How do you sense the world around you?
How do you know about the world outside your brain? You see it. You touch it. You smell it. You hear it, and you taste it. Sensory organs, such as your eyes and ears, or sensors like those in your skin and muscles are windows to the world. The world is what your senses tell you it is. This section explores how your nervous system gathers information about your body and the world around you through the flow of sensory information.
Think about taking a hike with your dog. It is late afternoon and there is a beautiful sunset. You stop to look at the sunset, and the dog sniffs around in the bushes. The dog does not see colors, but it has a keener sense of hearing, and its sense of smell is 100 times greater than yours is. At the same moment, in the same place, the world that you and your dog are experiencing is different.
Your brain associates a sensation with stored information in your brain memories. Write a journal entry about your favorite color. Include descriptions of how the color might feel, taste, smell, and sound. What association does your brain make with your favorite color?
What is sensation? A sensation is your experience of what the sensors tell the brain about you and your world. Sensations include seeing, hearing, touching, smelling, balancing, and more. The cerebral cortex enables you to identify, distinguish, and feel sensations. Nerve impulses are all the same. But they mean different things depending on where they go in your brain. Impulses for different sensations, such as touch, pain, sound, light, and smell, go to different parts of your cerebral cortex.
Sensory neurons and sensory receptors, such as cells in the eyes and ears, code information as nerve impulses. These nerve impulses carry information to the spinal cord and brain. When the information gets to your cortex, it is interpreted as a sensation.
Activity 5-1: Using Your Sensors
What would you be like without the use of your eyes, ears, or other senses? Your sensors are the important “windows” and “doors” to your environment. Your sensors help you receive information from the world around you. Your sensors do this by responding to a stimulus and turning it into nerve impulses sent to your brain. In this activity you will design an experiment to investigate what happens when you sort objects without using one of your sensors.
- Objects of a variety of shapes and sizes
- Gloves (wool or thick cotton)
- Activity Data Table
- Activity Report
Step 1 With your lab partners, discuss how the sense of touch helps you receive information from your environment.
Step 2 Your task is to sort a group of objects that have similar but different sizes and shapes. The sorting must be done within a limited time period, both with and without the sense of touch and vision.
Step 3 Consider the following questions as you design your experiment. Remember that the sense of touch is your variable, the factor that you are investigating.
- How will you sort the objects both with and without your variable-the sense of touch?
- Determine the number and size of objects needed for sorting.
- Which extra materials will you need?
- How much time should be allowed for each sorting?
- Who on your lab team will be the sorter? Who on your lab team will be the timer? Who on your lab team will be the recorder?
- What kind of data table will you need to summarize your data?
Step 4 Write down a description of your materials, experimental procedure, and a data table. Show this to your teacher for suggestions and approval.
Step 5 Write a hypothesis, your best guess, to predict what you think will happen in your experiment. Be sure to include reasons in support of this hypothesis.
Step 6 Carry out the experiment you have designed and record the data on your data sheet. Make notes of any questions or problems as you are working.
Step 7 Repeat Steps 1 through 6 for the sense of vision.
Step 8 When you are finished, clean up your materials as directed by your teacher.
Step 9 Complete the Activity Report.
Brain tissue itself has no pain sensors. A surgeon can operate on the brain using only a local anesthetic for the tissues surrounding the brain.
How Do Sensory Nerves Work?
Imagine sitting at NASA's Mission Control. You pick up a ringing phone. A voice says, “The space shuttle is now over Sydney, Australia.” With this information, you can plan the future orbit of the space shuttle. In this case, the phone line works like a sensory nerve. The sensor is the phone in Sydney. It codes the speaker's words into electrical pulses. The phone in your hand translates these pulses back into speech you can understand. The pulses move through the phone line as electricity, not as words. Your sensory nerves work in a similar way.
Figure 5.1 The right side of your brain controls the left side of your body.
