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2.1: Thinking about the Nervous System

Difficulty Level: At Grade Created by: CK-12
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Your Brain is Amazing

What is the most complex thing you can think of? Some candidates might be a 747 jumbo jet, a super computer, NASA mission control, an aircraft carrier, and an ecosystem. None of those things are as complex as your brain. Starting in this section and continuing through this unit, we are going to explore how the brain works. This exploration is really one of the most exciting areas in all of science.

“If your brain is bigger, are you smarter?”

“How can the brain think?”

“How does the brain keep memories and thoughts?”

“Why do we have feelings?”

“Why do I feel so different?”

-7th graders, wondering about the nervous system

Your Brain Is a Super Computer

The human brain is the most complex matter in the known universe! It weighs 1.4 kilograms (a little more than three pounds). That's about the weight of a cantaloupe or a pineapple. It has the appearance and consistency of grayish-cream-colored pudding or tofu. The brain is complex because it is made up of billions of cells that have hundreds of billions of connections between them. The connections make it possible for the cells to work together as a really super computer. Your brain is truly amazing. It makes it possible for you to learn all there is to know about a jumbo jet, a super computer, an aircraft carrier, an ecosystem, or a NASA mission. But, the human brain cannot yet understand itself completely. That is one of our great challenges for the future.

“... if Annabel's mind was in Annabel's body, which it certainly seemed to be, and Annabel's mind was also in Ma's body, which it most definitely was (me is me, no matter what I look like), then where did Ma's mind get to? Somebody else's body, I suppose-but whose?”

-Freaky Friday,

Mary Rodgers

Your brain processes all the information that comes from your senses of seeing, hearing, smelling, touching, and balancing. Your brain stores lots of information as memories and then uses those memories to evaluate or even invent new information. Your brain is responsible for your personality, your feelings, your beliefs, and, most importantly, your ability to think and to communicate your thoughts. Who you are depends more on your brain than on anything else. You have probably heard of heart, kidney, and liver transplants, but there is no such thing as a brain transplant.

Suppose for a minute that brain transplants were possible, as in a science fiction movie. If you had a brain transplant, would you be you, or would you be the person who donated the brain? Why?

The Brain Is Only One Part of the Nervous System

To work and to control the body, the brain needs to receive information from sensors, and it needs to send information out to the muscles and organs of the body. To understand how the brain receives and sends information, we need to consider the other parts of the nervous system.

Continuous with the brain is the spinal cord, which leaves the head through a hole in the bottom of the skull. The spinal cord runs down the center of the backbone or spinal column. The spinal cord is like the main cable of a huge telephone system. It contains thousands of information tracks carrying signals to and from the brain.

The brain and the spinal cord are called the central nervous system, or CNS. The CNS communicates with the rest of the body through the peripheral nervous system, or PNS.

Did You Know?
The neuron is the basic building block of your nervous system. You were born with about 15 billion neurons in your body. Since your birth, your neurons have been busy storing memories and information about your world. Although other cells in your body die and are replaced, your neurons are not. If a neuron dies, it is gone forever.

The peripheral nervous system is a huge network of nerve fibers that run throughout all parts of your body. They transmit information between all tissues and the nervous system. The nerve fibers are made up of nerve cells called neurons, which are the building blocks of the nervous system. If you could dissolve away all of the cells of your body except neurons, you could see the brain and spinal cord as solid structures. The rest of your body would be a ghost made up of thousands and thousands of fibers ranging in size from the diameter of thick string to fibers so small they can only be seen with a microscope.

Did You Know?
The sperm whale has the largest brain of any animal. An average sperm whale brain weighs about 9 kilograms (20 pounds).

Your brain is more complicated than any computer anywhere. As you learned earlier, it's about the size of a cantaloupe, weighs 1.4 kilograms (about 3 pounds), and contains billions of neurons. Your brain keeps all parts of you working smoothly and lets you do different things at the same time. For example, because of the way your brain works, you can chew gum, ride a bicycle, and think about a movie all at once.

We have compared the brain to a computer, but there is a really big difference between how the brain works and how most computers work. A computer is good at handling one piece of information at a time, but doing it very fast. The brain actually handles each piece of information-such as information from a flash of light, a sound, or a touch-rather slowly. But unlike a computer, the brain handles thousands of separate pieces of information at the same time! That is why you can chew gum, ride a bicycle, wave to your friend, watch out for traffic, and think about your homework all at once.

