The Special Senses

The senses connect humans to the real world, allowing them to interpret what is happening around them and respond accordingly. The color of the sky at dusk, the sound of laughter at a party, the scent of eucalyptus and pine, the taste of freshly baked bread—all would be meaningless without the senses. They not only provide pleasure, but warn of danger. Traditionally, sight, hearing, smell, taste, and touch have been considered the five main senses of the body. However, touch (along with the senses of pressure, temperature, and pain) is one of the general senses that has small sensory receptors scattered throughout the body in the skin (for a further discussion, see chapter 4). The other four "traditional" senses—sight, hearing, smell, and taste—are the special senses.

DESIGN: PARTS OF THE SPECIAL SENSES

The abilities to see, to hear, to smell, and to taste are all possible because of sensory receptors, or special nerve cells or endings of the peripheral nervous system (part of the nervous system consisting mainly of nerves that extend from the brain and spinal cord to areas in the rest of the body). Sensory receptors respond to a stimulus by converting that stimulus into a nerve impulse. The impulse is then carried by sensory nerves to a specific part of the brain, where the sensation of sight, sound, smell, or taste is perceived or "felt."

Sensory receptors are classified according to the type of stimulus that arouses or excites them. The receptors for the sense of sight are photoreceptors that are sensitive to light. The receptors for the sense of hearing are mechanoreceptors that are sensitive to sound waves or vibrations. The receptors for the senses of smell and taste are chemoreceptors that are sensitive to various chemicals.

The special sensory receptors for sight and hearing are located in large, complex sensory organs—the eyes and the ears. Those for smell and taste are located in organs that function in other systems—the nose in the respiratory system and the mouth in the digestive system.

Anatomy of the eye

The eye is the organ of sight or vision. Each eye works with the brain to transform light waves into visual images. Eighty percent of all information received by the human brain comes from the eyes.

The Special Senses: Words to Know

Accommodation (ah-kah-mah-DAY-shun):

Process of changing the shape of the lens of the eye to keep an image focused on the retina.

Aqueous humor (AYE-kwee-us HYOO-mer):

Tissue fluid filling the cavity of the eye between the cornea and the lens.

Binocular vision (by-NOK-yoo-lur VI-zhun):

Ability of the brain to create one image from the slightly different images received from each eye.

Ceruminous glands (suh-ROO-mi-nus GLANDZ):

Exocrine glands in the skin of the auditory canal of the ear that secrete earwax or cerumen.

Chemoreceptors (kee-moe-re-SEP-terz):

Receptors sensitive to various chemicals substances.

Choroid (KOR-oid):

Middle, pigmented layer of the eye.

Ciliary body (SIL-ee-air-ee BAH-dee):

Circular muscle that surrounds the edge of the lens of the eye and changes the shape of the lens.

Cochlea (KOK-lee-ah):

Spiral-shaped cavity in the inner ear that contains the receptors for hearing in the organ of Corti.

Cones:

Photoreceptors in the retina of the eye that detect colors.

Cornea (KOR-nee-ah):

Transparent front portion of the sclera of the eye.

Conjunctiva (kon-junk-TIE-vah):

Mucous membrane lining the eyelids and covering the front surface of the eyeball.

Eardrum (EER-drum):

Thin membrane at the end of the outer ear that vibrates when sound waves strike it.

Eustachian tube (yoo-STAY-she-an TOOB):

Slender air passage between the middle ear cavity and the pharynx, which equalizes air pressure on the two sides of the eardrum.

External auditory canal (ex-TER-nal AW-di-tor-ee ka-NAL):

Also called the ear canal, the tunnel in the ear between the pinna and eardrum.

Gustation (gus-TAY-shun):

The sense of taste.

Gustatory cells (GUS-ta-tor-ee CELLS):

Chemoreceptors located within taste buds.

Iris (EYE-ris):

Pigmented (colored) part of the eye between the cornea and lens made of two sets of smooth muscle fibers.

Lacrimal gland (LAK-ri-muhl GLAND):

Gland located at the upper, outer corner of each eyeball that secretes tears.

Lens:

Clear, oval, flexible structure behind the pupil in the eye that changes shape for the focusing of light rays.

Mechanoreceptors (mek-ah-no-re-SEP-terz):

Receptors sensitive to mechanical or physical pressures such as sound and touch.

Olfaction (ol-FAK-shun):

The sense of smell.