Remember that your sensors can only send one kind of message-nerve impulses. So how would you know (without looking) whether a creature standing on your foot is a mouse or an elephant? You can figure out the answers to these questions based on things you have already learned. First, if the stimulus comes into contact with a larger area of your body, more sensors will send nerve impulses to the cerebral cortex. Remember the map of the body surface on the cerebral cortex? More of that map will receive messages when more of the body is stimulated. The elephant's footprint on the cortex map will be larger than the mouse's footprint! How hard the stimulus is pressing on your foot is coded by how fast each sensor produces nerve impulses. Think of how you might let a performer know how much you enjoyed the performance. If it was just OK, you might clap your hands slowly just to be polite. But if you thought the performance was terrific, you would clap your hands much faster. A single sensor codes the strength of a stimulus by how frequently it sends nerve impulses along the sensory neuron.
Now think again about the map in your cerebral cortex. Remember that parts of your body are represented by maps in your brain. Figure 5.1 shows how the right half of your brain represents the left side of your body. The sensory nerves from one side of your body go to the opposite side of your brain. Similarly, the motor nerves from one side of your brain go to the opposite side of your body.
Messages from sensors pass through sensory pathways all the way to the cerebral cortex. Look back at the cortex map in Figure 2.12. Remember the strip of cortex that receives information from your skin, your joints, and your muscles. This strip contains a map of your body with the head represented on the lower part and the lower part of the body represented on the top. Also remember that the left side of the body is represented on the right side of the cortex, and the right side of the body is represented on the left side of the cortex. Now, recall the areas of the cortex that receive messages from your eyes and ears. The visual area is in the very back, and the hearing areas are on the sides. In the rest of this section you will explore the eyes and ears as examples of how your senses pick up information from the outside world and transmit it to your brain.
A person who has lost a limb can sometimes still feel pain sensations as if it were still there. This is known as phantom pain. Sensory neurons still go from the missing limb to the spinal cord. At the spinal cord, the sensory neurons connect with interneurons that travel to the cerebral cortex. If something causes the cut sensory neurons to produce nerve impulses, those impulses will travel to the cortex area for the limb. The person will feel the sensations as if they came from sensors in the missing limb. It's the map in the brain that feels the sensations, not the limb itself.
Use Your Sensors
Sit quietly and close your eyes for a few minutes. When you close your eyes, you shut down an important sensory “window.” Now you can pay attention to other sensors your body has that send your brain information. Perhaps you hear a quiet sound you didn't hear when your eyes were open. Maybe you notice your shoelaces are too tight. You may realize that you are thirsty. Make a list of all the things you sense without sight or movement.
Seeing the World
Your eyes bring you a great deal of information about the world around you. They work constantly while you are awake. People with sight depend heavily on that sense to survive, while those without sight develop their other senses to compensate. How do eyes work? How does your brain handle all the information the eyes provide? Let's start by discussing how the eye works, and the role of the nervous system in vision.
Figure 5.2 You can see some eye parts by just looking into the mirror at your own eyes. Turn off the lights for a minute. Then turn them on while looking into the mirror. Can you see your iris close to protect the retina from too much light?
Activity 5-2: Designing and Building a Model of the Eye
What are the parts of the human eye? How do these different parts work? What can happen if some parts of the eye are not working properly? What can you do to help keep your eyes healthy and safe? In this activity you design and build your own model of the human eye to answer these questions.
- Construction materials (as requested by students)
- Activity Report
Step 1 Within your group, discuss the structure of the human eye and how it functions. Use the Resource and Figure 5.3 as a guide.
Step 2 Do the following tasks to prepare to build a three-dimensional model of the human eye that is both accurate and realistic.
- Brainstorm ideas for how you could design a three-dimensional model of the eye.
- Draw a sketch of your design.
- Prepare a preliminary list of materials you will need.
- Decide who will do what.
Step 3 Present your design to the rest of the class. Ask for suggestions on how to improve your model. Add these changes to your plans.
Step 4 Create a final list of materials you will need to build your model eye. Check with your teacher for approval of the list before continuing.
Step 5 Follow your teacher's directions to obtain the materials you need.
Step 6 When you have finished your model eye, use it to discuss the following questions with your lab partners.
- What are the functions of the eye?
- What are the important parts of the eye?
- How does the eye work to help us see?
- How is your model eye similar to and different from a real human eye?
Pupils in a Different light
How do your eyes control the amount of light entering the eye to help you see? Complete the steps below to answer this question.
