Lesson Summary
Humans can normally see both distant and close-up objects clearly, and we also see in three dimensions and color.
Light entering the eye is focused by the lens on the retina, which sends messages to the brain through the optic nerve.
Visible light is electromagnetic radiation that can be detected by the human eye.
Vision problems such as myopia and hyperopia can be corrected with lenses that help focus light on the retina.
Further Reading / Supplemental Links
CK12 High School Biology, Chapter 35 http://www.fda.gov/CDRH/LASIK
Body Atlas. Nerves, Brain and Senses. Ticktock Media Ltd., 2004.
Donald B. Light. The Senses. Chelsea House Publications, 2004.
Christopher Sloan. The Human Story: Our Evolution from Prehistoric Ancestors to Today. National Geographic Children’s Books, 2004.
http://www.veterinaryvision.com/See.htm
http://en.wikipedia.org/wiki
Review Questions
What is vision?
Describe the lens of the eye and what it does.
What happens when light is focused on the retina of the eye?
Describe visible light.
What is hyperopia?
Explain how humans can see in three dimensions.
Why were depth perception and color vision important for early primates?
Black is sometimes defined as the absence of light. Why?
Assume you see a bright red apple. Why does the apple look red?
What causes myopia, and what type of lens corrects it?
Vocabulary
cornea
Clear, protective covering on the outside of the eye that helps focus light.
hyperopia
Vision problem in which distant objects are clear but nearby objects look blurry; also called farsightedness.
iris
Colored structure at the front of the eye.
lens
Clear, curved structure in the eye that focuses light on the retina.
myopia
Vision problem in which nearby objects are clear but distant objects look blurry; also called nearsightedness.
pupil
Black opening in the iris that lets light enter the eye.
retina
Layer of light-sensing cells that covers the back of the eye.
visible light
Electromagnetic radiation that humans can detect with their eyes.
vision
Ability to see light.
Points to Consider
The sense of sight is important to humans and other animals, but other senses may be equally important. What are some of our other senses?
Why are these other senses important to us? For example, what are some ways we depend on our sense of hearing?
Lesson 20.3: Other Senses
Lesson Objectives
Explain how the ears hear and help maintain balance.
Outline how we sense pressure, temperature, and pain.
Describe how we identify different tastes and smells.
Explain why hearing, balance, touch, taste, and smell are important.
Check Your Understanding
What is the role of the nervous system?
How do signals ("messages") get from one area of the body to the brain?
Introduction
Imagine walking through the fruit market shown in Figure below. Your sense of sight would be stimulated by all the brightly colored fruits. But your other senses would be stimulated, too. You would hear the noisy bustle of the market. As you checked to see if a piece of fruit was firm, you would feel its smooth skin. If you tried a sample of the fruit, you would taste its juicy sweetness and smell its appetizing aroma. Clearly, a market like this is a feast for all of the senses. In this lesson, you will read how your nervous system senses the sound, feel, taste, and smell of a market like this—and of everything else around you.
Figure 20.23
This outdoor fruit market stimulates all the sensessight, sound, smell, taste, and touch.
Hearing and Balance
What do listening to music and riding a bike have in common? It might surprise you to learn that both activities depend on your ears. The ears are sense organs that detect sound. They also sense the position of the body and help maintain balance.
Hearing
Hearing is the ability to sense sound. Sound travels through the air in waves, much like the waves you see in the water in Figure below and the light waves described in Lesson 2. Sound waves in air cause vibrations inside the ears. The ears detect the vibrations.
Figure 20.24
Sound waves travel through the air in all directions away from a sound like waves traveling through water away from where a pebble was dropped.
What the human ear looks like is shown in Figure below. As you read about it below, trace the path of sound waves through the ear. You can also see an animation of the ear sensing sound at http://www.sumanasinc.com/webcontent/animations/content/ soundtransduction.html.
Assume a car horn blows in the distance. Sound waves spread through the air from the horn. Some of the sound waves reach your ear. The steps below show what happens next. They explain how your ears sense the sound. Each numbered step refers to a structure with the same number in Figure below and Table (below).
Figure 20.25
Read the names of the parts of the ear in the key (Table ), then find each of the parts in the diagram, referring to the diagram as you read about the parts of the ear.
Number in diagram Part of the ear
1 pinna
2 ear canal
3 eardrum
4 hammer
5 anvil
6 stirrup
7 oval window
8 cochlea
9 auditory nerve
10 semicircular canals
The sound waves are gathered by the pinna, or outer ear. This is the part of the ear you can see.
The sound waves are channeled into the ear canal. This is a tube-shaped opening in the ear.
