Normally, we don't even think about how it's possible for our bodies
to move -- it just happens when we want it to.

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?

Your brain and spinal cord - a message pathway that turns thought
into motion.
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

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 .


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.

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 . 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.

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.

Healthy balance of dopamine and acetylcholine.
One way to illustrate how the muscle control process works is as
follows: two buckets - one for the dopamine system and one for the
acetylcholine system - balanced on either end of a seesaw (figure
5). This depicts the situation at rest when the dopamine and
acetylcholine systems are balanced. When you decide to move, your
brain understands the movement you want to make and it sends out a
balance of dopamine and acetylcholine messages to keep that movement
smooth.








.