1. Lesson 20 What is the nervous system?

2. Every school has an office. It is a very important place
Every school has an office. It is a very important place. Messages come into the office, and messages go out. Most plans for the school are made in the office.

3. In your body, the job of sending and receiving messages is done by the nervous system..
The nervous system is made up of the brain, spinal cord, and branching nerves.

4. The central nervous system is made up of the brain and spinal cord.
You have learned that the sense organs receive stimuli from the world around you. What happens next?

5. For example, how do you decide to answer the phone, or raise your hand in class?
This is how it works:

6. Stimuli from the sense organs change to electrical signals.
These electrical signals do not stay in the sense organs. Nerves carry the signals to the brain and spinal cord.

7. The brain decides what each stimulus is
The brain decides what each stimulus is. The brain also decides how to respond to each stimulus.

8. Nerves carry “what to do” messages away from the brain
Nerves carry “what to do” messages away from the brain. The messages go to the part of the body that will answer or respond to the stimuli.

9. Most “what to do” messages go to muscles. Some, however, go to glands.
Most of the responses our body has are carried out by muscles.

10. A neuron is another word for a nerve cell.

11. Neurons make paths all over your body.

12. Your nerves send electrical messages through the neurons.
At one end of the path is a sense organ, at the other, a muscle or gland that responds to the stimuli.

13. Thirty-one pairs of nerves branch out of your spinal cord.
They go to all parts of your body.

15. Some emergency responses must happen extra fast
Some emergency responses must happen extra fast. There is no time for the brain to decide how to respond. Delay could cause severe injury, or even death.

16. Think of yourself riding your bike down a winding hill
Think of yourself riding your bike down a winding hill. You have drifted into the middle of the road and round a corner.
Suddenly………
BEEEEEPPPPPPPPP

17. Before your brain even knows what is going on, you have quickly moved your bike to the side of the road.

18. How did you know to get to the side of the road if your brain knows nothing about what is happening?
When you are in severe danger, your spinal cord tells your muscles what to do.

19. After it sends its urgent message to the muscles, it sends a message to the brain to let it know what is going on.
These responses are called reflexes.