Your brain is always doing hundreds, even thousands, of things at the same time. Sometimes you are aware of what it is doing, but most of the time you are not. For example, imagine the following situation. You are home alone taking a bath when the phone rings. You think it could be an important call from your study buddy. You spring out of the warm water, dry yourself quickly, and dash out the door for the phone. As you hit the floor running, you stumble over the toy your little brother left on the floor, but you don't fall. You keep your balance, hurrying to answer the phone. You're still drying off when you get to the phone and pick it up. It's a wrong number.

Figure 1.1 The human brain contains billions of nerve cells. They process and store information, keep the organs of the body running smoothly, and allow us complex thought, memory, and speech.

What did your brain do while all this was going on? Signals coming from your sensors told you about the bath water temperature, the smell of the soap, where the towel was, and the sound of the phone. As these signals came into your brain, they triggered other signals that helped you recall emotions, memories, thoughts, and plans. You knew the sound was the phone not the doorbell, because you remembered the difference. You thought the caller was your friend, because you planned to do homework together. You felt happy because you imagined yourself talking to your friend. Almost immediately, signals coming from your brain told other parts of your body what to do. You stood up and stepped carefully out of the tub. You reached for the towel, dried yourself, and began to run. You tripped on the toy but kept your balance. You got angry at your lime brother for leaving the toy. But you still got to the phone before it stopped ringing.

While you were running to answer the phone, your brain also did many other things. For example, it made you breathe, controlled your heart rate and blood pressure, and noticed that your skin temperature cooled. Your brain even sent out commands to keep you from falling when you tripped on the toy-all without your having to think about it.

Figure 1.2 Your nervous system is made up of your brain, spinal cord, and a network of nerve fibers.

Did You Know?
Many neurons have long extensions. They are the longest cells in your body. Some neurons go all the way from your spinal cord to the tips of your toes or fingers.
Did You Know?
When you are born your spinal cord is about as long as your bony spinal column. As you grow, your spinal column gets longer, but your spinal cord does not. That is why as an adult your spinal cord ends about in the middle of your back.

As you can see, your brain has to be able to do many things at once. It has to tell your body to do all the work of maintaining itself 24 hours a day, every day. Your brain also controls all of your activities, such as playing basketball, writing a poem, speaking a language, riding a bicycle, playing the piano, or just daydreaming. Your brain has a lot to do. That's why you need so many brain cells.

The study of the nervous system is called neurobiology. What do you call someone who studies neurobiology? What subjects do you think a person would have to study to become a scientist who studies neurobiology?

Figure 1.3 Follow the diagram to see how information moves in your nervous system. The left side of the diagram, labeled Sensory, shows how your body sends information to the brain. The right side of the diagram, labeled Motor, shows how the brain sends instructions back out about what to do to keep things running smoothly.

We Can Compare the Nervous System to Mission Control

Your nervous system works to control your body like NASA's Mission Control works to control the flight of the space shuttle. The computers at Mission Control receive information from all over the world. The computer programmers, flight controllers, and other people use this information to make decisions and tell the astronauts how to do their work and return safely to Earth.

Billions and Billions

Fill up a measuring cup with kernels of corn (or peas, or beads). Count how many kernels are in the cup. Calculate how many cups you would need to fill before you had one billion kernels of corn.

Just as NASA's Mission Control receives all kinds of information about weather patterns, the orbits of satellites and space debris, the conditions at landing sites, and the conditions inside the space shuttle, your CNS continuously receives information about the world around you and the conditions inside your body. The CNS uses this information to tell your organs what to do to keep the conditions inside your body within safe limits. The maintenance of fairly constant conditions in your body is called homeostasis. Even when conditions in the external environment change, your nervous system controls the activities of your organs to keep your internal environment constant.

For example, your body temperature stays about the same in cold weather or hot weather. When it gets cold outside, your brain tells you to put on a sweater and seek a warm place. If that isn't enough, your brain makes you shiver to produce heat. When it gets hot outside, your brain tells you to take clothes off, seek shade, and sweat to cool down. To understand homeostasis think about balancing a seesaw. If one end starts to go down, pushing on the other end will make the seesaw balance. In a similar way, if conditions in your body begin to change, your body takes action to restore the balance. Your nervous system collects information about the changing conditions. Then, your nervous system tells your muscles, organs, and glands what to do to keep conditions inside the body constant.

