35–4 The Senses - Downtown Magnets High School

Section 35–4
1 FOCUS
Objectives
35.4.1 Name the five types of sensory
receptors.
35.4.2 Identify the five sense
organs.
Vocabulary Preview
Ask: Which Vocabulary terms refer
to parts of the eye (Pupil, lens,
retina, rod, and cone) Which terms
refer to parts of the ear (Cochlea,
semicircular canal)
Reading Strategy
Have students preview the section by
studying the figures and reading the
captions.
2 INSTRUCT
Build Science Skills
Applying Concepts Ask students
to imagine they are at a picnic on a
beautiful summer day with a picnic
basket full of their favorite foods.
Then, ask: How might the different
categories of your sensory receptors
be stimulated at the picnic
(Students may say, for example, that
their thermoreceptors might be stimulated
by holding a cold drink and their
chemoreceptors by smelling and tasting
food.)
Vision
Make Connections
Physics Explain that light is part of
the electromagnetic spectrum, which
includes electromagnetic waves of
different wavelengths. Add that
humans can see only light that falls
within a very limited range of wavelengths
and that light in this range is
called visible light.
Print:
35– 4 The Senses
BI 9.e. Students know the roles of sensory neurons, interneurons, and motor neurons in sensation, thought, and
response.
Key Concept
• What are the five types of
sensory receptors
Vocabulary
sensory receptor
pupil
lens
retina
rod
cone
cochlea
semicircular canal
taste bud
Reading Strategy:
Outlining Before you read,
use the headings of the section
to make an outline about the
five sense organs. As you read,
fill in the subtopics and smaller
topics. Then, add phrases or a
sentence after each to provide
key information.
(magnification: 2000)
SECTION RESOURCES
The body contains millions of neurons that react directly to
stimuli from the environment, including light, sound,
motion, chemicals, pressure, and changes in temperature. These
neurons, known as sensory receptors, react to a specific
stimulus such as light or sound by sending impulses to other
neurons, and eventually to the central nervous system. Sensory
receptors are located throughout the body but are concentrated
in the sense organs. These sense organs include the eyes, the
inner ears, the nose, the mouth, and the skin. Sensory receptors
within each organ enable it to respond to a particular stimulus.
There are five general categories of sensory
receptors: pain receptors, thermoreceptors, mechanoreceptors,
chemoreceptors, and photoreceptors. Pain
receptors are located throughout the body except in the brain.
Pain receptors respond to chemicals released by damaged cells.
Pain is important to recognize because it usually indicates
danger, injury, or disease. Thermoreceptors are located in the
skin, body core, and hypothalamus. Thermoreceptors detect
variations in temperature. Mechanoreceptors are found in the
skin, skeletal muscles, and inner ears. They are sensitive to
touch, pressure, stretching of muscles, sound, and motion.
Chemoreceptors, located in the nose and taste buds, are sensitive
to chemicals in the external environment. Photoreceptors, found
in the eyes, are sensitive to light. Figure 35–12 shows how photoreceptor
cells appear under a scanning electron microscope.
Vision
• Teaching Resources, Lesson Plan 35–4,
Adapted Section Summary 35–4, Adapted
Worksheets 35–4, Section Summary 35–4,
Worksheets 35–4, Section Review 35–4,
Enrichment
• Reading and Study Workbook A, Section
35–4
Time
Saver
The world around us is bathed in light. The sense organs that
we use to sense light are the eyes. The structures of the eye are
shown in Figure 35–13. Light enters the eye through the cornea,
a tough transparent layer of cells. The cornea helps to focus the
light, which then passes through a chamber filled with a fluid
called aqueous (AY-kwee-uhs) humor. At the back of the chamber
is a disklike structure called the iris. The iris is the colored
part of the eye. In the middle of the iris is a small opening called
the pupil. Tiny muscles in the iris adjust the size of the pupil
to regulate the amount of light that enters the eye. In dim light,
the pupil becomes larger so that more light can enter the eye.
In bright light, the pupil becomes smaller so that less light
enters the eye.
