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Knee Jerks
I got the idea for this week's experiment
while trying to find a time to schedule my annual physical examination. Of all
the strange things that doctors check, probably one of the best known is the
knee jerk reflex. In addition to being the source of uncounted slapstick
comedy routines, it is also an interesting, painless part of a physical exam.
To try this, you will need:
Sit in a tall chair or on the edge of a tall
counter. Your feet should not touch the floor. To locate the right spot,
feel your knee cap. The knee cap is a round bone called the patella. The
spot we are looking for is just below the bottom edge of the patella.
Once you have located the right spot, you are
going to give it a gentle tap. No matter what you may have seen the Three
Stooges do, DO NOT USE A HAMMER! Instead, use the side of your hand. Hold
your hand out, with all of the fingers straight and together. You are going to
gently tap the edge of your hand across your leg at the spot just below the
patella. Do not hit too hard. It should not hurt. You may have to tap a few
times to find exactly the right spot. When you do, your leg should kick
slightly.
How did that happen? You didn't mean for
your leg to kick. Your brain did not tell it to kick. Instead, the kick was
triggered by a reflex. Reflexes are automatic reactions that help control and
protect your body. They jerk your hand away from something sharp or hot to
minimize the injury. When you touch something hot, if you had to wait until
the nerve impulse got all the way to your brain, decide what to do about it and
then wait for the impulse to get back to the muscles, your finger would be very
badly burned before you got moving. Instead, the nerve impulse goes to the
spinal cord and instantly triggers the muscles to jerk your hand away. You
move your hand before you actually feel the pain. The knee jerk reaction only
takes about 50 milliseconds, much faster than you could do it on purpose.
When the doctor taps your knee, he is checking
your nervous system. Both knees should give a single kick. If the response
is too strong or too weak, or if one leg is much different from the other, then
the doctor may check further to be sure you are OK.
There are several things that you can do to
change this reflex. One is called the Jendrassick maneuver. Cup your hands
and hook them together in front of you. Pull apart with your arms while using
your cupped hands to hold them together. While you are doing this, have a
friend tap your knee reflex point. This should give you a stronger reaction.
You can also try keeping your brain
occupied. Try counting backwards from 100 while a friend tests your knee jerk
reflex. Again, you should notice a stronger reaction.
Doctors use the knee jerk to test your nervous
system, but what is the real reason for this reflex? If the amount of weight
on your leg suddenly increases, this reflex pushes back, helping you absorb the
shock and keep your balance. It plays an important part in helping you walk
and run, without falling on your face.
Other reflexes serve other purposes. They
cause your eyes to blink when something moves towards them quickly. They cause
us to jump when we hear an unexpected, loud noise. They cause us to sneeze if
the inside of your nose is irritated. They cause your mouth to water when you
think about eating a big bowl of Rocky Road ice cream, with hot fudge sauce and
chopped walnuts, topped with some whipped cream and a cherry. Still, walking
to the freezer to get the ice cream is a voluntary action, so you can't blame
your spinal cord for making you eat an extra dessert.
Have a wonder filled week.
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