Figure 1.4 You can control some nervous system functions, such as walking or running. But you're not even aware of many of your nervous system functions, such as controlling heart rate or controlling digestion.

What Does Your Brain look like?

If you cut back the skin covering the skull (the scalp), and saw off the top of the skull, you can see the brain. Actually, what you see is the part of the brain called the cerebral cortex. It looks like grayish-beige, squiggly custard or tofu wrapped in a thin sheet of plastic. The squiggly custard-like material consists of billions of brain cells. The membranes that cover and protect the brain look like a thin sheet of plastic.

Figure 1.5 If you removed the skin and bone covering the top of the head, you would see the part of the brain that scientists call the cerebral cortex.

These membranes are called the meninges (meh-NIN-jeez). A serious disease called meningitis (meh-nin-JI-tis) occurs when these membranes become infected or inflamed, causing terrible headaches. If you could see the brain, you would notice blood vessels running all over the surface, between the meninges. Those blood vessels bring oxygen and sugar to the brain cells and take away their wastes.

It is hard to imagine what the brain does just by looking at it. Think about looking inside a television set. Does seeing the picture tube, the wires, and the electronics help you understand how TV programs are produced, broadcast, received, and shown? You may not know exactly how all the stuff in a TV makes it work, but it is a place to start. The same is true with investigating the brain. Look at Figure 1.5. This is an illustration of what your brain looks like, as seen from the top of your head.

Did You Know?
During infancy and childhood, 29 separate bones join together to make up the skull. Of these, 14 make up your facial structure, giving you some of the features you see in the mirror.

How Your Brain Is Protected from Damage

Since the brain is soft like custard or tofu, it is very delicate and must be protected. Your skull protects your brain. This armor-like covering of bone prevents your head from splitting open like a pumpkin whenever you fall. However, the skull can break or fracture if it is hit hard enough. A blow to the head may not fracture the skull, but could cause a concussion (kon-KUH-shun). A concussion is a brain injury somewhat like a bruise. To prevent fractured skulls and concussions in possibly dangerous situations, we need to wear bicycle helmets, football helmets, seat belts, and other kinds of protective gear.

In addition to the strong bony skull, a cushion of fluid protects your brain from damage when you fall, bump, or even shake your head. This fluid, that is found between the skull and the membranes covering the brain, works like a water bed to cushion the brain. It is called cerebrospinal (ser-REE-broh-spi-nul) fluid. This fluid is formed from the blood.

What Do You Think?
Think about what you have learned about the brain so far. Now think about the bike helmet laws in effect in many areas. These laws, which are sometimes controversial, require anyone riding a bicycle to wear a helmet. Do you think a bike helmet law is a good idea? Is such a law fair? Explain your responses.

The skull and the cerebrospinal fluid protect the brain from some physical damage. But the brain needs to be protected in other ways. For example, germs and dangerous chemicals that can damage the brain may enter the bloodstream. The brain needs extra protection from these dangers because, unlike other body tissues, the brain usually cannot grow new brain cells to replace damaged ones. A system called the blood-brain barrier keeps germs and some chemicals from entering the brain.

Did You Know?
Because brain cells cannot replace themselves, any damage from chemicals, drugs, or physical injury may be permanent.

Figure 1.6 Egghead apparatus.


Get two raw eggs. Put one carefully into a screw-top jar. Put the other into a similar jar filled to the top with water. Now shake both jars. What happens? What do the egg, the water, and the glass jar represent?

Activity 1-1: The Blood-Brain Barrier (BBB)


This activity helps explain what the blood-brain barrier (BBB) is and how it protects the cells of your brain. Like all of the other cells of your body, your brain cells obtain nutrients from the blood. Tiny blood vessels go through all parts of your brain. The blood also can transport substances other than nutrients. For example, blood can contain bacteria, drugs, toxic chemicals, alcohol, and nicotine. Substances such as these can alter the function of brain cells, and they can kill brain cells. Since you cannot replace the brain cells you lose, it is important to protect brain cells. To protect brain cells, the tiny blood vessels that pass through the brain are much less leaky than are the tiny blood vessels in other parts of your body. Also, these blood vessels are wrapped by special cells called glia. The tight blood vessels and their covering of glial cells create a barrier between the blood and the brain-the blood-brain barrier. But substances can get through the blood-brain barrier depending on their chemical characteristics. Since the barrier consists of the membranes of blood vessel cells and glial cells, and membranes consist largely of lipids (fats), anything that is more soluble in lipids than in water can get through the BBB more easily than something that is soluble in water but not in lipids. Of course, some things are soluble in both water and lipids. Alcohol is an example.

