Lesson Summary
The nervous system controls all the other systems of the body.
Neurons are nerve cells that carry nerve impulses. The central nervous system is made up of the brain and spinal cord.
The peripheral nervous system consists of all the rest of the nerves in the body.
Review Questions
List three functions of the nervous system.
Describe a neuron and identify its three main parts.
What structures make up the central nervous system?
Name the lobes of the cerebrum and state one function of each lobe.
What are the two major divisions of the peripheral nervous system? (Beginning
Explain how one neuron transmits a nerve impulse to another neuron. (Intermediate
Compare and contrast the three main parts of the brain.
Why is the spinal cord like a two-way highway?
A baby girl sees a toy and reaches out to grab it. Describe the path of messages through the baby’s nervous system, from her eyes to her hand.
Assume you are so startled by a sudden loud noise that your heart starts pounding fast. Explain what controls your reaction to the loud sound.
Further Reading / Supplemental Links
CK12 High School Biology, Chapter 35 http://biology.clc.uc.edu/Courses/bio105/nervous.htm.
Body Atlas. Nerves, Brain and Senses. Ticktock Media Ltd., 2004.
Chris Hawkes. The Human Body: Uncovering Science. Firefly Books, 2006.
F. Fay Evans-Martin. The Nervous System. Chelsea House Publications, 2004.
H.P. Newquist. The Great Brain Book: An Inside Look at the Inside of Your Head. Scholastic Nonfiction, 2005.
Treays, Rebecca. Understanding Your Brain. Usborne Books, 2004.
http://www.pbs.org/wgbh/nova/magnetic/animals.html
http://www.pelagic.org/overview/articles/sixsense.html
http://en.wikipedia.org/wiki/Autonomic_nervous_system
http://en.wikipedia.org/wiki/Brain_stem
Vocabulary
autonomic nervous system
Part of the motor division that carries nerve impulses to internal organs and glands.
axon
Part of a neuron that receives nerve impulses from the cell body and passes them on to other cells.
brain
Control center of the nervous system that is located inside the skull.
brain stem
Part of the brain that controls basic body functions such as breathing, heartbeat, and digestion.
cell body
Part of a neuron that contains the nucleus and other organelles.
central nervous system
Part of the nervous system that includes the brain and spinal cord.
cerebellum
Part of the brain that controls body position, coordination, and balance.
cerebrum
Part of the brain that controls awareness and voluntary movements.
dendrite
Part of a neuron that receives nerve impulses from other cells and passes them on to the cell body.
hemisphere
One of the two halves of the cerebrum.
motor division
Division of the peripheral nervous system that carries messages from the central nervous system to internal organs, glands, and muscles.
motor neuron
Neuron that carries nerve impulses from the central nervous system to internal organs, glands, or muscles.
nerve
Bundle of individual nerve cells.
nerve impulse
Electrical signal that is transmitted by neurons.
nervous system
Body system that controls all the other systems of the body.
neuron
Nerve cell that carries electrical messages.
neurotransmitter
Chemical that carries nerve impulses from the axon of one neuron to the dendrite of the next neuron.
parasympathetic division
Division of the autonomic nervous system that controls body processes under nonemergency conditions.
peripheral nervous system
All the nerves of the body that lie outside the central nervous system.
sensory division
Division of the peripheral nervous system that carries messages from the sense organs and internal organs to the central nervous system.
sensory neuron
Neuron that carries nerve impulses from sense organs or internal organs to the central nervous system.
somatic nervous system
Part of the motor division that carries nerve impulses to muscles that control voluntary body movements.
spinal cord
Long, tube-shaped bundle of neurons that carry nerve impulses back and forth between the body and brain.
sympathetic division
Division of the autonomic nervous system that prepares the body for fight or flight in emergencies.
synapse
Place where the axon of one neuron meets the dendrite of another neuron.
Points to Consider
The sensory division of the peripheral nervous system carries messages from sense organs to the central nervous system. What are some examples of sense organs?
Do you know how sense organs work? For example, do you know how your eyes sense light?
Lesson 20.2: Eyes and Vision
Lesson Objectives
Describe how humans see and explain why vision is important.
Explain how the eye works to produce images.
Describe the nature of light.
Explain how lenses correct vision problems.
Check Your Understanding
What are some ways that people use their eyes?
Which part of the nervous system carries messages from the eyes to the central nervous system?
Which part of the brain interprets messages from the eyes?