First look into the eyes of a classmate, a friend, or a family member to locate the pupil. The pupil is the small black circle surrounded by the colored iris. Note the size of the pupil and use a ruler to estimate its diameter. Under which conditions are the pupils the largest? Under which conditions are the pupils the smallest?
Figure 5.3 You can see more parts of the eye by looking at its cross section.
Parts of the Eye
Figures 5.2 and 5.3 show the parts of the eye. The eyeball has a hole in the front to let in light. The hole is the dark spot in the center of the eye, called the pupil. The cornea (KOR-nee-uh) is a clear protective sheath that covers the iris and pupil. The iris is the colored ring around the pupil. Light passes through the cornea and the pupil to get inside the eyeball. Depending on the amount of light around you, the iris opens or closes the pupil. In bright light, the pupil is small. In the dark, it is bigger. Just behind the iris is the lens. The lens is an elastic, naturally curved tissue controlled by tiny muscles (ciliary muscles). When you look at something far away, these muscles relax and the lens has only a slight curve. When you look at a nearby object, the muscles contract, causing the lens to curve out more.
Humans have two eyes on the front of their heads in order to have good depth perception. But this eye position means that humans have a limited field of view. Some animals, such as rabbits, have their eyes positioned on the sides of their heads, giving them a huge field of view, but limited depth perception. A large field of view strengthens an animal's ability to protect itself. Better depth perception strengthens an animal's ability to hunt.
Together, the cornea and the lens focus light on the back inside part of the eye, the retina (RET-ihn-uh). Figure 5.3 shows a cross section of the eye and Figure 5.4 shows a cross section of the retina. Sensor cells sensitive to light line the retina and capture the light energy that falls on them. These sensor cells are specialized neurons called rods and cones. You have about 120 million rod cells and 7 million cone cells. When stimulated, these cells send impulses to your cortex. Other neurons in the retina process the information collected by the rods and cones before sending nerve impulses to the brain. All of the axons taking these nerve impulses to the brain leave the back of the eyeball at the same place, making up the optic nerve. Have you ever heard of someone having a blind spot? As a matter of fact, you have one blind spot on each retina. The blind spot is an area on the retina where the optic nerve begins. There are so many axons where the optic nerve begins that there is no room for rods and cones. So it is a blind spot.
Figure 5.4 (a) The retina is layers of neurons that line the inside rear of the eye. Cone cells are the optic nerve sensors. (b) The position of the blind spot is on the retina where the optic nerve begins.
What Are the Advantages of Two Eyes?
Find a partner to play catch with. Before playing, have each person cover one eye with a piece of paper taped over the eye. Toss a ball back and forth. What happens? Find something smaller to play catch with, such as a table tennis ball. Can you do it? What is difficult? What can two eyes provide that one cannot?
How does your cerebral cortex distinguish light, dark, and color? At night you see with your rods. They work in dim light, allowing you to see light and dark areas. However, rods are not sensitive to color. In brighter light you see with your cones. Cones enable you to see color. Cones also help you see sharply focused images such as the letters on this page. Have you ever tried to read in dim light? It is difficult because you can only use your rods. Cones need much more light to work than rods.
You experience a shift between seeing with your cones and seeing with your rods at various times. For example, at sunset and at sunrise or when the lights are dimmed in a movie theater before the movie comes on, you shift from seeing with your cones to seeing with your rods. The next time you watch a sunset, notice that as the sky gets darker, the colors of objects nearby seem to become less vivid and then fade out completely. In dim light, you can't tell a blue book or car from a red one or a green one. You can only say one is lighter or darker than the other. As the light dims, it gets too low to stimulate your cone cells. Finally, at a specific level of light, you can no longer see color. But you can still see, because your rod cells are more sensitive to light than are cone cells. Instead of color, though, you can only see a world of different shades of gray-no colors. The reverse happens, of course at sunrise or when the bright picture of the movie comes on the theater screen.
Just as you are right- or left-handed, you are also right- or left-eyed. Make a circle with your finger and your thumb. Hold the circle at arm's length in front of your face. Look through the circle and focus on a distant object, such as a clock. Now, close your right eye. What do you see? Close your left eye. What do you see? The eye which sees the object best is the dominant eye.