At the end of the ear canal, the sound waves strike the eardrum. This is a thin membrane that vibrates like the head of drum when sound waves hit it.
The vibrations pass from the eardrum to the hammer. This is the first of three tiny bones that pass vibrations through the ear.
The hammer passes the vibrations to the anvil, the second tiny bone that passes vibrations through the ear.
The anvil passes the vibrations to the stirrup, the third tiny bone that passes vibrations.
From the stirrup, the vibrations pass to the oval window. This is another membrane like the eardrum.
The oval window passes the vibrations to the cochlea. The cochlea is filled with liquid that moves when the vibrations pass through, like the waves in water when you drop a pebble into a pond. Tiny hair cells line the cochlea and bend when the liquid moves. When the hair cells bend, they release neurotransmitters.
The neurotransmitters trigger nerve impulses that travel to the brain through the auditory nerve. The brain interprets the sound and “tells” you what you are hearing.
No doubt you’ve been warned that listening to loud music or other loud sounds can damage your hearing. It’s true. In fact, repeated exposure to loud sounds is the most common cause of hearing loss. The reason? Very loud sounds can kill the tiny hair cells lining the cochlea. The hair cells do not generally grow back once they are destroyed, so this type of hearing loss is permanent. You can protect your hearing by avoiding loud sounds or wearing earplugs or other ear protectors.
Balance
Did you ever try to stand on one foot with your eyes closed? Try it and see what happens, but be careful! It’s harder to keep your balance when you can’t see. Your eyes obviously play a role in balance. However, your ears play an even bigger role. The gymnast in Figure below may not realize it, but her ears—along with her cerebellum—are primarily responsible for her ability to perform on the balance beam
.
Figure 20.26
This gymnast is using the semicircular canals in her ears, along with the cerebellum in her brain, to help keep her balance on the balance beam.
The parts of the ears involved in balance are the semicircular canals. In Figure above, the semicircular canals are the structures numbered 10. The canals contain liquid, and are like the bottle of water in Figure below. When the bottle tips, the water surface moves up and down the sides of the bottle. When the body tips, the liquid in the semicircular canals moves up and down the sides of the canals. Tiny hair cells line the semicircular canals. Movement of the liquid inside the canals triggers the hair cells to send nerve impulses. The nerve impulses travel to the cerebellum in the brain. In response, the cerebellum sends commands to muscles to contract or relax so the body stays balanced.
Figure 20.27
This bottle of water models the semicircular canals in your ears; when you tip the bottle, the water moves up or down the sides of the bottle; when you tip your head, the liquid inside the semicircular canals moves up and down the sides of the canals; tiny hair cells lining the canals detect the movement of liquid and send messages to the brain.
Touch
When you look at the prickly cactus in Figure below, does the word ouch come to mind? Touching the cactus would no doubt be painful. Touch is the sense of pain, pressure, or temperature. It depends on sensory neurons in the skin. The skin on the palms of the hands, soles of the feet, and face has the most sensory neurons and is especially sensitive to touch. The tongue and lips are very sensitive to touch, as well. Neurons that sense pain are also found inside the body in muscles, joints, and organs. If you have a stomach ache or pain from a sprained ankle, it’s because of these internal sensory neurons.
Figure 20.28
The spines on this cactus are like needles, they help keep away animals that might want to eat the cactus.
The following example shows how messages about touch travel from sensory neurons to the brain, as well as how the brain responds to the messages. Suppose you wanted to test the temperature of the water in a lake before jumping in. You might stick one bare foot in the water. Neurons in the skin on your foot would sense the temperature of the water and send a message about it to your central nervous system The frontal lobe of the cerebrum would process the information. It might decide that the water is really cold and send a message to your muscles to pull your foot out of the water.
In some cases, messages about pain or temperature don’t travel all the way to and from the brain. Instead, they travel only as far as the spinal cord, and the spinal cord responds to the messages by giving orders to the muscles. When messages bypass the brain in this way, it forms a reflex arc, like the ones shown in Figures below, below and below.
First image:
Figure 20.29
Reflex Arc: When you touch something hot, you may jerk your hand away without even thinking about it; the nerve impulse from your hand travels to the spinal cord and the spinal cord sends a message to your muscles to pull back your hand.
Second image:
Figure 20.30
Reflex Arc: When you touch something hot, you may jerk your hand away without even thinking about it; the nerve impulse from your hand travels to the spinal cord and the spinal cord sends a message to your muscles to pull back your hand.