Introduction
Think about all the ways that students use their sense of sight during a typical school day. As soon as they open their eyes in the morning, they may look at the clock to see what time it is. Then, they might look out the window to see what the weather is like. They probably look in a mirror to comb their hair. In school, they use their eyes to read the board, their textbooks, and the expressions on their friend’s faces. After school, they may keep their eye on the ball while playing basketball (Figure below). Then they might read their homework assignment and the text messages from their friends. If you aren’t visually impaired, you probably use your sense of sight in all of these ways, as well. In fact, you may depend on your sight so much that you have a hard time thinking of anything you do without it, except sleep. Why is sight so important?
Figure 20.11
All eyes are on the ball in this basketball game; think about how we use the sense of sight in other games.
The Nature of Human Vision
Sight, or vision, is the ability to see light. It depends on the eyes detecting light and forming images. It also depends on the brain making sense of the images, so we know what we are seeing. Human beings—and other primates—depend on vision more than many other animals. It’s not surprising, then, that we have a better sense of vision than many other animals. Not only can we normally see both distant and close-up objects clearly. We can also see in three dimensions and in color.
Seeing in Three Dimensions
Did you ever use 3-D glasses to watch a movie, like the boy in Figure below? If you did, then you know that the glasses make people and objects in the movie appear to jump out of the screen. They make images on the flat movie screen seem more realistic because they give them depth. That’s the difference between seeing things in two dimensions and three dimensions.
Figure 20.12
This boy is wearing 3-D glasses; when you look at objects and people in the real world, your eyes automatically see in three dimensions.
We are able to see in three dimensions because we have two e
yes facing the same direction but a few inches apart. As a result, we see objects and people with both eyes at the same time, but from slightly different angles. Hold up a finger a few inches away from your face, and look at it first with one eye and then with the other. You’ll notice that your finger appears to move against the background. Now hold up your finger at arm’s length, and look at it with one eye and then the other. Your finger seems to move less against the background than it did when it was closer. Although you aren’t aware of it, your brain constantly uses such differences to determine the distance of objects.
Seeing in Color
For animals like us that see in color, it may be hard to imagine a world that appears to be mainly shades of gray. You can get an idea of how many other animals see the world by looking at a black-and-white picture of colorful objects. For example, look at the apples on the tree Figure below. In the top picture, they appear in color, the way you would normally see them. In the bottom picture they appear without color, in shades of gray (Figure below).
Figure 20.13
Humans with color vision see the apples on this tree; the bright red color of the apples stands out clearly from the green background of leaves.
Figure 20.14
Dogs and cats would see the green and red colors as shades of gray; they are able to see blue, but red and green appear the same to them.
Evolution and Primate Vision
Why do you think primates, which include humans, evolved the ability to see in three dimensions and in color? To answer that question, you need to know a little about primate evolution. Millions of years ago, primate ancestors lived in trees. To move about in the trees, they needed to be able to judge how far away the next branch was. Otherwise, they might have a dangerous fall. Being able see in depth was important. It was an adaptation that would help tree-living primates survive.
Primate ancestors also mainly ate fruit. They needed to be able to spot colored fruits among the dense leafy background of the trees (Figure below). They also had to be able to judge which fruits were ripe and which were still green. Ripe fruits are usually red, orange, yellow, or purple. Being able to see in color was important for finding food. It was an adaptation that would help fruit-eating primates survive.
Figure 20.15
With color vision, you can tell which cherries in this picture are ripe, because cherries turn red as they ripen.
Knowing about primate evolution helps explain why we see the way we do. However, it doesn’t explain how we see as we do. What allows us to see in three dimensions and in color? To answer that question, you need to know how the eye works.
How the Eye Works
The function of the eye is to focus light. The parts of the eye, shown in Figure below, suit it for that function. Follow the path of light through the eye as you read about it below.
You can also watch an animation of the eye at http://pennhealth.com/health_info/ animationplayer/seeing.html.
Figure 20.16
The human eye is a complex structure that detects light; the light passes through the cornea, pupil, and lens, and is focused on the retina.
Vision involves detecting and focusing light from people and objects. First, light passes through the cornea of the eye. The cornea is a clear, protective covering on the outside of the eye. Next, light passes through the pupil. The pupil is a black opening in the eye that lets light enter the eye. Surrounding the pupil is the iris, more commonly brown, blue, grey, or green.
