How does the brain work?
The answer to that question depends on what you look for, the tools you use, and the questions you ask. In this section you will explore the brain to find out how it works. You will be able to answer questions you have now and more questions that come up as you explore the workings of the brain.
Imagine that an alien spacecraft landed on the top of the Seattle Kingdome sports complex during a baseball game. Then imagine that the aliens' job is to find out what's going on inside the dome. Each alien in the group has a different set of tools. One drills a hole in the roof of the sports complex and lowers a camera down to take pictures. The pictures show that the people inside are organized in a way that suggests they are doing different things. Those in the center are mostly dressed in two types of uniforms and are arranged in a pattern. One person is dressed in black. The people all around the sides are arranged in rows and sections but look very different. In the aisles are some people carrying big boxes with the words “Hot Dogs” and “Drinks” written on them.
Figure 2.1 An alien looks into the Kingdome and wonders what is going on.
Another alien lowers several very small microphones into different parts of the sports complex to record the sounds. A microphone placed near one of the people records, “Peanuts!” “Get-chur red-hots here!” Near the person in black, the alien hears “Strike three-you're out!” Just after that, a microphone lowered into one of the rows of people records “Get rid of the umpire!” When this microphone is moved over to another row it picks up, “Did you hear-the weather's s'pose to be nice for our picnic?”
A third alien places a dish-shaped microphone on the surface of the Kingdome. This dish record big bursts of noise, and then periods of low-level random noise. How difficult do you think it would be for the aliens to figure out the rules of baseball using their techniques? Scientists who want to understand how the brain works face a challenge much greater than our imaginary aliens on the Kingdome. But they use a similar strategy of applying different tools and methods to gather facts.
Figure 2.2 Another alien lowers a microphone into the Kingdome to find out what kinds of sounds are being made.
Figure 2.3 One alien tries to measure the sounds of the entire crowd inside the Kingdome.
The rules of baseball would probably be pretty difficult for the aliens to figure out from the clues above. But they are no comparison to figuring out how the brain works. Neurobiologists (NUR-oh-bi-AHL-uh-jists) are scientists who study the brain. They use a variety of different techniques to learn new information about the brain. Each new piece of information they learn is only a very small part of the whole story of the brain. Some neurobiologists study the structure and organization of the brain. Others study chemistry of the brain. Still others use tools like the microphones of the aliens to record what single nerve cells or huge numbers of nerve cells are doing.
Doctors can investigate problems inside the brain and body without having to cut a patient open. Do library research to find out what MRI scans, CAT scans, and PET scans are. Write a report describing how they relate to the brain and other parts of the nervous system.
How Do We Study How the Brain Works?
Neurobiologists have methods that enable them to observe how the brain works. Think about how our imaginary aliens lowered microphones into the Kingdome. The brain, like all tissues, is made up of cells. Brain cells called neurons receive and send electrical signals called nerve impulses. By lowering very fine wires called electrodes (ee-LEK-trohdes) into the brain of a living person, it is possible to record the nerve impulse sent by a single neuron. On a loudspeaker, nerve impulses sound like crackles or pops. Viewed on a screen, as shown in Figure 2.4, they look like spikes.
Other kinds of electrodes can be put on the surface of the brain or just on the scalp. These electrodes work somewhat like the big microphones used by our imaginary aliens. These electrodes measure the electrical activity of huge numbers of neurons. A record of these measurements is called an electroencephalogram (ee-LEK-troh-en-CEF-loh-gram), or EEG for short. Your EEG shows big changes when you go to sleep and when you dream during sleep.
Neurobiologists can visualize brain cells by cutting brain tissue into very thin slices that are thinner than an onionskin. They use dyes to stain the slices so that the cell parts can be seen with a microscope. Different kinds of microscopes give different views of the cells that make up the brain. Microscope images show that the brain is made up of neurons and of other kinds of cells that help the neurons do their jobs. Using microscopes and thin slices of brain tissue, it is possible to find out which neurons are connected to which other neurons. This information helps the scientist determine where the electrical signals come from and go to.