Most people have three types of cones that detect color. One type of cone is sensitive to red light, another to blue light, and another to green light. All three types of cones are needed for normal color vision. Most people can distinguish between 150 and 200 colors. A person born with only one or two types of cones cannot see certain colors, a condition known as color blindness. In humans, color blindness is a genetic condition that is inherited from a parent. Many animals, especially those animals that are active at night, are completely color-blind.
The eyelids, which protect, moisten, and clean your eye, blink in 110 of a second.
Your tear glands constantly produce tears to cleanse and moisten the eye. When you cry, your nervous system tells your tear glands to produce more tears. Some of the tears run down your face, but most drain into your nose and throat. That's why you often get a runny nose when you cry.
Owls are completely color-blind. What does this imply about the structure of the eyes of owls?
If the lens in your eye doesn't work just right, you might need glasses. If your lens doesn't bend light so that you can see objects up close, you are farsighted. If you can't see faraway objects because your lens bends light too much or in the wrong way, you're nearsighted.
Your eye works a little like a camera. The lens turns the image of the world upside down and makes it small so that it comes into focus on your retina. In one central area of your retina, called the fovea (FOH-vee-uh), there is a high concentration of cones. The fovea lines up with the lens so a focused image of the world in front of you falls on this part of the retina. The picture from your retina is turned into nerve impulses that pass through sensory nerves. The nerve impulses go from the retina to the cortex of the occipital lobes of the cerebrum, called the visual cortex. Your visual cortex interprets the nerve impulses and tells you what you are seeing.
If you can read all the letters in the smallest line of letters on an eye chart at a distance of 20 feet, you have 2020 vision. What does 2040 vision mean?
Activity 5-3: Exploring a Mammalian Eye
What does an eye look like? How does it work? How could you find out? Biologists often study the organs, processes, and behaviors of other animals to increase their knowledge of humans. In this activity you design a procedure to examine and dissect an eye specimen. You describe the procedure you will use and list the materials needed. Finally, you carry out your dissection and record your work with drawings, labels, and written comments.
- Sheep or cow eye
- Paper towels
- Dissection pan
- Scalpel (single-edged cutting tool)
- Needle or metal probe
- Activity Report
Step 1 With your partners, discuss the structure and function of the human eye. Use your text and other references to make sure you are familiar with each part of the eye and what it does. Consider the following questions.
- How would you examine the structure of an eye?
- Which questions would you want to ask?
- Where could you go for more resources?
Step 2 With your partners, design a procedure that describes where you will obtain an eye specimen and how you will dissect it. Include a list of the materials needed and indicate which person is responsible for each task involved in the dissection.
Step 3 Show the procedure to your teacher and ask for additional comments, suggestions, and approval.
Step 4 Complete the eye dissection as indicated in your procedure. Be sure to include written statements and drawings as you work. Also, add any suggested changes to improve the procedures.
Step 5 When you have finished this activity, turn in your lab report and follow your teacher's directions for cleanup.
Figure 5.5 Diagram of the eye
Hearing the World
What do your ears do? Ears convert sounds into nerve impulses. The converted nerve impulses travel to the temporal lobes where they stimulate the areas that are responsible for hearing. These areas of the cerebral cortex are called the auditory cortex.
A noise or sound starts when an object vibrates (moves back and forth). The vibrating makes pressure waves in the air. Have you ever felt a guitar string vibrate? Have you ever placed your finger on a ringing bell or on a piano while someone plays it? Have you ever gently touched the speaker of a stereo while music was playing? If you have done any of these things, you felt the vibrations that create pressure waves in the air. Those pressure waves are sound waves.
Most species of animals do not make or hear sounds.
Have you ever tossed a stone into water? Sound waves are somewhat like the ripples a stone causes when thrown in water. The ripples move out from where the stone made the original splash. In a similar way, sound waves move out from a vibrating source. A toy spring can also help you think about sound waves. Hold one end of the spring with the other end on the ground. If you wiggle the end in your hand, waves move down the spring as it is compressed and stretched. Like the waves in the spring, sound waves are compression waves. Air molecules tend to be evenly spaced. But they can be pushed back and forth by something such as a vibrating stereo speaker. As the vibrating speaker moves back and forth, it pushes air molecules together and then pulls them farther apart. This push and pull of the air molecules creates a compression wave in the air.