Olfactory epithelium (ol-FAK-ter-ee ep-e-THEE-leeum):

Section of mucous membrane in the roof of the nasal cavity that contains odor-sensitive olfactory nerve cells.

Organ of Corti (OR-gan of KOR-tee):

Structure in the cochlea of the inner ear that contains the receptors for hearing.

Ossicles (OS-si-kuls):

Three bones of the middle ear: hammer, anvil, and stirrup.

Papillae (pah-PILL-ee):

Projections on the tongue that contain taste buds.

Photoreceptors (fo-to-re-SEP-terz):

Receptors sensitive to light.

Pinna (PIN-nah):

Commonly referred to as the ear, the outer, flaplike portion of the ear.

Pupil (PYOO-pil):

Opening in the center of the iris though which light passes.

Receptors (re-SEP-terz):

Specialized peripheral nerve endings or nerve cells that respond to a particular stimulus such as light, sound, heat, touch, or pressure.

Retina (RET-i-nah):

Innermost layer of the eyeball that contains the photoreceptors—the rods and cones.

Rods:

Photoreceptors in the retina of the eye that detect the presence of light.

Saccule (SAC-yool):

Membranous sac in the vestibule of the inner ear that contains receptors for the sense of balance.

Sclera (SKLER-ah):

Outermost layer of the eyeball, made of connective tissue.

Semicircular canals (sem-eye-SIR-cue-lar ka-NALZ):

Three oval canals in the inner ear that help to maintain balance.

Taste buds:

Structures on the papillae of the tongue that contain chemoreceptors that respond to chemicals dissolved in saliva.

Utricle (YOO-tri-kuhl):

Membranous sac in the vestibule of the inner ear that contains receptors for the sense of balance.

Vestibule (VES-ti-byool):

Bony chamber of the inner ear that contains the utricle and the saccule.

Vitreous humor (VIT-ree-us HYOO-mer):

Transparent, gellike substance that fills the cavity of the eye behind the lens.

The human eyeball is about 0.9 inch (2.3 centimeters) in diameter and is not perfectly round, being slightly flattened in the front and back. Only about one-sixth of an eye's front surface can normally be seen. The rest of the eye is enclosed and protected by a cushion of fat and the walls of the orbit, a cavity in the skull formed by facial and cranial bones. The eye wall consists of three covering layers: the sclera, the choroid, and the retina.

THE SCLERA. The sclera, the outer layer made of fibrous connective tissue, encases and protects the eyeball. The visible portion of the sclera is seen as the "white" of the eye. When that portion is irritated, the small blood vessels contained in the layer enlarge, producing a "bloodshot eye." In the center of the visible portion of the sclera is the cornea, which projects slightly forward. The cornea is transparent and has no capillaries. It is the "window" or the first part of the eye through which light enters. A delicate mucous membrane, the conjunctiva, covers the cornea and visible portion of the sclera. It secretes mucus to lubricate the eyeball and keep it moist.

THE CHOROID. The choroid is a thin membrane lying underneath the sclera. It is composed of a dark pigment that absorbs light within the eye (preventing glare) and numerous blood vessels that nourish the internal tissues of the eye. At the front end of the choroid is the ciliary body. Running like a ring around the visible portion of the eye, the ciliary body connects the choroid with the iris. The ciliary body contains muscles that are connected by ligaments to the lens behind the iris.

The iris is the visible portion of the choroid. It gives the eye its color, which varies depending on the amount of pigment present in the iris. Dense pigment makes the iris brown, while little pigment makes the iris blue. If there is no pigment the iris is pink, as in the eye of a white rabbit. The rounded opening in the center of the iris is the pupil, through which light passes. In bright light, muscles in the iris constrict the pupil, reducing the

amount of light entering the eye. Conversely, the pupil dilates (enlarges) in dim light, increasing the amount of light entering. Extreme fear, head injuries, and certain drugs can also dilate the pupil.

THE LENS. The lens is a crystal-clear, oval, flexible body that is biconvex (curving outward on both surfaces). It is made up of approximately 35 percent protein and 65 percent water. The entire surface of the lens is smooth and shiny, contains no blood vessels, and is encased in an elastic membrane. The lens sits behind the iris and focuses light on the retina. In addition to holding the lens in place, the muscles of the ciliary body contract and relax, causing the lens to either fatten or become thin. As the shape of the lens changes, so does its focus.

