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Unit 19: Biology Part 2 Page 105 of 137

Activities and Experiments

Lesson 5: Controlling the Body

Experiment: Measuring Reflexes

Usually our brain sends messages to the muscles of our body, causing them
to expand or contract. This makes our body move the way we need it too.
Sometimes, such as when something unexpected happens, our body doesn’t

take the time to send the message all the way to the brain. Instead, the
message goes as far as your spinal cord, and the spinal cord gives the
muscles the messages they need. This is called a reflex. How good are your
reflexes?

Materials:

 Yardstick
 Partner

1. Have a partner hold a yardstick in front of you.

2. Put your fingers around the bottom of the yardstick, but don’t touch it.
3. Without warning you, have your partner let go of the yardstick.
4. Try to stop the falling yardstick by squeezing it with your fingers as
soon as you notice it falling.
5. When you stop it, record how high up on the yardstick you are (in
inches or centimeters.) The smaller the number, the quicker your
reflex.

What’s Happening: Stopping the ruler is a reflex. Your brain was not
involved. Other reflexes include removing your hand from a hot or painful
object.

Experiment: Message Received

Your brain sends messages to your muscles, as well as to all the other
organs in your body. Sometimes, you have no control over these messages,
such as when the brain controls breathing, digestion, or blood circulation.
Other times, such as when you are exercising or just walking around, you

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