To visualize this process, picture the brain as the body's communications system. It receives stimuli from the sense organs and, in return, sends messages to various parts of the body. These messages transmit instruction which govern specific bodily functions. If, in any way, there is an interference in this transmittal process, then, like a damaged communications system, some breakdown occurs. Messages pass between the cells in the substantia nigra and the cells in the striatum through the aid of dopamine, a chemical substance which acts as a transmitter.
Dopamine is one of several such chemical transmitters found in the brain.
It is the most important chemical transmitter governing the activity of the substantia nigra and the striatum, and, therefore, of movement, balance, and walking. In PD, many of the cells of the substantia nigra, and a few cells in the striatum are damaged. These damaged cells may contain pink staining spheres called Lewy bodies. Why the Lewy body appears, and how it is involved in damaging cells is not known. Paralleling the degree of cell loss in the substantia nigra is a loss of dopamine in the striatum.