THE RETINA. The retina, the innermost layer of the eye, is thin, delicate, sensory tissue composed of layers of light-sensitive nerve cells. The retina begins at the ciliary body (not at the front of the eye) and encircles the entire interior portion of the eye. Rods and cones are the photoreceptors of the retina. In each eye there are about 126 million rods and 6 million cones.

Rods function chiefly in dim light, allowing limited night vision: it is with rods that a person sees the stars. Rods cannot detect color (that is why objects in dim light appear in shades of gray), but they are the first cells to detect movement. They are most abundant toward the edge of the retina and provide people with peripheral (or side) vision. Cones function best in bright light and are sensitive to color. They are most abundant in the center of the retina. Scientists believe three types of cones—red, blue, and green—exist in the eye. The perception of different colors is the result of the stimulation of various combinations of these three types.

CAVITIES AND FLUIDS OF THE EYE. Between the cornea and the lens is a small cavity. This cavity is filled with a clear watery fluid known as aqueous humor, formed by capillaries in the ciliary body. This fluid aids good vision by helping maintain eye shape, providing support for the internal structures, supplying nutrients to the lens and cornea, and disposing of cellular wastes produced by the eye.

The large cavity in back of the lens (the center of the eyeball) is filled with a transparent, gellike substance called vitreous humor. Light passing through the lens on its way to the retina passes through the vitreous humor. The vitreous humor is 99 percent water and contains no cells. It helps to maintain the shape of the eye and support its internal components.

WHY DO ALL NEWBORN BABIES HAVE BLUE EYES?

The amount of pigment in the iris is what determines its color. In newborns, most of the pigment is concentrated in the folds of the iris. Since only a little bit of pigment exists on the visible portion of the iris, it appears blue. When a baby is a few months old, the rest of the pigment begins moving to the surface of the iris, giving the baby his or her permanent eye color.

ACCESSORY STRUCTURES OF THE EYE. The lacrimal gland, which lies immediately above each eyeball at the outer corner of the eye socket, produces tears. Tears are mostly water, but also contain antibodies and an enzyme that prevents the growth of most bacteria on the wet, warm surface of the eye. Tears flow through numerous ducts from the lacrimal gland to the area beneath the upper eyelid. Blinking spreads the tears across the cornea's outside surface, keeping it moist and clean. Tear fluid then either evaporates or drains into two small pores in the inner corner of the eye that connect to a larger duct, which eventually drains tears into the nasal cavity.

Eyelids and eyelashes help to protect the eye. The blinking movement of eyelids keeps the front surface of the eye lubricated and free from dust and dirt. The rate of blinking varies. On average, the eye blinks once every five seconds (or 17,280 times a day or 6.3 million times a year). The eyelids can also close firmly to protect the eye. Eyelashes, hairs that project from the border of each eyelid, help to keep dust, dirt, and insects out of the eye.

Extending from the bony surface of the orbit to the outside of the eyeball are six small muscles that contract and relax, allowing the eye to move in various directions. Four of the muscles move the eyeball up and down and side to side. The other two muscles rotate the eye.

Anatomy of the ear

The human ear is the organ responsible for hearing and for equilibrium or balance. The ear consists of three regions or areas: the outer (external) ear, the middle ear, and inner (internal) ear. The mechanoreceptors for hearing and balance are all found in the inner ear.

THE OUTER EAR. The outer ear collects external sounds and funnels them through the auditory system to the eardrum. The outer ear is composed of three parts—the pinna (or auricle), the external auditory canal, and the eardrum (tympanic membrane).

What are commonly called ears—the two flaplike structures on either side of the head—are actually the pinnas of the outer ear. Pinnas are skin-covered cartilage, not bone, and are therefore flexible. In many species of animals, the pinnas act to collect and funnel sound waves into the external auditory canal. In humans, however, the pinnas do not serve this purpose. In fact, humans could lose their pinnas and hearing would not be adversely affected.

WHAT ARE "FLOATERS" IN THE EYE?

Floaters are semi-transparent or dark little specks that float across the field of vision and can be mistaken for flies in the room. Some floaters originate with red blood cells that have leaked out of the retina. The blood cells swell into spheres—some forming strings—and float around the areas of the retina. Others are shadows cast by microscopic structures in the vitreous humor.

A sudden appearance of dark floaters, if accompanied by bright little flashes, could indicate that the retina has detached, a serious problem that requires medical treatment.