Figure 35–12 There are two types of lightsensitive
photoreceptor cells in the retina—rods
and cones. This color-enhanced scanning electron
micrograph shows the rod cells of an eye.
• Adapted Reading and Study Workbook B,
Section 35–4
• Lab Worksheets, Chapter 35 Real-World Lab
Technology:
• iText, Section 35–4
• Transparencies Plus, Section 35–4
906 Chapter 35

Fovea
Optic nerve
Blood vessels
Retina
Inner layer of eye
that contains
photoreceptors.
Choroid
Middle layer of
eye that is rich in
blood vessels.
Vitreous humor
Just behind the iris is the lens. Small muscles attached to
the lens change its shape to help you adjust your eyes’ focus to
see near or distant objects. Behind the lens is a large chamber
filled with a transparent, jellylike fluid called vitreous (VIHtree-uhs)
humor.
The lens focuses light onto the retina. Photoreceptors are
arranged in a layer in the retina. The photoreceptors convert
light energy into nerve impulses that are carried to the central
nervous system. There are two types of photoreceptors: rods and
cones. Rods are extremely sensitive to light, but they do not
distinguish different colors. Cones are less sensitive than rods,
but they do respond to light of different colors, producing color
vision. Cones are concentrated in the fovea. The fovea is the site
of sharpest vision. There are no photoreceptors where the optic
nerve passes through the back of the eye. This place is called the
blind spot.
The impulses assembled by this complicated layer of interconnected
cells leave each eye by way of an optic nerve. The
optic nerves then carry the impulses to the appropriate regions
of the brain. The brain interprets them as visual images and
provides information about the external world.
Where are the photoreceptors located in the eye
Sclera
Outer layer of eye that maintains its
shape. Serves as point of attachment
for muscles that move the eye.
Muscle
Lens
Aqueous humor
Cornea
Pupil
Iris
Ligaments
Figure 35–13 The eye is a
complicated sense organ. The
sclera, choroid, and retina are three
layers of tissue that form the inner
wall of the eyeball. Interpreting
Graphics What is the function of
the sclera
NSTA
For: Links on the
senses
Visit: www.SciLinks.org
Web Code: cbn-0354
Use Visuals
Figure 35–13 Ask students to locate
the three layers of tissue (sclera,
choroid, and retina) that form the
inner wall of the eyeball. Then, have
students trace the path of light
through the eye. Call on students to
identify each of the structures the
light passes through. Finally, ask:
What purpose do the muscles
around the lens serve (They change
the shape of the lens to help the eye
focus to see near or distant objects.)
Where does the lens focus the
light (On the retina) What happens
to the impulses after they leave the
eye by way of the optic nerve
(They go to the appropriate regions of
the brain, where the visual images are
interpreted.)
Build Science Skills
Using Analogies Ask: If the lens of
the eye is analogous to a projector,
what part of the eye is analogous
to the screen (The retina) How is
the image projected on the screen
different from the image projected
on the retina (There are no receptors
on the screen to convert the image into
electrical impulses.)
NSTA
Download a worksheet
on the senses for students to complete,
and find additional teacher
support from NSTA SciLinks.
UNIVERSAL ACCESS
Inclusion/Special Needs
Help students understand the terms for sensory
receptors by writing the following prefixes on the
board: thermo-, mechano-, chemo-, and photo-.
Challenge students to think of words that begin
with these or similar prefixes, such as thermometer
and mechanic. List the words on the board.
Then, ask students to define the prefixes based
on the meanings of those words.
Advanced Learners
Give students who need an extra challenge an
opportunity to investigate surgical methods for
correcting vision problems, including photorefractive
keratectomy (PRK) and laser in situ
keratomileusis (LASIK). Encourage students to
present their findings to the class in an oral
report, with diagrams showing how the procedures
correct specific vision problems.
Answers to . . .
The retina
Figure 35–13 The function of the
sclera is to maintain the shape of the
eye and serve as a point of attachment
for muscles that move the eye.
