Hearing And Balance

The ear performs two main functions: detecting sound and maintaining balance. The fleshy structure of the external ear directs sound vibrations into the ear. As Figure 49-10 shows, the auditory canal connects the external ear with the tympanic (tim-PAN-ik) membrane, or eardrum. Vibrations in the air of the auditory canal cause the tympanic membrane to vibrate. Air pressure in the chamber beyond the tympanic membrane, the middle ear, is regulated by the amount of air passing through the Eustachian tube to the middle ear. The Eustachian (yoo-STAY-kee-uhn) tube is an opening to the throat that equalizes the pressure on both sides of the tympanic membrane during a sudden change in atmospheric pressure, such as occurs when an airplane takes off or lands.

The vibrating tympanic membrane sets in motion three small bones of the middle ear: the hammer, the anvil, and the stirrup. The stirrup transfers vibrations to a membrane called the oval window. The oval window separates the middle ear from the inner ear. The inner ear contains the cochlea (KAHK-lee-uh), a coiled tube consisting of three fluid-filled chambers that are separated by membranes. The middle chamber contains the organ of Corti, which is the organ of hearing. The organ of Corti rests on the bottom membrane in the cochlea and contains mechanoreceptors known as hair cells. Vibrations of fluid in the cochlea move the bottom membrane and cause the hair cells to bend against a second membrane, which covers the hair cells. The bending of hair cells activates ion channels. The resulting change in the electric potential of the hair cells causes the release of neurotransmitters. The neurotransmitters stimulate neurons in the auditory nerve. Action potentials are sent to the auditory region of the brain stem, then to the thalamus, and finally to the auditory cortex, which interprets sound.

Word Roots and Origins tympanic from the Greek tympanon, meaning "drum"

Outer ear

Outer ear

Semicircular canals

Auditory nerve

Cochlea

Auditory canal

Tympanic membrane

(eardrum)

Stirrup

Auditory canal

Tympanic membrane

(eardrum)

Semicircular canals

Auditory nerve

Cochlea

Stirrup figure 49-10

Sound waves, which are vibrations in the air, cause the tympanic membrane to move back and forth. This motion causes the small bones of the middle ear to move as well, transferring vibrations to the oval window. Mechanoreceptors in the inner ear translate these vibrations to action potentials. These, in turn, travel through the auditory nerve to auditory processing centers in the brain.

figure 49-11

Hair cells are arranged in orderly rows inside the organ of Corti in the middle chamber of the cochlea. In the damaged section shown here, the hair cells in the top and center rows are relatively intact. Many of the hair cells of the bottom row, however, are frayed and bent. The cluster of hair cells in the middle of the bottom row has been completely destroyed.

figure 49-11

Hair cells are arranged in orderly rows inside the organ of Corti in the middle chamber of the cochlea. In the damaged section shown here, the hair cells in the top and center rows are relatively intact. Many of the hair cells of the bottom row, however, are frayed and bent. The cluster of hair cells in the middle of the bottom row has been completely destroyed.

Observing a Lens

Materials beaker, water, newspaper, 4 drops cooking oil Procedure Observe the newspaper through the sides of an empty beaker. Fill the beaker with water, and observe the newspaper through the water. Add four drops of oil to the top of the water. Observe the newspaper through the oil drops and water. Note any difference in print size.

Analysis Infer why the print size changes when the newspaper is viewed through water. Which structure of the eye does the oil on the water represent?

The hair cells that line the cochlea are delicate and vulnerable. Repeated or sustained exposure to loud noise destroys hair cells in the organ of Corti. Once destroyed, hair cells do not generally regenerate, and the sound frequencies they interpret are no longer heard. Figure 49-11 shows rows of hair cells in a damaged section of the organ of Corti. Hair cells that respond to high-frequency sound are especially vulnerable to destruction. The loss of these cells typically leads to difficulty understanding human voices. Much of this type of permanent hearing loss is avoidable by reducing exposure to loud noises, such as machine noise and loud music.

Besides detecting sound, the ear also helps maintain balance. Balance is maintained by mechanoreceptors in the three semicircular canals of the inner ear. The semicircular canals are filled with fluid. Their interiors are lined with hair cells that have tiny particles of calcium carbonate on top of them. When the head moves, the hair cells are bent by the action of gravity or inertia on the calcium carbonate particles. The brain decodes how far and in what direction the hair cells bend. It interprets the head's motion and orientation in space and sends out the proper orders to help the body maintain balance.

The eyes are specialized organs that detect light and transmit signals to visual processing areas of the brain. The eye is basically a hollow sphere filled with a clear fluid. The structures of the eye act together to focus light on the retina, the light-sensitive inner layer of the eye.

Light passes first through a clear, protective layer called the cornea. Light then passes through the pupil, the opening to the interior of the eye. The pupil becomes larger when light is dim and smaller when light is bright. Muscles in the pigmented iris that surrounds the pupil control these involuntary responses.

After light passes through the pupil, it travels through a crystalline structure called the lens. Muscles attached to the lens adjust the shape of the lens to bend the rays of the incoming light. This bending focuses the image formed by the light onto the retina.

Lying within the retina are rods and cones, photoreceptors that translate light energy into electrical signals that can be interpreted by the brain. Rods contain rhodopsin, a light-sensitive pigment that allows the rods to respond to dim light. Cones in the retina are stimulated by bright light. The cones initiate the production of sharp images and respond to different colors. Humans have three kinds of cones. Each kind of cone contains a pigment that absorbs different wavelengths of light. When the brain integrates signals from these three kinds of cones, a person perceives all the colors in the visible spectrum. Colorblindness, which is the inability to distinguish certain colors, is caused by faulty or missing cones.

Each photoreceptor responds to light from a single location in the visual field. Signals from the stimulated photoreceptors in the deepest layer of the retina travel to neurons on the surface of the retina. From these neurons, millions of axons, which form the optic nerve, exit the eye. The optic nerve carries visual information in the form of action potentials from the retina to the thalamus. The cortex of the occipital lobe ultimately processes visual information into meaningful patterns of shape and color. Figure 49-12 shows the structure of the eye.

Lens

Optic nerve

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