The external auditory canal or ear canal is a passageway that begins at the pinna and extends inward and slightly upwards, ending at the

eardrum. In the adult human it is lined with skin and hairs and is approximately 1 inch (2.5 centimeters) long. The outer one-third of the canal is lined with wax-producing ceruminous glands and fine hairs. The purpose of the earwax and hairs is to protect the eardrum by trapping dirt and foreign bodies and keeping the canal moist.

The eardrum or tympanic membrane is a thin, concave membrane stretched across the inner end of the auditory canal much like the skin covering the top of a drum. The eardrum marks the border between the outer ear and middle ear. In the adult human, the eardrum has a total area of approximately 0.1 square inch (0.6 square centimeter). The middle point of the eardrum—called the umbo—is attached to the stirrup, the first of three bones contained within the middle ear.

THE MIDDLE EAR. The middle ear transmits sound from the outer ear to the inner ear. The middle ear consists of an oval, air-filled space approximately 0.1 cubic inch (2 cubic centimeters) in volume. Contained in this space are three tiny bones called ossicles. Because of their shapes, the three ossicles are known as the hammer (malleus), the anvil (incus), and the stirrup (stapes).

Connecting the middle ear to the throat is the eustachian tube. This tube is normally closed, opening only as a result of muscle movement during yawning, sneezing, or swallowing. The eustachian tube causes air pressure in the middle ear to match the air pressure in the outer ear.

HOW HEARING AIDS WORK

Hearing aids are tools that amplify sound for people who have a hard time hearing. Millions of hearing aids are sold annually, especially to people over the age of sixty-five. More than 1,000 different models are available in the United States.

A typical hearing aid contains a microphone that picks up sounds and changes them into electric signals. The hearing aid's amplifier increases the strength of the electric signals. Then the receiver converts the signals back into sound waves that can be heard by the wearer.

The entire mechanism is housed in an ear mold that fits snugly in the ear canal. The power to run the electronic parts is provided by a small battery. There are a variety of designs to fit the needs of the wearer, some small enough to be completely concealed by the ear canal.

Many modern hearing aids have miniature computer chips that allow the aid to selectively boost certain frequencies. This means that a person could wear such a hearing aid to a loud party and screen out unwanted background noise while tuning in to a private conversation. The hearing aid can also be programmed to conform to a person's specific hearing loss. Some models can be further programmed to allow the wearer to choose different settings depending on the noise of the surroundings.

The most noticeable example of eustachian tube function occurs when there is a quick change in altitude, such as when a plane takes off. Prior to takeoff, the pressure in the outer ear is equal to the pressure in the middle ear. When the plane gains altitude, the air pressure in the outer ear decreases, while the pressure in the middle ear remains the same, causing the ear to feel "plugged." In response to this the ear may "pop." The popping sensation is actually the quick opening and closing of the eustachian tube and the equalization of pressure between the outer and middle ear.

THE INNER EAR. The inner ear is responsible for interpreting and transmitting sound and balance sensations to the brain. The inner ear, located just behind the eye socket, is small (about the size of a pea) and complex in shape. With its series of winding interconnected chambers, it has been called a labyrinth. The main components of the inner ear are the vestibule, semicircular canals, and the cochlea.

The vestibule, a round open space, is the central structure within the inner ear. The vestibule contains two membranous sacs—the utricle and the saccule (the saccule is the smaller of the two). These sacs, lined with tiny hairs and attached to nerve fibers, function as an individual's chief organs of balance.

Attached to the vestibule are three loop-shaped, fluid-filled tubes called the semicircular canals. These canals, arranged perpendicular to each other, are a key part of the vestibular system. Two of the canals help the body maintain balance when it is moving vertically, such as in falling and jumping. The third maintains horizontal balance, as when the head or body rotates.

The cochlea is the organ of hearing. The cochlea consists of a bony, snail-like shell that contains three separate fluid-filled ducts or canals. The middle canal contains the basilar membrane, which holds or supports the organ of Corti, named after Italian anatomist Alfonso Giacomo Gaspare Corti (1822–1876) who discovered it. The organ contains over 20,000 hair cells (mechanoreceptors) connected at their base to the auditory nerve. The organ is the site where sound waves are converted into nerve impulses, which are then sent to the brain along the auditory nerve.

Anatomy of the sense of smell

Smell, called olfaction, is the ability of an organism to sense and identify a substance by detecting tiny amounts of the substance that evaporate and produce an odor. Smell is the most important sense for most organisms.