Third Image
Figure 20.31
Reflex Arc: When you touch something hot, you may jerk your hand away without even thinking about it; the nerve impulse from your hand travels to the spinal cord and the spinal cord sends a message to your muscles to pull back your hand.
Taste and Smell
Your sense of taste is controlled by sensory neurons on your tongue that detect chemicals in food. The neurons are grouped in bundles within taste buds (Figure below). There are five different types of taste neurons on the tongue. Each type detects a different taste. The tastes are sweet, salty, sour, bitter, and umami, which is a meaty taste. When taste neurons detect chemicals, they send messages to the brain about them. The brain, in turn, decides what tastes you are sensing.
Figure 20.32
Tiny bumps that cover the tongue contain taste buds, bundles of sensory neurons that allow you to detect different types of tastes, such as sweet and salty tastes.
Your sense of smell also involves sensory neurons that detect chemicals. The neurons are found in the nose, and they detect chemicals in the air. Unlike taste neurons, which can detect only five different tastes, the sensory neurons in the nose can detect thousands of different odors.
Have you ever noticed that you lose your sense of taste when your nose is stuffed up? That’s because your sense of smell contributes greatly to your ability to taste of food. As you eat, airborne molecules of food chemicals enter your nose. You experience the taste and smell at the same time. Being able to smell as well as taste food greatly increases the number of different tastes you are able to sense. For example, you can use your sense of taste alone to learn that a food is sweet, but you have to use your sense of smell as well to learn that the food tastes like strawberry cheesecake.
Why These Senses Matter
The senses of hearing, balance, touch, taste, and smell enrich our lives each day. The sense of hearing lets us listen to our favorite music. The sense of balance helps us play the sports we like. The sense of touch allows us to use a keyboard to text our friends. The senses of taste and smell allow us to enjoy the flavor and aroma of our favorite foods.
These five senses not only enrich our life. They also help us sense danger. For example, being able to stay balanced on a icy sidewalk might prevent a nasty fall. Being able to hear a fire alarm could alert us to flee from a burning building. Being able to taste and smell might warn us that food that is spoiled and could make us sick. The sense of smell could also warn us of dangers such as fires and gas leaks.
Being able to feel pain is especially important for preventing injury. It might not seem that pain is a good thing–until you think about what might happen if you couldn’t feel pain. For example, what if you couldn’t feel a hot iron? You might be badly burned before you realized you were touching it. What if you couldn’t feel the pain of a sprained ankle? You might keep using the ankle and make the injury worse.
Lesson Summary
The ears detect sound waves and help maintain balance. The skin senses pain, pressure, and temperature.
Sensory cells on the tongue and in the nose detect tastes and smells.
The senses of hearing, balance, touch, taste, and smell enrich our life and help keep us safe.
Further Reading / Supplemental Links
CK12 High School Biology, Chapter 35.
Autumn Libal. The Ocean Inside: Youth Who Are Deaf and Hard of Hearing. Mason Crest Publishers, 2007.
Body Atlas. Nerves, Brain and Senses. Ticktock Media Ltd., 2004.
Donald B. Light. The Senses. Chelsea House Publications, 2004.
Elaine Landau. The Sense of Touch. Children’s Press, 2008.
http://en.wikipedia.org/wiki/Taste_buds
Review Questions
What are the two main functions of the ears?
Which structure in the ear changes sound waves in air to vibrations?
What happens after the oval window in the ear passes vibrations to the cochlea?
Which parts of the ear sense changes in the body’s position?
What are the five tastes sensed by neurons on the tongue?
Why does death of hair cells in the cochlea cause hearing loss?
Explain the statement, “You listen with your ears, but you hear with your brain.”
How and why do reflex arcs occur?
Why is your sense of taste affected when you have a stuffy nose?
How could the ability to feel pain help prevent serious injury? Give an example.
Vocabulary
anvil
Second of three tiny bones that pass vibrations through the ear.
auditory nerve
Nerve that carries nerve impulses generated by sound waves from the ear to the brain.
cochlea
Liquid-filled structure in the ear that senses vibrations and generates nerve impulses in response.
ear
Sense organ that detects sound.
ear canal
Tube-shaped opening in the ear that carries sound waves to the eardrum.
eardrum
Membrane in the ear that vibrates when sound waves hit it.
hammer
First of three tiny bones that pass vibrations through the ear.
hearing
Ability to sense sound.
oval window
Membrane in the ear that passes vibrations from the stirrup to the cochlea.
pinna
Outer part of the ear that gathers sound waves.
reflex arc
Path of nerve impulses that bypass the brain for a quicker response.
CK-12 Life Science Page 54
