progression of the parkinson's disease as the pathophysiology develops further

The study of disease is called pathology which involves the determination of the cause (etiology) of the disease, the understanding of the mechanisms of its development (pathogenesis), the structural changes associated with the disease process (morphological changes) and the functional consequences of those changes. Correctly finding the cause of a disease is necessary for the proper course of treatment.
Humans, other animals, and plants are all susceptible to diseases of some sort. However, that which disrupts the normal functioning of one type of organism may have no effect on the other types.

The main pathological characteristics of Parkinson's disesase are cell death in the basal ganglia (affecting up to 70% of the dopamine secreting neurons in the substantia nigra pars compacta by the end of life) of brain and the presence of Lewy bodies (accumulations of the protein alpha-synuclein) in many of the remaining neurons. This loss of neurons is accompanied by the death of astrocytes (star-shaped glial cells) and a significant increase in the number of microglia (another type of glial cell) in the substantia nigra.

There are five major pathways in the brain which connects other brain areas with the basal ganglia. These are known as the motor, associative, limbic and orbitofrontal circuits, with names indicating the main projection area of each circuit. All of them are affected in parkinson's disease and their disruption explains many of the symptoms of the disease, since these circuits are involved in a wide variety of functions, including movement, attention and learning, the motor circuit has been examined the most intensively.

The basal ganglia normally exert a constant inhibitory influence on a wide range of motor neurons, preventing them from becoming active at inappropriate times. When a decision is taken to perform a particular action, there is reduced inhibition for the required motor system, thereby releasing it for activation. Dopamine facilitate this release of inhibition. The high levels of dopamine  tend to promote motor activity while the low levels of dopamine, such as occur in parkinson's disease, demands for greater exertions of effort for any given movement. Thus, the net effect of dopamine reduction is to produce hypokinesia, an overall reduction in motor output. Drugs that are used to treat parkinson's disease may produce excessive dopamine activity, allowing motor systems to be activated at inappropriate times and thereby producing dyskinesias.