In humans, the sense of smell differs from the other senses (sight, hearing, and taste) in its directness. People actually smell microscopic bits or chemicals of a substance that have evaporated and made their way through the nostrils into the nasal cavity. In the roof of the nasal cavity is a section of mucous membrane called the olfactory epithelium. It covers an area of roughly 0.75 square inch (4.8 square centimeters), or about the size of a postage stamp.

The olfactory epithelium contains millions of odor-sensitive olfactory receptor cells (chemoreceptors) that are connected to the olfactory nerves. The olfactory receptors have long olfactory hairs that protrude outward from the epithelium. Beneath the olfactory epithelium lie olfactory glands that produce mucus that covers the epithelium and bathes the olfactory hairs. The mucus keeps the area moist and clean and prevents the buildup of potentially harmful or overpowering chemicals.

Anatomy of the sense of taste

Taste, called gustation, is the sense for determining the flavor of food and other substances (taste comes from the Latin word taxare, meaning "to touch" or "to feel"). It is one of the two chemical senses (the other being smell) and it is stimulated when taste buds on the tongue come in contact with certain chemicals. The sense of taste is also influenced by the smell and texture of substances, hereditary factors, culture, and familiarity with specific taste sensations.

HEARING COLORS AND SEEING SOUNDS

Normally, when people experience the world through their senses, they do so in an orderly fashion. They see with their eyes, hear with their ears, and taste and smell with the chemoreceptors in their mouths and noses.

For some people, however, the basic rules of sensory perception do not apply. They tend to perceive stimuli not only with the sense for which it was intended, but with others as well—sight mingles with sound, taste with touch. They may see musical notes as color hues or feel flavors as different textures on the skin.

This rare condition is known as synesthesia (sines-THE-zee-ah), and the people who have it as synesthetes. Women are about six times as likely as men to be synesthetes.

Some scientists believe that the condition is the result of associations learned at an early age. Other scientists disagree, believing that a unique physical condition exists in the brains of synesthetes. Some brain studies have shown that during synesthetic experiences, blood flow to some parts of the brain decreases. Normally, that blood flow would have been increased by sensory stimuli. Synesthesia also appears to run in families, leading some scientists to theorize that it has a genetic basis.

Clusters of small organs called taste buds are located in the mouth. Of the almost 10,000 taste buds, most are located on the upper surface of the tongue (a few are located on the soft palate and on the inner surface of the cheeks). Taste buds (named so because under the microscope they look similar to plant buds) lie in small projections on the tongue called papillae. Within the taste buds are taste receptors known as gustatory cells. Each gustatory cell projects slender taste hairs into the surrounding fluids through a narrow taste pore. As food is broken down in the mouth, these receptors come into contact with chemicals dissolved in the saliva. They then send messages along nerves to the brain, which interprets the flavor as sweet, sour, salty, or bitter.

Taste buds for all four taste groups can be found throughout the mouth, but specific kinds of buds are concentrated in certain areas (the areas tend to overlap each other). Sweetness is detected by taste buds on the tip of the tongue. The buds for sour tastes are on the sides of the tongue. Those for salty are toward the front. Bitter taste buds are located on the back of the tongue. Bitterness can make many people gag, which is a defense mechanism. Since many natural poisons and spoiled foods are bitter, gagging helps prevent poisoning.

New taste buds are produced every three to ten days to replace the ones worn out by scalding or frozen foods. As people grow older, their taste buds are replaced at a slower rate, and more of a substance is needed to experience its full flavor.

WORKINGS: HOW THE SPECIAL SENSES FUNCTION

An individual's total experience of the external world—what he or she "feels"—is a blending of all the senses. The senses take in information about the world in the form of light waves, sound waves, and dissolved chemicals.

They convert those various forms into nerve impulses that are sent to the brain, which then interprets the impulses and gives them meaning in such a way that an individual sees, hears, smells, and tastes.

Sight

Light is a form of electromagnetic radiation—a form of energy carried by waves. The term electromagnetic radiation refers to a vast range of energy waves, including gamma rays, X rays, ultraviolet rays, visible light, infrared radiation, microwaves, radar, and radio waves. Of all these forms, only one can be detected by the human eye: visible light.

The human eye detects two types of visible light: direct light and indirect light. Direct light is light coming directly from a source such as the Sun or a lightbulb. Indirect light is light that has bounced off an object. People see trees, buildings, cars, and even the Moon through indirect light. Regardless of the type of light, an individual can see only if light rays are focused on the retina and the resulting nerve impulses are transmitted to the brain.

