Nervous System33

392 Chapter 33 Nervous System Reading Essentials

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section ● The Senses3

Before You ReadThink of the last time you had a bad cold. On the lines below, describe how your appetite was affected by your cold. Read the section to learn how your senses of taste and smell work together.

Sensory receptors allow you to detect your surroundings.

What You’ll Learn■ different sensory structures■ how each sense organ is able

to send nerve impulses■ the relationship between smell

and taste

Make a Chart Create a three-column chart. Label the columns: Sense, Sensory Receptors, and Location of Receptor. As you read, record information in each column to help you organize what you are reading.

1. Defi ne What is the name for the sensory receptors that allow you to taste?

Read to LearnTaste and Smell

Specialized neurons in your body allow you to taste, smell, hear, see, and touch. You also use specialized neurons to sense motion and temperature. Specialized neurons that allow you to detect your surroundings are known as sensory receptors.

Taste buds are sensory receptors on the tongue that identify the tastes of sweet, sour, salty, and bitter. Taste buds work by sensing chemical combinations in food. They send this information to the brain.

Sensory receptors designed for taste and smell are located in your mouth and nasal cavity. These receptors work together. If you hold your nose while you eat, the food will seem to have less fl avor.

SightLight fi rst enters your eyes through the cornea. The cornea

helps focus the light through an opening called the pupil. The size of the pupil changes to let in more or less light. Muscles in the iris, the colored part of your eye, control the size of the pupil opening.

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Reading Essentials Chapter 33 Nervous System 393

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.Where are the eye’s sensory receptors located?

Behind the iris is the lens, which inverts the image and projects it onto the retina. Between the lens and the retina is a gelatinlike liquid called the vitreous humor. The eye’s sensory receptors—rods and cones—are located in the retina. Rods are sensory receptors that respond to low levels of light. Cones provide information about color to the brain. The structures of the eye are shown in the fi gure below. Action potentials travel from rods and cones to neurons in the optic nerve and fi nally to the brain. The brain interprets the signals and forms a visual image.

Hearing and BalanceHearing and balance are the two major functions of the

ear. Sensory receptors in the ear detect the volume of sounds and the pitch of sounds. The pitch is the highness or lowness of sounds in your environment. The inner ear controls your sense of balance.

What structures in the ear detect sound waves?Particles in the air vibrate as a result of sound waves. Sound

waves enter the auditory canal of the ear. The waves cause thetympanic membrane (eardrum) at the end of the canal to vibrate. These vibrations travel through three bones in the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup). As the stapes vibrates, it causes a membrane separating the middle ear from the inner ear to move back and forth.

The cochlea (KOH klee uh) in the inner ear is fi lled with fl uid and lined with tiny hair cells. When the vibrations reach the inner ear, the fl uid in the cochlea moves like waves against the hair cells. The hair cells produce nerve impulses in the auditory nerve and send the impulses to the brain.

Picture This 2. Explain Add the label

rods and cones beside the structure where rods and cones are located.

3. Identify The sensory receptors for sound are located in what structure?

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How do structures in the ear monitor balance?There are three semicircular canals in the inner ear,

as shown in the fi gure below. Semicircular canals send information about body position and balance to the brain. The canals are positioned at right angles to each other. Like the cochlea, the semicircular canals are fi lled with fl uid and lined with hair cells. As you move your head, fl uid moves within the canals. The fl uid movement causes the hair cells to bend, sending nerve impulses to the brain. The brain uses this information to determine your body position. The brain can also tell whether or not your body is in motion.

TouchThe epidermis and dermis layers of the skin contain

many sensory receptors. These sensory receptors respond to temperature, pressure, and pain.

Where are sensory receptors located?Sensory receptors are not distributed evenly throughout

your body. The tips of your fi ngers have many sensory receptors to detect light touch. The soles of your feet have sensory receptors that respond to heavy pressure. However, the soles of your feet have few sensory receptors to respond to a light touch. Consider how this difference helps you respond to your environment.

What is the purpose of pain receptors?Pain receptors are found in all body tissues except the brain.

Pain receptors are free nerve endings. They send pain signals to the brain, and the brain responds to help ease the pain.

5. Draw ConclusionsWhy do you think your fi ngertips have more sensory receptors for light touch than your elbows?

Picture This4. Name The structures that

monitor balance are located in which part of the ear? (Circle your answer.)

a. outer ear b. middle ear c. inner ear

394 Chapter 33 Nervous System Reading Essentials

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