Figure 2.4 When tiny electrodes are used to record the activity of single neurons, they detect “spikes” of electrical activity.
Figure 2.5 Because there are so many neurons in the brain producing electrical signals, electrical changes can be recorded on the surface of the brain or even the scalp. An electroencephalogram (EEG) shows brain activity in the form of waves.
Figure 2.6 This magnetic resonance image (MRI) shows a detailed view showing brain damage following a stroke. The damage appears as the dark area in the lower left cerebrum (right on image).
Neurobiologists can also visualize the brain at work m other ways as well. You may have heard of magnetic resonance imaging (MRI). Doctors use MRI to look inside the brain and other parts of the body. For example, doctors can use MRI to find out which parts of a patient's brain has been damaged by a stroke. A stroke is a medical condition that occurs when a part of the brain stops working because its blood supply was cut off. The lack of blood supply can be caused by a clogged or broken blood vessel that stops the blood from flowing to a part of the brain.
Parts of the Brain
Let's explore the brain by starting with some brain “geography.” Then we can explore what all the parts of the brain do. One way to remember the major parts of the nervous system is to compare them to the gearshift of a sports car. Look at Figure 2.7. Using the gearshift as a model, the spinal cord is the stick. Just as the stick ends in a knob, the spinal cord ends in the brain stem. The brain stem includes the medulla (meh-DOOL-luh), pons (PAHNS), and midbrain. All messages between the brain and the body pass through the brain stem.
Your cerebral hemispheres (ser-REE-bruhl HEM-i-sfeers) sit behind your forehead, between your ears, and above the bump on the back of your skull. Your brain stem sits beneath your cerebral hemispheres and narrows into your spinal cord. Your spinal cord extends from the bottom of your skull and down your back in the bony tube formed by your vertebrae.
Without the cerebral cortex, your brain would be pretty much like a lizard's. Reptiles have the same parts of the brain as we do in our brain stems. So reptiles have basic feelings such as fear and hunger. But the human cortex allows for much more complex thinking.
If we opened the top of the skull, we would first see the squiggly, grayish-beige material that makes up the cerebral cortex. The word cortex means outer layer. The cortex contains about 80% of the neurons in your brain. The density of neuron cell bodies is what makes the cortex look gray. That is the reason the cerebral cortex is sometimes called the gray matter. The cortex is very thin. It is only about as thick as a cracker. Even though it is very thin the cortex does a lot, including all of your thinking. The cerebral hemispheres (cerebrum) are behind the forehead, between the ears, and extend down to the bump you can feel on the back of your head. Each of the cerebral hemispheres of the cerebrum is composed of four different lobes. These four lobes are called the frontal, parietal, temporal, and occipital lobes. Each of these lobes is associated with different body functions. You will investigate each of those functions a little later. The cerebral cortex covers the cerebral hemispheres. The cerebrum completely covers the brain stem. Think back to the analogy of the gearshift (spinal cord) ending in a knob (the brain stem). The cerebrum is like a hand covering the knob of the gearshift.
Figure 2.7 The brain and spinal cord are shaped somewhat like the gearshift in a sports car.
Figure 2.8 The control of most body functions (which you're usually not aware of) happens in the brain stem. Most of your thinking and information processing occurs in the cortex.
Another part of the brain is called the cerebellum. Your cerebellum sits at the back of your brain stem below the bump at the back of your head. It sits on top of the brain stem.
For our visual model of the brain to be complete, we need to add two more parts. These two parts are called the thalamus and the hypothalamus (hypo means below). The thalamus and the hypothalamus are at the center of the cerebral hemispheres and join the cerebrum to the brain stem.
Figure 2.9 The Brain.
Activity 2-1: Big Brain on a Stick
How do the parts of the nervous system fit together? In this activity you will have an opportunity to use and/or build and use a model of the brain and spinal cord. A good way to describe this model is a “Big Brain on a Stick.” Soon you will learn what each of the parts does, so it is important to know how they fit together. As you are putting this model together, think about the following question. What is the best way to take care of each part of the nervous system so it will continue to work well?