In short, sight occurs when light waves enter the eye by passing through the conjunctiva, cornea, aqueous humor, pupil, then the lens behind the iris. The lens focuses the light waves through the vitreous humor onto the retina. The rods and cones in the retina convert the light energy into electrical impulses. These impulses are then carried via the optic nerves to the brain, where they are interpreted as images.

When light passes from one substance to another of greater or lesser density, its waves or rays are refracted or bent. This is what happens when light waves pass from the surrounding air through the cornea and lens. The function of the lens is to focus the waves precisely on the retina. When the lens fails to do so, visual problems occur (see "Ailments" section below).

SEEING INTO THE FUTURE

Researchers at Harvard University have found that everyone, not just fortune tellers, can see into the future. Tennis players routinely react to balls travelling over 100 miles (160 kilometers) per hour. The researchers conducted studies to determine how an individual could respond to an object that was moving faster than the eye had time to transmit its image to the brain.

The researchers found that the retina of the eye contains cells, called ganglion cells, that can calculate the future position of a moving object. The cells send nerve messages to the brain about the position of an object thousandths of a second before it actually arrives in that place in space.

This finding contradicts the notion that the eye acts like a simple camera, recording an image placed directly before it.

The lens changes shape depending on whether an object being viewed is far or near. The light waves from distant objects, generally over 20 feet (6 meters) away, approach the eye as parallel rays. In order to focus those rays on the retina, the lens does not change its normal, relatively flat shape, and the muscles of the ciliary body are relaxed. On the other hand, light waves from close objects tend to scatter and diverge or spread out. In order to focus those rays on the retina, the lens must bulge or become more spherical. To achieve this, the muscles in the ciliary body contract and the body forms a smaller circle. The elastic lens recoils and bulges in the middle (becoming convex). This process of changing the shape of the lens to keep an image focused on the retina is called accommodation.

When light rays strike the retina, they stimulate chemical reactions in the rods and cones. Different wavelengths of visible light (which result in different colors) bring about different types of chemical reactions. The reactions generate electrical impulses that are transmitted from the rods and cones by neurons. These neurons come together to form the optic nerve, which passes out through the rear wall of the eyeball.

The optic nerves from the two eyes then converge and nerve fibers from the inside portion of each optic nerve cross over to the other side. The nerves then separate again and travel to the occipital lobes of the cerebral cortex, the outermost layer of the cerebrum (largest part of the brain). The visual area in the right occipital lobe (on the right side or right hemisphere of the brain) receives nerve fibers from the right side of each eye. The visual area in the left occipital lobe receives nerve fibers from the left side of each eye.

When an image is formed on the retina by the light-bending action of a lens, it is an upside-down likeness of the object the light rays bounced off of. The image is also smaller than the original object (the farther away an object, the smaller its image on the retina) and reversed left to right. When the brain receives impulses containing information about these small, upside-down, reversed images from the eyes, it corrects the images to produce normal vision. Scientists still do not know how the brain does this.

Another ability of the brain is to bring together the two separate images (one from each eye) to form a single image. Each eye forms its own flat, twodimensional (with height and width) image from a slightly different angle. The brain compares and combines the two images to create a single image that is three-dimensional (one that also has depth). This ability of the brain, producing normal vision using both eyes, is known as binocular vision.

Hearing

Sound is produced by a vibrating object or body. As an object vibrates or moves back and forth, it causes molecules in the surrounding air to be pushed together or stretched apart. This disturbance in the air radiates outward in the form of waves that can travel through air, water, and solids. When these waves enter the ear, the effect is perceived as sound.

Frequency is the number of back-and-forth motions (called oscillations) a wave makes in a unit of time. Frequency is usually measured in cycles (vibrations) per second, and the unit used to measure frequency is the hertz (abbreviation: Hz). For example, 1,000 Hz is equal to 1,000 cycles per second.

Objects can vibrate with a great range of frequencies, from only a few cycles (or vibrations) per second to millions of times per second. However, the human ear is able to detect only a limited range of those vibrations, generally those between 20 to 20,000 Hz. When the rate of vibration is less than 20 Hz, the sound is said to be infrasonic; when the rate is above 20,000 Hz, it is said to be ultrasonic.

When sound waves reach the outer ear, the pinna funnels the waves into the external auditory canal and toward the eardrum. As the waves strike the eardrum, it vibrates in response to the pressure or force of the sound waves. The initial vibration causes the eardrum to be pushed inward by an amount equal to the intensity of the sound. Once the eardrum is pushed inward, the pressure within the middle ear causes the eardrum to be pulled outward, setting up a back-and-forth motion.