Did You Know?
Your outer ear canal is about 2.5 centimeters (1 inch) long.
Parts of the Ear
Look at the parts of the ear shown in Figure 5.6. The only part of your ear that you can see from the outside is the flap (auricle) or pinna. The pinna is the part that surrounds the hole that goes into your skull. The pinna collects sound waves and funnels them into the ear hole, which is the beginning of the ear canal. At the end of each ear canal is the eardrum. The eardrum is a thin piece of skin stretched across the ear canal. Sounds traveling through the ear canal make the eardrum vibrate.
Figure 5.6 The pinna and ear canal are parts of the outer ear.
Figure 5.7 Sound waves move through the outer and middle ear to the inner ear, where they cause tiny hairs to wave back and forth. These tiny hairs attach to sensors, which send nerve impulses to the brain.
Behind the eardrum is a space filled with air. This space is called the middle ear. The air that fills the space comes from the back of your throat, up, and through your Eustachian (yoo-STAY-shun) tube. Three tiny bones called the hammer, anvil, and stirrup cross the middle ear from the back of the eardrum. These tiny bones move whenever the eardrum moves. The bones carry vibrations from the eardrum to two tiny “windows” that lead into the inner ear, which is called the cochlea (KOH-klee-uh). The inner ear is a coiled tube filled with fluid. A membrane lined with tiny hairs that are attached to sensors runs down the center of the fluid-filled tube.
Why do deer and rabbits have such oversized ear flaps? Why do you cup your hands behind your ears when you want to hear better?
How Does Hearing Happen?
Here are the steps in the hearing process.
- Sound waves come into the ear, pass through the ear canal, and strike the eardrum. The eardrum vibrates in response to the sound waves. In fact, it vibrates differently depending on the kind of sound. For example, high-pitched sounds, such as a scream, cause the eardrum to vibrate very quickly. Low-pitched sounds, such as a drum, cause the eardrum to vibrate very slowly.
- The vibrations of the eardrum cause the hammer, anvil, and stirrup to move.
- The stirrup attaches to a membrane-covered window leading to the inner ear. When the stirrup moves, it causes the membrane over the window to move in and out.
- Movements of the window cause the fluid of the inner ear to move. The cochlea is a long, curving tube. Down the center of the tube is a flexible membrane lined with hairs attached to sensors. When fluid in the cochlea moves, it causes the membrane to flex, which bends the sensor hairs. The sensors are sensitive to how much the hairs bend. Loud sounds bend them more than soft sounds.
- Sounds of different frequencies cause different sensors to bend. Some sensors code high-pitched sounds. Other sensors code the low-pitched sounds. When the window to the cochlea moves in and out very fast, the membrane in the cochlea tube flexes close to the window. This flexing causes sensors in the region to bend. When sound has a lower pitch, the cochlear window moves in and out slower. The sensors farthest from the window flex when the window moves in and out slowly. The sensors close to the window flex when the window moves faster. So the sensors close to the window code high-pitched sounds (treble). And the sensors farthest from the window code low-pitched sounds (bass).
- These sensors send impulses to the brain, where your auditory cortex interprets them as sound. Sensors stimulated by high-pitched sounds (located close to the windows) send their nerve impulses to one area of the hearing map in the auditory cortex. Sensors stimulated by low-pitched sounds send their nerve impulses to another part of the hearing map on the auditory cortex.
How do you think the loudness of a sound is communicated to the auditory cortex?
Your two ears hear sounds coming from one direction at slightly different times. One ear hears the sound about 11,500 of a second sooner than the other ear does. Your brain interprets this difference as position. Another clue your brain can use is a difference in the loudness of the sound arriving at your two ears. When a sound comes from the side, your head acts as a sound barrier for the ear on the opposite side.
How Can loud Sounds Damage Your Ears?