- Illustration of brain and spinal cord (Figure 2.8) “Big Brain on A Stick” construction materials (See the Resource.)
- “Big Brain on A Stick” parts for assembly (See the Resource.)
- Masking tape (3/4 inch)
- Colored pencils
- Black marker with fine point
- Bike helmet (optional)
- Activity Report
Step 1 Review the illustration in Figure 2.8. Draw the brain and spinal cord on your Activity Report.
Step 2 Build your model.
a. If you are going to make the model parts for “Big Brain on a Stick,” use the directions on the Resource.
b. If you are given the prepared model parts for “Big Brain on a Stick,” use the directions on the Resource to assemble the model.
Step 3 Use a fine-point, black marking pen to make the following labels with masking tape. Place the appropriate masking tape label on the correct part of your model.
These parts are
- Thalamus and Hypothalamus
- Spinal Cord
Step 4 Complete the Activity Report.
A Closer look at the Cerebral Cortex
The brain cells in your cortex are packed into a thin sheet about the thickness of a cracker. But there are so many neurons in the cortex that this sheet would be about one-meter square if you spread it out on a table. How does a sheet of neurons one-meter square fit into your skull? It is crumpled up. Imagine that the cerebral sheet is a square of soft rubber. Now imagine that you have to fit it inside a soccer ball. You would create folds, ridges, and valleys by crumpling the rubber cerebral sheet up to fit it in the soccer ball.
Figure 2.10 If you spread out the sheet of neurons that makes up your cerebral cortex, you would have a one-meter square sheet, about the thickness of a cracker.
Neurobiologists have figured out what different parts of the cortex do. Look at Figure 2.10. Use the information in the table to make a map of cortex functions.
Table: Brain Regions and Responsibilities
Controls breathing, heart rate, throat functions (swallowing, coughing), and many other functions of the body that you don't have to think about.
Part of the brain stem where the cerebellum is connected. It controls traffic between the cerebellum and the rest of the nervous system. Helps with breathing.
Coordinates complex movements and balance.
Directs messages from the sensors to the cortex.
Controls thirst, hormones, hunger, salt intake, body temperature, and metabolism; generates daily rhythms.
Responsible for thought, sensation, speaking, tasting, smelling, seeing, hearing, movement, learning, and dreaming.
Many years ago, some violent criminals were given frontal lobotomies. This was an operation in which the frontal lobes were destroyed or severed from the rest of the brain. Thank goodness such procedures were not used often. The procedure did make agitated inmates calmer, but it also destroyed the patient's personality.
Remember that the cerebrum and its cortex are divided into four lobes. Let's start at the front of the brain, the area behind your forehead. This area, known as the frontal lobe, is responsible for motion, speech, judgment, personality, and some memory. Just below the frontal lobe are the olfactory (ohl-FAK-tor-ee) bulbs. This part of the brain is the area where your nose sends information about things it smells. People with tumors in the olfactory parts of the brain may smell strange smells.
Serious damage to the most anterior part of the frontal lobe can reduce mood swings and make a person very passive. Damage to other parts can impair speech or the ability to move a part of the body.
What is your earliest memory? Describe the memory and pay close attention to the role your senses play in this memory. For example, can you smell or hear or see something as part of the memory? Why do you think this memory has stuck with you for so long?
The temporal lobe is a section of the brain located along the sides of your head, just above the ears. The temporal lobe is responsible for hearing and understanding speech. A tumor in the hearing area may make someone hear things that are not there. This part of the brain contains areas that process memories. These areas are called the association cortex and are found in other lobes too. One area on the bottom of the temporal lobe is necessary to remember faces of people you know! If this part of the brain is damaged, you can still remember people's names and everything you know about them, but you cannot recognize them by their faces.
The parietal lobe runs from the middle of the top of your head to the back of your head. This part of the cortex receives information about touch, pressure, and pain. The parietal lobe is also involved in language, reading, and body awareness.
