Parkinson's Disease

How Our Brain Controls Our Movement

 

In order to understand Parkinson's Disease, it is helpful to examine the function of the brain in controlling our movements.

 

Many different areas within the brain are involved in a complex chain of decisions required for even the smallest muscular movement. Take walking for example. The brain must first gather all the information it needs about your body position. 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. Will you be crossing an open field of grass or a busy street (This information is sent by your eyes to the brain)? Is the ground is easy to walk on or could you lose your balance because it is bumpy or slippery (brain enlists the information sensed by your feet for this)?

 

Figure 1. Your spinal cord acts as a giant message highway for transmitting the messages from the brain into motion.

All this information is compiled by the brain 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 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).

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.

 

 

Figure 3

When a dopamine message is needed, a nerve cell that produces dopamine gathers packets within itself filled with dopamine particles. These packets carrying the dopamine move to the end of the nerve cell, open a "window," and release the dopamine particles into the synapse. The dopamine particles flow across the synapse and fit into special pockets on the outside of the neighboring, or receiving, nerve cell (figure 3). The receiving cell is now stimulated to send on the message, so it gathers its own packets of dopamine and passes along the message to the next nerve cell in the same way.

 

After the receiving cell has been stimulated to pass along the message, the pockets then release the dopamine back into the synapse. To fine-tune coordination of movement, these "used" dopamine particles, along with any excess dopamine that did not originally fit into a pocket on the receiving cell, are broken down by a chemical in the synapse called MAO-B (figure 4). This is an important step in the precise control of muscle movement. Too much or too little dopamine can disrupt the normal balance between the dopamine system and another neurotransmitter system, and interfere with smooth, continuous movement.

 

 

Figure 4

Acetylcholine is another neurotransmitter system that works in conjunction with the dopamine system to produce smooth movement.  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.

 

What Happens in Parkinson's Disease

 

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/acetylchol ine balance, since the level of acetylcholine remains normal.and there is not enough dopamine to keep balance with the acetylcholine. The basal ganglia are thus prevented from modifying the nerve pathways that control muscle contraction. As a result, the muscles are overly tense, causing tremor, joint rigidity, and slow movement. Most drug treatments increase the level of dopamine in the brain or oppose the action of acetylcholine.

 

 

Healthy state During movement, signals pass from the brain's cortex, via reticular formation and spinal cord (pathway A), to muscles, which contract. Other signals pass, by pathway B, to the basal ganglia; these damp the signals in pathway A, reducing muscle tone so that movement is not jerky. Dopamine, a nerve transmitter made in the basal ganglia, is needed for this damping effect. Another transmitter, acetylcholine, inhibits the damping effect.

Parkinson's disease   In Parkinson's disease, degeneration of parts of the basal ganglia causes a lack of dopamine within this part of the brain. The basal ganglia are thus prevented from modifying the nerve pathways that control muscle contraction. As a result, the muscles are overly tense, causing tremor, joint rigidity, and slow movement. Most drug treatments increase the level of dopamine in the brain or oppose the action of acetylcholine.