35â4 The Senses - Downtown Magnets High School

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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
Page 1: Section 35-4 1 FOCUS Objectives 35.

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35â4 The Senses - Downtown Magnets High School