The movement of the eardrum sets all three ossicles in motion. The vibrating pressure of the stirrup (the last ossicle) on the small opening leading to the inner ear sets the fluid in the cochlea in motion. The fluid motion causes a corresponding, but not equal, wavelike motion of the basilar membrane.

When the basilar membrane moves, it causes the small hairlike fibers on the top of the hair cells of the organ of Corti to bend. The bending of the hair cells causes chemical reactions within the cells themselves, creating electrical impulses in the nerve fibers attached to the bottom of the hair cells. The nerve impulses travel up the auditory nerves to auditory areas in the temporal lobes of the cerebral cortex of the brain, which interpret those nerve impulses as "sounds."

Sound waves usually reach the two ears at different times, allowing an individual to hear "in stereo." The brain uses this difference to determine from where in the surrounding environment the sound waves are coming. Scientists are not quite sure how the brain determines whether a sound is high-pitched or low-pitched, but they believe the sensation of pitch is dependent on which area of the basilar membrane vibrates (causing certain hair cells of the cochlea to bend). They also believe that the amount of vibration of the basilar membrane determines whether the brain will interpret a sound as loud or soft.

BALANCE. Besides hearing, the inner ear is also concerned with balance. The utricle and saccule (the membranous sacs in the vestibule of the inner ear) provide the brain with information about the body at rest. The tiny hair cells within these sacs project into a jellylike material that contains tiny mineral crystals. As the position of the head changes (while the body stays at rest), gravity causes the crystals to shift, pulling on the jellylike material. This movement bends the hair cells, and they are stimulated to generate nerve impulses that are sent to the cerebellum of the brain, informing it of the position of the head. The brain then uses this information to maintain equilibrium or balance.

Helping to maintain balance while the body moves through space are the fluid-filled semicircular canals. They provide the brain with information concerning rotational movements of the head. The three semicircular canals are arranged at right angles to each other, oriented in the three planes of space (up and down, right and left, forward and back). An area at the base of each semicircular canal contains sensory hair cells. Depending on the direction in which the body moves, the thick fluid in one or more of the semi-circular canals moves in the opposite direction and the hair cells are bent. This stimulates the hair cells to generate nerve impulses that are sent to the cerebellum. The brain then uses the information to maintain balance (by coordinating muscle movements) while the body is moving.

Smell

Scientists believe there are only a few basic odors or scents that combine to form all other odors. However, they have found it hard to agree on the number of basic odors. Estimates range from seven to fifty or more. Although many animals have a sense of smell far superior to that of humans, the human nose is capable of detecting over 10,000 different odors, even some that occur in extremely minute amounts in the air.

For smelling to occur, chemicals in the air must enter the nostrils into the nasal cavity. There, they must be dissolved in the mucus covering the olfactory epithelium. Once dissolved, the chemicals bind to the olfactory hairs, stimulating the olfactory receptors to send nerve impulses along the olfactory nerves to the olfactory areas in the temporal lobes of the cerebral cortex. The brain interprets the impulses as a specific odor or odors.

Olfactory information is also sent to the limbic system, a horseshoe-shaped area of the brain concerned with emotional states and memory. Thus, the impressions formed by odors or smells are long-lasting and very much a part of an individual's memories and emotions.

Taste

Although there are only four general types of taste receptors—sweet, sour, salty, and bitter—people experience many different tastes because the chemicals in foods stimulate different combinations of the four receptors.

The mechanism for tasting is similar to that for smelling. Chemicals from foods and liquids that are dissolved in the saliva come in contact with the taste hairs of the gustatory cells. The cells are then stimulated to generate nerve impulses that are carried by three cranial nerves to the parietal and temporal lobes of the cerebral cortex. The brain then interprets those impulses as taste sensations.

Scientists have discovered that individual tasting abilities and preferences for specific foods are partially hereditary. Some people are genetically programmed to have more taste buds than others and, as a result, taste more flavors in a particular food. Additionally, culture and familiarity with foods greatly influence taste preferences. Foods that are a tradition in certain cultures may be unappealing to those who are unfamiliar with them. A taste for a particular food usually develops as a person consumes it more frequently.

The smell, texture, and temperature of foods also affect taste. People often first experience the flavor of a food by its odor. When an individual's sense of smell is decreased due to congestion from a cold or flu, that person frequently experiences a reduced ability to taste. Some people will not eat pears because of the fruit's gritty texture, while others would not think of drinking cold coffee.