Your eyes send messages to the back part of your brain, the occipital lobe. A blow to the back of your head can make you see stars or flashing lights. This is because such a blow stimulates nerve cells in the visual cortex without involving the eyes. The visual cortex receives information from the eyes and is responsible for vision. Remember the gearshift model of the brain and spinal cord. You can think of the cerebrum as the hand on the gearshift knob. The area controlling vision would be near the wrist, and the area controlling hearing would be in the thumb.
Maps in Your Brain
There are maps of your body in your brain. The area of the parietal cortex closest to the frontal cortex receives information about touch, pain, the temperature of your skin, and the position of your muscles and joints. The connections in this region are organized like a map of the body. Similarly, there is a muscle map at the back of the frontal lobe. Neurons in the muscle map control muscles in corresponding parts of the body.
Now you will learn a really strange fact about the connections between your cerebral cortex and your body. Your left cerebral cortex is connected to the right side of your body, and your right cerebral cortex is connected to the left side of your body. So messages from sensors on the right side of your body go to the sensory map on the left side of your brain. Messages from the left side of your body go to the right side of your brain.
Figure 2.11 The parts of the brain and their functions.
Similarly, the muscles on the left side of your body are controlled from the muscle's map on the right side of your brain, and muscles on the right side of your body are controlled from the left side of your brain. Communication between the two sides of the brain lets you coordinate both sides of your body so they work together. There are conditions that interfere with communication between the two sides of the brain. In one of these situations, a person may ignore one side of the body and behave strangely. For example, a man might shave the beard on one side of his face, and leave the other side all hairy.
How do you think the cerebral cortex could control our behavior? How do you think the cerebral cortex could control our personalities? How do all those cells make us what we are? What is intelligence?
Your whole body maps onto parts of the cerebral cortex as shown in Figure 2.12. Notice that the map looks very distorted. Sensitive areas of your body, such as your lips and fingers, occupy bigger areas on the map. Less sensitive parts of your body, such as your legs or back, occupy smaller areas. To better understand the features of the cortex, you can make a model of it in Activity 2-2: Thinking Cap.
Brain on Your Hand
You can use your fist as a model of one side of your brain. Make labels with masking tape to label the parts of the brain on your hand. Explain to at least three other people, what the parts are and what they do. Make a fist with your right hand. What part of the brain would the thumb represent? Which side of the brain would your right fist represent? Where are the frontal lobe, the parietal lobe, the occipital lobe, the cerebellum, and the spinal cord?
Figure 2.12 Information from your senses and from different parts of your body maps onto your cerebral cortex, where information is received and processed. Commands from the cerebral cortex muscle map go to specific muscles in the body.
Activity 2-2: Thinking Cap
You may have heard people say “Well, just put on your thinking cap and you can answer that question!” In this activity you make a paper model of a “thinking cap” with parts to represent regions of the brain. For example, your model will show how your eyes and ears relate to parts of your brain and spinal cord.
- Large paper grocery bag
- Transparent tape
- Colored pencils and/or marking pens
- Clear plastic wrap
- Activity Report
Step 1 Use the information in your text to complete items 1 and 2 on the Activity Report.
Step 2 Using the information on the Resource, cut out your own paper model of a “thinking cap.” It is important to remember that on biological diagrams, the words “left” and “right” mean the specimen's left or right. Ask your teacher if you need help with this idea.
Step 3 Use the Resource to complete item 3 on the Activity Report.
Step 4 With transparent tape, permanently fasten the top and sides of your “thinking cap” together. Try it on. With a mirror, observe your “thinking cap.” Then observe your partner's “thinking cap.”
Step 5 Cover your “thinking cap” with clear plastic to represent the three layers of meninges.
Step 6 Complete the remaining items on your Activity Report.
What might you guess is the difference between a species of mammal that has a large temporal cortex compared to one with a large occipital cortex?
The cerebral cortices of other mammals are smaller and less complex than ours are. Do you think animals have feelings? Do they think? Do you think some animals, such as dolphins and chimpanzees, are intelligent? Explain.