Nervous System 907

35–4 (continued)
Hearing and Balance
Use Visuals
Figure 35–14 Name the structures
that sound waves pass through after
they enter the ear. As you name each
structure, ask: What role does this
structure play in hearing (Students
might say, for example, that the auditory
canal channels the sound waves to
the tympanum and that the tympanum
vibrates in response to the sound
waves.)
Make Connections
Health Science Explain that the
region of the ear called the middle
ear, which is the area between the
tympanum and the semicircular
canals, is prone to infections. This is
because of the close connection
between the middle ear and the
eustachian tube, which originates in
the throat. Viruses and bacteria in the
throat can easily travel to the middle
ear through the eustachian tube and
cause infections, inflammation, and
pain.
Build Science Skills
Inferring Tell students that middleear
infections often block the
transmission of sounds from the outside
world but not sounds, such as
chewing sounds, that originate within
the head. Ask: Why can people
hear “head” sounds even when
their ears are blocked because of
an infection (Because the sound
waves are transmitted directly to the
inner ear through the bones of the
head)
Demonstration
Half fill a glass container with water
and, as students watch, slowly tilt
the container from side to side.
Because the water always stays
parallel to the floor due to gravity,
students will observe it move up and
down the sides of the glass as the
glass tilts. Explain that this is also
how the fluid inside the semicircular
canals moves as the head changes
position.
Auditory canal
Hammer
(magnification: about 3500)
Figure 35 –14 The diagram (top)
shows the structures in the ear that
transmit sounds. The scanning electron
micrograph shows hair cells (yellow) in
the inner ear. The motion of these hairs
produces nerve impulses that travel to
the brain through the cochlear nerve.
Predicting How would frequent
exposure to loud noise affect a person’s
threshold for detecting sound
TEACHER TO TEACHER
Tympanum
Anvil
When I teach about the senses, I give students a
chance to experience sensory “fatigue.” I have
students rest a penny on the inside of the forearm
against the skin and measure how long it
takes until they can no longer sense the presence
of the coin. When everyone is finished, we discuss
why sensory fatigue occurs and when it is
useful (for example, when you are wearing clothing).
I also demonstrate sensory fatigue with the
sense of smell. I ask a volunteer to come to the
Oval window
Stirrup
Semicircular canals
Round window
Hearing and Balance
Cochlear nerve
Cochlea
Bone
Eustachian tube
The human ear has two sensory functions. One of these functions
is hearing. The other function is detecting positional
changes associated with movement.
Hearing Sound is nothing more than vibrations in the air
around us. The ears are the sensory organs that can distinguish
both the pitch and loudness of those vibrations. The
structure of the ear is shown in Figure 35–14.
Vibrations enter the ear through the auditory canal. The
vibrations cause the tympanum (TIM-puh-num), or eardrum,
to vibrate. These vibrations are picked up by three tiny bones,
commonly called the hammer, anvil, and stirrup. The last of
these bones, the stirrup, transmits the vibrations to the oval
window. Vibrations of the oval window create pressure waves
in the fluid-filled cochlea (KAHK-lee-uh) of the inner ear.
The cochlea is lined with tiny hair cells that are pushed
back and forth by these pressure waves. In response to these
movements, the hair cells produce nerve impulses that are
sent to the brain through the cochlear nerve.
Balance Your ears contain structures that help your central
nervous system maintain your balance, or equilibrium. Within
the inner ear just above the cochlea are three tiny canals at right
angles to one another. They are called semicircular canals
because each forms a half circle. The semicircular canals and
the two tiny sacs located behind them monitor the position of
your body, especially your head, in relation to gravity.
front of the room and hold one nostril closed
while I hold a bottle of oil of wintergreen (available
at pharmacies) under the other nostril. The
class measures the time it takes until the volunteer
can no longer distinguish the smell of
wintergreen.
—Duane Nichols
Biology Teacher
Alhambra High School
Alhambra, CA
908 Chapter 35

The semicircular canals and the sacs are filled with fluid
and lined with hair cells. As the head changes position, the fluid
in the canals also changes position. This causes the hair on the
hair cells to bend. This action, in turn, sends impulses to the
brain that enable it to determine body motion and position.
