of nuclei situated deep within the cerebral hemispheres, lying laterally to the thalamus (Gazzeniga, George, and Mangun, 2008). The basal ganglia are important subcortical structures. They are composed of five nuclei: caudate, putamen, globus pallidus, subthalamic nucleus, and substantia nigra. These nuclei are not thought of a single anatomical entity; rather, they form a functional unity whose contribution is greatly to motor control. Interestingly, this system does not influence movement through spinal cord pathways, but rather acts as part of a feedback loop to all areas of the cerebral cortex, with primary input into motor areas (Aird,...