http://www.parkinsonsinfo.com/about_parkinsons/brains.html
In Parkinson's disease, for reasons that are not fully understood, nerve cells in the part of the brain that produces dopamine, the substantia nigra, begin to decrease in number. This causes a decrease in the amount of the available dopamine. Also, the chemical in the synapse that breaks down the dopamine (MAO-B) continues to deplete what little dopamine is left. The overall effect is a large loss of dopamine in the brain. This throws off the normal dopamine/acetylcholine balance, since the level of acetylcholine remains normal (figure 1).
In Parkinson's disease, there is not enough dopamine to keep balance with the acetylcholine. The result of this imbalance is a lack of coordination of your movement that often appears as tremor, stiff muscles and joints, and/or difficulty moving. Currently, there is no way to stop the loss of nerve cells that produce dopamine or to restore those that have already been lost. However, there are several methods, including drug therapy, that can help you manage the slow decline in function that occurs with Parkinson's disease.
Normally, we don't even think about how it's possible for our bodies to move -- it just happens when we want it to. But before reading more about Parkinson's disease and its treatment, you will probably find it helpful to understand a little more about how the movement of your body is controlled.
Many different areas within the brain are involved in a complex chain of decisions required for even the smallest muscular movement. For an action like walking, for example, the brain must first gather all the information it needs about your body position. For example, are you sitting, lying down, or already standing up? Where are your feet? Do you have your balance? Then, the brain must add in what it knows about where you will be going. For example, do your eyes tell your brain that you'll be crossing an open field of grass or a busy street? Do your feet detect that the ground is easy to walk on or that you could lose your balance because it is bumpy or slippery?
This information comes together in a central area of the brain, called the striatum, which controls many aspects of bodily motion. The striatum works with other areas of the brain, including a part called the substantia nigra, to send out the commands for balance and coordination. These commands go from the brain to the spinal cord through nerve networks to the muscles that will then help you to move (figure 1).
The entire nervous system is made up of individual units called nerve cells. Nerve cells actually serve as a "communication network" within your body. To communicate with each other, nerve cells use a variety of chemical messengers called neurotransmitters. Neurotransmitters carry messages between nerve cells by crossing the space between cells, called the synapse (figure 2).
Neurotransmitters also allow the nervous system to communicate with the body's muscles and translate thought into motion. One especially important messenger is dopamine, which is manufactured in the substantia nigra. Dopamine is crucial to human movement and is the neurotransmitter that helps transmit messages to the striatum that both initiate and control your movement and balance. These dopamine messages make sure that muscles work smoothly, under precise control, and without unwanted movement.
The other neurotransmitter system that works in conjunction with the dopamine system to produce smooth movement uses a messenger called acetylcholine. Some of the nerve cells in the brain are specialized to use either dopamine or acetylcholine to send different messages, depending on what it is you want to do.