After passing into the eye through the pupil, light passes next through the lens. Like a hand lens, the lens of the eye is a clear, curved structure. Along with the cornea, the lens helps focus light at the back of the eye. This is shown in Figure below. The lens must bend light from nearby objects more than it bends light from distant objects. The lens changes shape to bend the light by just the right amount to bring objects into focus.
Figure 20.17
Light from objects at different distances is focused by the lens of the eye; muscles in the eye control the shape of the lens so the light is focused on the back of the eye no matter how far the object is from the lens.
The lens focuses light on the retina, which covers the back of the inside of the eye. The retina consists of light-sensing cells called rods and cones. Rods let us see in dim light. Cones let us detect light of different colors. When light strikes rods and cones, it causes chemical changes. The chemical changes start nerve impulses. The nerve impulses travel to the brain through the optic nerve (Figure below). The brain interprets the nerve impulses and tells you what you are seeing. You know that the eyes sense light. But do know what light is? You need to understand the nature of light to fully understand vision.
The Nature of Light
Visible light is a type of electromagnetic (EM) radiation. It’s the only type of EM radiation that can be detected by the human eye. To be visible to humans, EM radiation has to travel in waves of certain wavelengths. Wavelength is the distance from any point on one wave to the same point on the next wave. The different types of electromagnetic radiation are shown in Figure below. Just a small part of the full range of EM radiation is visible to the human eye.
Electromagnetic Radiation
Figure 20.18
This diagram shows the wavelengths of electromagnetic radiation, from shortest (extreme left) to longest (extreme right); the human eye can detect only visible light, which falls in a narrow range of wavelengths, but the eyes of some animals can detect radiation of different wavelengths; bees can see ultraviolet radiation.
Colors of Light
Visible light from the sun is colorless. However, if you bend visible light by passing it through a prism, it produces a “rainbow” of light of different colors (Figure below). Why does this happen? Different colors of visible light have slightly different wavelengths. Light of different wavelengths bends by different degrees when it passes through a prism. This separates visible light into all of its colors.
Figure 20.19
A prism bends white light to create a rainbow of red, orange, yellow, green, blue, indigo, and violet light.
Light and Vision
Except for objects that give off their own light, we don’t see things just because light strikes them. We see things because light strikes them and then reflects, or bounces back, from their surface. What we see is the reflected light.
Some things reflect all the light that strikes them. These things appear white. Some things do not reflect any light. Instead, they absorb all the light that strikes them. These things appear black. Other things, like the beads in Figure below, reflect just one wavelength of light. Whatever wavelength they reflect is the color we see. For example, beads that reflect only red light look red to us.
Figure 20.20
These plastic beads reflect light of different wavelengths, so they appear to be different colors.
Lenses and Vision Correction
You probably know people that need eyeglasses or contact lenses to see clearly. Maybe you need them yourself. Lenses are used to correct vision problems. Two of the most common vision problems are myopia and hyperopia. To watch an animation that shows how these two vision problems occur and how they can be corrected, go to http://pennhealth.com/health_info/animationplayer/seeing.html.
Myopia
Myopia is also called nearsightedness. It affects about one third of people. People with myopia can see nearby objects clearly, but distant objects appear blurry. How a person with myopia might see two boys that are a few meters away is shown in Figure below.
Figure 20.21
This is how a person with normal vision sees the two boys(Left)Normal Vision (Right)Myopia.
In myopia, the eye is too long. As shown in Figure below, this results in images being focused in front of the retina. Myopia is corrected with a concave lens, which curves inward like the inside of a bowl. The lens changes the focus so images fall on the retina as they should.
Figure 20.22
The eye of a person with myop
ia is longer than normal and as a result, images are focused in front of the retina (top); concave lens is used to correct myopia to help focus images on the retina (bottom)(Left)Normal Vision (Right)Myopia.
Hyperopia
Hyperopia is also called farsightedness. It affects about one fourth of people. People with hyperopia can see distant objects clearly, but nearby objects appear blurry. In hyperopia, the eye is too short. As shown in Figure 11b, this results in images being focused in back of the retina. Hyperopia is corrected with a convex lens, which curves outward like the outside of a bowl. The lens changes the focus so images fall on the retina as they should.
In addition to lenses, many cases of myopia and hyperopia can be corrected with surgery. For example, a procedure called LASIK uses a laser to permanently change the shape of the cornea so light is correctly focused on the retina.
CK-12 Life Science Page 53