Smell and Taste
You may never have thought of it this way, but your sense of
smell is actually an ability to detect chemicals. Chemoreceptors
in the lining of the nasal passageway respond to specific chemicals
and send impulses to the brain through sensory nerves.
Your sense of smell is capable of producing thousands of
different sensations. In fact, much of what we commonly call the
“taste” of food and drink is actually smell. To prove this to
yourself, eat a few bites of food while holding your nose. You’ll
discover that much of the taste of food disappears until you open
your nose and breathe freely.
Like the sense of smell, the sense of taste is a chemical sense.
The sense organs that detect taste are the taste buds. Most of
the taste buds are on the tongue, but a few are found at other
locations in the mouth. The surface of the tongue is shown in
Figure 35–15. The tastes detected by the taste buds are classified
as salty, bitter, sweet, and sour. Sensitivity to these different
categories varies on different parts of the tongue.
Touch and Related Senses
The sense of touch, unlike the other senses you have just read
about, is not found in one particular place. All of the regions of
the skin are sensitive to touch. In this respect, your largest
sense organ is your skin. Skin contains sensory receptors that
respond to temperature, touch, and pain. Not all parts of the
body are equally sensitive to touch, because not all parts have
the same number of receptors. The greatest density of touch
receptors is found on your fingers, toes, and face.
35 – 4 Section Assessment
1. Key Concept Name the
five types of sensory receptors
and list where they are found in
the body.
2. Identify the functions of the
cornea, pupil, lens, retina, and
optic nerve.
3. What are the four basic tastes
detected by the tongue
4. Explain why you can’t “taste”
food when you have a bad cold.
5. Critical Thinking Applying
Concepts If you spin around for
a time, the fluid in your semicircular
canals also moves. When you
stop suddenly, you feel as though
you are still moving. Why do you
think you might feel dizzy
(magnification: 86)
Figure 35–15 This colorenhanced
scanning electron micrograph
shows the surface of the
tongue. The large pink objects are the
taste buds. Chemoreceptors
found in the taste buds are sensitive
to chemicals in food.
Creative Writing
Imagine that you have to do
without your sense of taste
for one day. How would this
influence your food choices
Write a 3- to 4-paragraph
essay describing how the
absence of this sense organ
would affect your day.
Smell and Taste
Demonstration
Have volunteers taste and try to identify
a variety of different fruit juices
while wearing blindfolds and pinching
their noses shut. (Without sight
cues and the sense of smell, students
will find it difficult to distinguish the
tastes.)
Touch and Related
Senses
Build Science Skills
Designing Experiments Challenge
students to design an experiment to
determine the distribution of heat
and cold receptors in a small area of
skin on the back of the hand.
3 ASSESS
Evaluate Understanding
Provide students with copies of
Figure 35–13 without the labels.
Have students label each part of the
eye shown in the figure.
Reteach
On the chalkboard or an overhead
transparency, list five general categories
of sensory receptors. Have
students give examples of each category
of receptors at work. (For
chemoreceptors, for example, students
might say smelling a flower or tasting
food.)
Answers will vary. For example,
students might describe how their
sense of taste is affected and how
this, in turn, affects their enjoyment
of food and their appetite.
35 – 4 Section Assessment
1. Pain receptors: everywhere except the brain;
thermoreceptors: skin, body core, hypothalamus;
mechanoreceptors: skin, skeletal
muscles, inner ears; chemoreceptors: nose,
taste buds; photoreceptors: eyes
2. Cornea: helps to focus light; pupil: controls
the amount of light that enters the eye; lens:
adjusts focus for near or far distances; retina:
rod and cone photoreceptors convert light
into electrical impulses; optic nerve: carries
the electrical impulses to the brain
3. The four basic taste receptors are receptors
for salty, bitter, sweet, and sour tastes.
4. Because much of the sense of taste is actually
due to interaction with the sense of smell
5. Sensory receptors lag behind the rapid
changes in position.
If your class subscribes to the iText,
use it to review the Key Concepts in
Section 35–4.
Answer to . . .
Figure 35 –14 Loud noises can
damage tiny hair cells and raise the
threshold for detecting sounds.
Nervous System 909