Now we can explain how loud sounds, such as amplified music, can damage your ears. Do you remember the flexible membrane running down the center of the fluid-filled cochlea? Do you remember also that depending on the pitch of a sound, this membrane vibrates at different places along its length? At each place, tiny hairs move and send nerve impulses to the brain. These sensor hairs are very fragile and can be damaged. Exposure to loud sound (more than 100 decibels) for long periods of time can damage hairs at specific places along the membrane, causing deafness to specific frequencies of sound. Study the decibel scale on this page to compare the loudness of different sounds.
Figure 5.8 The sound of a particular frequency causes the cochlear membrane to flex at a specific point, which bends sensory hairs.
40 kw siren (30 meters away)
Jet takeoff (80 meters away)
Riveting machine, large circular saw
Subway train (7 meters away)
Low aircraft flying over
Pneumatic drill (15 meters away)
Vacuum cleaner (3 meters away)
Light traffic (30 meters away), in-home noise, urban industrial area
In-home noise, urban residential area
In-home noise, quiet suburb
Threshold of hearing
Figure 5.9 Decibel scale for selected sources.
Protecting Your Hearing
Your hearing is very important to you. Put earplugs in your ears for a while and try to imagine what it would be like to have to live all of the time with greatly reduced hearing. It would be difficult, and it would be dangerous. Sometimes you may be able to hear danger coming before you can see it. We know how important our hearing is. Even so, sometimes we don't take very good care of it.
Sticky ear wax and tiny hairs protect your ears from dust and other particles that enter the outer ear. Your eardrum, the tiny bones in your middle ear, and the sensor cells in your inner ear are all very delicate and can be damaged easily. Poking things in your ear can damage your eardrum. Infections that come up your Eustachian tube from your throat can damage your middle ear. Swimmers sometimes have trouble with middle ear infections because dirty water gets into their mouths and up their Eustachian tubes.
Loud sounds can damage the middle ear bones and the sensors in the inner ear. People who work near jet engines at the airport wear ear protectors because loud sounds made by the engines can cause ear damage. Rock music can be as loud as jet engines. Many rock performers have damaged their hearing permanently. If you damage the sensor cells in your inner ears, they are gone forever. It is a good idea to protect your ears from very loud sounds.
What is noise pollution? Give some examples of noise pollution in your environment, and rank them according to how much they affect you. Are all sources of noise pollution the same for everyone? What can be done in your community to reduce noise pollution?
Your brain can “filter out” sounds so that you can concentrate on what you want to hear. Suppose you are at a party surrounded by people talking, loud music, and other sounds. Suddenly you hear your name. You can focus on just that conversation so you hear only what they say about you and not much else. What happened? Your brain filters what your ears are hearing. Your brain can switch your attention from all the party noises to the talk about you. Amazingly, your brain filters most sensory messages without your knowing it's happening.
A Balancing Act
Did you ever wonder how you manage to stand, walk, and run upright on just two feet? After all, a chair with two legs would fall over. Well, it's a part of your ear that helps you maintain your balance. Two organs that are built into your ear, next to the cochlea, help you to keep your balance. One organ senses sudden changes in movement. This organ is made of two little chambers. Lining the chambers are some sensory hairs that are embedded in thick stuff inside the chamber. When your head moves suddenly, the thick stuff moves and bends the hairs. These sensors send messages to the brain. The brain interprets these messages as movement. The other organ is called the semicircular canals. The semicircular canals detect changes in the position of your head and spine. These three looping canals are filled with fluid and lined with motion-sensitive hairs. Since each loop has a different orientation, the movement of fluids in them changes when the head changes position.
If you spin around in circles, you feel dizzy when you stop. Why?
What is motion sickness? How does it relate to your ears?
When talking about the senses we typically think of the five senses. They are taste, smell, sight, hearing, and touch. Expand your thinking a little and see how many other senses you can think of. Do library research to find examples of animals that use different senses than we do. Write about how animals use different sources of information about the environment than we do.
- Sample answers to these questions will be provided upon request. Please send an email to firstname.lastname@example.org to request sample answers.
- What role do sensors play in experiencing sensations?
- How do you know what type of sensation you're feeling?
- What is the difference between rods and cones? What do they do?
- In what ways does your eye work like a camera?
- How does your ear capture sound?