Re: What controls movment of the body, and sense of balance?

Nick,
     The short answer is nerves.  I am sure you have heard of nerves 
before but perhaps you didn't know what they did.  Basically all the 
information your body needs to do the things it has to do is 
transmitted from your brain and spinal cord by nerves.  The brain is the 
portion of you that makes sense of the world around us and relays the 
proper information to the body to take proper action.  You can try to 
imagine that the brain is the top of the spinal cord and sends nerves down 
through this cord like wires.  These nerves then project out of the spinal 
cord and end on things like skin, muscle and internal organs.  Some nerves 
carry information from these structures back to the brain so the brain can 
decide what to do like moving a muscle to move whatever part of the body 
is feeling the sensation.  
     Here's an example that might help.  Say you're walking barefoot in 
your living room and you step on a pin, OUCH!  Now you can probably answer 
what you would do without knowing anything about nerves.  You would 
quickly take all your weight off of the foot with the pin in it, balance 
on the other foot, and pull the pin out.  Here is what is going on in your 
nervous system.  Walking around the room your are already using certain 
nerves to make your muscles move the way you want them to and your balance 
is evenly on both legs(we'll get more into balance in a minute).  The pin 
stabs your foot and activates the nerves that sense pain to transmit this 
information to your brain through your spinal cord.  Now the real 
interesting thing is that not all nerves have to go to the brain in order 
to make your body move.  These are called reflex nerves.  Think about 
that, cause a reflex is something that you can't control and that is part 
of what is happening in this example.  OK, the pain information goes to 
the spinal cord and brain, but it is in the spinal cord where the reflex 
action takes place.  In the spinal cord the pain nerve splits into a few 
branches.  Some of it continues to the brain for processing, but other 
fibers of that nerve connect to another type of nerve in your spinal cord 
called a motor nerve.  The pain nerve "tells" the motor nerve what has 
happened using a special chemical, and this activates the motor nerves to 
make your leg muscles move so that you don't cause yourself more pain by 
stepping down harder on the pin.  Your brain has processed the pain, 
recognizing the exact location of where it is so that you can then do 
something to stop the pain, like pull the pin out.  Your brain tells motor 
nerves in your arms and hands to move to the pin, grab it, and yank it 
out!  The amazing thing is how fast all of this happens.  It takes very 
little time at all for your foot to signal your brain and spinal cord 
what is going on and for you to take action.  
     Now, balance is also controlled through nerves, but it also has to do 
with your ears.  Your ears actually can sense when the head is moving and 
send information to your brain so that you can adjust your body to keep 
from falling over.  Your ear is a pretty complex structure inside that has 
fluid filled canals inside.  When you move the fluid inside starts to move 
as well, signalling motion to your brain.  What happens if you make a wave 
at one end of the bath tub?  The wave heads off in the intended direction, 
hits the other side of the tub and bounces back and it keeps going for a 
while.  Well the same thing happens in your ear when you move.  With 
normal turns of the head the waves of fluid inide the ear are pretty small 
and subside quickly when you stop moving.  But when you spin in a circle 
you really shake things up, and when you stop the fluid keeps going for a 
while, so your brain is still getting signals that you are moving, but 
your eyes and your body is telling the brain that your are not and some 
major confusion occurs.  So you stumble around trying to stay up, but you 
fall over.  The fluid in the ear eventually settles down and your body 
returns to its normal sense of balance.  
      These are pretty simple explanations of these things, but they get 
the point across.  I apologize for taking so long to answer your 
questions, and now I hope you now have an appreciation for how your body 
uses nerves to work.

Mark Sullivan