How Do Messages from Your Body Get to the Correct Cortical Areas?
Messages travel up and down the spinal cord, carrying information from sensors to the cortex and to the muscles. Messages coming into the brain pass through the medulla and the pons. The medulla controls some automatic body functions such as heart rate and breathing, as well as swallowing, coughing, and hiccuping. The pons is where the cerebellum joins the brain stem, and messages travel through the pons from cerebrum to cerebellum and from spinal cord to cerebellum and in the opposite direction. The cerebellum helps to coordinate complex movements.
The cerebellum lies behind the pons and medulla and below the cerebrum. The cerebellum controls and coordinates movement and balance. It takes information from the cerebrum about what the body is supposed to do and compares it to what is really happening. It helps the body maintain balance and coordination. And the cerebellum deals with changes, error, and unexpected events.
At the top of the brain stem are the thalamus and hypothalamus. The thalamus works like a switchboard. It receives messages from sensory nerves and passes them on to the correct part of the cortex maps. Neurons coming from your cortex pass by your thalamus on the way down your brain stem to your spinal cord. These neurons form bundles that act like freeways through your brain stem and spinal cord.
The hypothalamus is below the thalamus. The hypothalamus regulates body temperature, eating, drinking, and sexual functions. For example, if one part of the hypothalamus of a rat is damaged, the rat will keep eating until it gets very fat. If another part of the hypothalamus is damaged, the rat will stop eating and get very skinny. The hypothalamus also reacts to certain changes in the blood. For example, if the blood is too salty, the hypothalamus stimulates thirst. The hypothalamus acts like a thermostat to control body temperature. If this small part of the brain is heated, the whole animal reacts as if it is too hot. If the hypothalamus is cooled, the animal shivers.
Technology has made it possible to keep a person's heart and lungs alive even when the brain is dead. When do you think a person is really dead? When he or she stops breathing? When the heart stops beating? When the brain stops functioning?
What parts of the brain do you think are essential when a person dances?
“Learning is a lifelong competition among brain cells . . . Learning is scary because it is real. Math struggled with is axons eagerly branching. Math learned is new connections made. Math recalled is old connections fortified. Math put to a new use is novel patterns of connections, inventive thoughts that had never been possible before.”
-The Body Book
Integrating the Messages
Your brain constantly makes connections between things you know, things you feel, and things you discover. Areas of your cortex are separate but communicate and work with each other all the time. For example, let's say you see a cat and then say the word, cat. Where do the messages go? The image of the cat forms in both eyes and is coded into neural messages that are sent to the visual area at the back of your cortex. Other neurons take this message to the parts of your brain where you store memories. You attach the image of the cat to the word cat by making a connection between the image stored in one place in the cortex with the word stored in another. Neurons carry this message to the speech area in your cortex. Your speech area then connects through the motor system to your tongue, face, and vocal cords, which lets you say cat. Connections can also be made to the part of your brain that controls your arm and hand, directing you to write the word cat. Trace the pathways used to speak and write cat when such an animal is seen. What about when you are blindfolded and identify it by its meow?
Some scientists say that because the average male and female brains are different, men and women tend to think differently. The real difference is that male brains tend to be more asymmetrical than female brains. The right cerebral cortex in males tends to be thicker than the left. In females, the difference is less. Language functions are largely in the left cerebral hemisphere and abilities to deal with spatial relations are largely in the right cerebral hemisphere. Therefore, it has been suggested that the brain differences between males and females explain why men tend to do better at spatial and mechanical tasks and women tend to do better at verbal tasks.
Imagine you are a doctor. Patient A comes to you and says that he is “seeing stars.” What part of the brain would you investigate? Patient B comes to you and says that she smells strange smells everywhere she goes. What part of the brain would you investigate?
- What part of the brain is responsible for thinking?
- Describe four major parts of the brain and explain what they do.
- If a surgeon stimulated the neurons for your big toe in the right sensory cortex, what would happen? Why? What if the surgeon stimulated “toe” neurons in your right frontal cortex?
- Describe several methods scientists use to learn how the brain works.