In this activity you explore the inability of oil (representing membranes) and water (representing blood) to mix, and you test the solubility of different substances in oil and water.


  • Safety goggles
  • 6 test tubes with stoppers
  • Test-tube rack
  • Marking pen
  • Masking tape
  • Clear cooking oil (to represent the membranes of the BBB)
  • Sesame or motor oil (to represent a fat-soluble substance)
  • Water with a little red food coloring (to represent blood)
  • Blue food coloring (to represent a water-soluble substance)
  • Alcohol
  • 3 eyedroppers
  • Paper towels
  • Colored pencils (red and blue)
  • Activity Report

CAUTION: Wear safety goggles anytime you are working with chemicals. Check your lab safety rules.


Step 1 Label 4 test tubes A, B, C, and D. You can use strips of masking tape for the labels.

Step 2 Pour equal amounts of clear cooking oil (representing BBB membranes) and redcolored water (representing blood) into each test tube.

Step 3 Complete the setup for each test tube as follows.

Test Tube A: Stopper tightly.

Test Tube B: Add 10 drops of blue food coloring (representing water-soluble substance). Stopper tightly.

Test Tube C: Add 10 drops of motor oil or sesame oil (representing a fat-soluble substance). Stopper tightly.

Test Tube D: Add 10 drops of motor oil or sesame oil and 10 drops of blue food coloring. Stopper tightly.

Step 4 Answer questions 1, 2, and 3 on the Activity Report.

Step 5 Predict what would happen in test tube D if you shook the test tube for 10 seconds. Record your predictions on the Activity Report. Note any differences between your predictions and your results. Offer explanations for any differences.

Step 6 Prepare 2 new test tubes labeled E and F. Put clear cooking oil in test tube E. Put an equal amount of red-colored water in test tube F. Put equal amounts of alcohol into both test tubes E and F. Record your observations on the Activity Report.

Step 7 Review and complete the remaining questions on the Activity Report. Follow instructions for cleanup in the laboratory.

Did You Know?
The membranes of all your cells are made up of lipids (fats) and proteins.

Small blood vessels, called capillaries (KAP-ih-layr-ees), in the brain form the blood-brain barrier. Throughout your body, capillaries carry blood that supplies your cells with oxygen and sugar and takes away carbon dioxide and other wastes. Most capillaries have many tiny holes that allow these substances to move between the bloodstream and your cells. The capillaries in the brain are much less leaky, so it is harder for molecules and germs circulating in the blood to pass through.

Star-shaped glial cells called astrocytes (AS-trow-sites) are also part of the blood-brain barrier. Astrocytes are one kind of glial cell. Glial cells play many supportive roles in the brain, and there are more of them than there are nerve cells. Parts of these astrocytes wrap around and cover the brain's capillaries and help control which substances enter the brain. Astrocytes can help certain substances cross the blood-brain barrier but help keep other substances out.


Your nervous system is as unique as your fingerprint. Your brain makes you who you are. Think about the following questions and write a few responses to share with your class. Who are you? What makes you unique? Do the things that make you unique involve your brain?

The blood-brain barrier can protect your brain from many substances, but it cannot protect against everything. For example, substances that dissolve in lipids (fats) can cross the blood-brain barrier very easily. You may remember that the membranes of all your cells, including those in your capillaries, are made up of lipids. Substances that dissolve in lipids can cross these cell membranes simply by dissolving in the membrane. Substances that can pass through the blood-brain barrier in this way include alcohol, nicotine, caffeine, anesthetics (painkillers), and heroin. These substances can be harmful because they can pass through the cell membranes into the brain cells.

What aspects of yourself and your brain interest you the most? Do you ever surprise yourself! Have you ever heard yourself say something and wonder where the thoughts came from? Explain your responses.

Review Questions/Answers

  • Sample answers to these questions will be provided upon request. Please send an email to teachers-requests@ck12.org to request sample answers.
  1. What are the three major divisions of the nervous system?
  2. In general, what does the nervous system do?
  3. List three aspects of the nervous system and brain you find remarkable. Explain why they are remarkable.
  4. How is the brain protected from damage?
  5. Why is wearing a bicycle helmet important?
  6. What substances does the blood-brain barrier not keep out? Why?
  7. Why can drugs or alcohol affect behavior so quickly?

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