AILMENTS: WHAT CAN GO WRONG WITH THE SPECIAL SENSES

Most problems that afflict the special senses are the result of normal aging. As people grow older, their lacrimal glands become less active. The eyes become dry and are more prone to infection and irritation. The muscles of the iris also become less efficient, and the lens tends to lose its crystal clarity. Both of these conditions cause less light to reach the retina, thereby reducing vision.

The ears are affected by few problems during childhood and adult life (except for ear inflammations or infections). However, after the age of sixty, the organ of Corti begins gradually to deteriorate and the ability to hear high tones and human speech decreases.

Smell and taste, the chemical senses, usually stay sharp throughout childhood and adult life. They gradually begin to decrease when a person reaches middle age because of a loss in the number of chemoreceptor cells. During life, impairment of smell and taste are usually the result of a nasal cavity inflammation (due to a cold, an allergy, or smoking) or a head injury. More serious infections in the nasal or oral cavity, such as oral candidiasis (yeast infection of the mucous membranes of the oral cavity), can obviously impair smell and taste.

The following are just a few of the disorders or diseases that can afflict some of the special senses.

Astigmatism

Astigmatism is a condition brought about by an uneven curvature of the cornea. As a consequence, some light rays entering the eye focus on the retina while others focus in front or behind it. The result is an indistinct or slightly out-of-focus image. Some cases of astigmatism are caused by problems in the lens. Minor variations in the curvature of the lens can result in minor degrees of astigmatism. In these cases, the cornea is usually normal in shape.

SPECIAL SENSE DISORDERS

Astigmatism (ah-STIG-mah-tiz-um): Incorrect shaping of the cornea that results in an incorrect focusing of light on the retina.

Cataract (KAT-ah-rakt): Condition in which the lens of the eye turns cloudy, causing partial or total blindness.

Conjunctivitis (kon-junk-ti-VIE-tis): Inflammation of the conjunctiva of the eye.

Farsightedness: Known formally as hyperopia, the condition of the eye where incoming rays of light reach the retina before they converge to form a focused image.

Glaucoma (glaw-KOE-mah): Eye disorder caused by a buildup of aqueous humor that results in high pressure in the eyeball, often damaging the optic nerve and eventually leading to blindness.

Meniere's disease (men-ee-AIRZ): Ear disorder characterized by recurring dizziness, hearing loss, and a buzzing or ringing sound in the ears.

Nearsightedness: Known formally as myopia, the condition of the eye where incoming rays of light are bent too much and converge to form a focused image in front of the retina.

Otitis media (oh-TIE-tis ME-dee-ah): Infection of the middle ear.

Astigmatism is a condition that may be present at birth. It may also be acquired if something is distorting the cornea. The upper eyelid resting on the eyeball, trauma or scarring to the cornea, tumors in the eyelid, or a developing condition in which the cornea thins and becomes cone shaped can all cause distortion. Diabetes can also play a role. High blood sugar levels can cause changes in the shape of the lens, resulting in astigmatism.

The main symptom of astigmatism is blurring. People with the condition can also experience headaches and eyestrain.

Astigmatism can generally be corrected by the use of cylindrical lenses, which can be in eyeglasses or contact lenses. The lenses are shaped to counteract the shape of the sections of the cornea that are causing the difficulty. In 1997, the U.S. Food and Drug Administration (FDA) approved laser treatment of astigmatism.

Cataracts

A cataract is a cloudiness in the normally transparent crystalline lens of the eye. This cloudiness can cause a decrease in vision and may lead to eventual blindness. Because cataracts are so common in the elderly, they are thought to be a normal part of the aging process. Of those people who are seventy years of age or older, at least 70 percent are affected by cataracts.

As people age, the lens hardens and changes shape less easily. The materials making up the lens also tend to degenerate. Changes in the proteins, water content, enzymes, and other chemicals of the lens are some of the reasons for the formation of a cataract. Some medical studies have determined that smoking, high alcohol intake, and a diet high in fat all increase the likelihood of cataract formation.

Common symptoms of cataracts include poor central vision, changes in color perception, increased glare from lights, poor vision in sunlight, and the painless onset of blurry or fuzzy vision. If the cataract forms in the area of the lens directly behind the pupil, vision may be significantly impaired. If it occurs on the outer edge or edges of a lens, vision loss is less of a problem.

When a cataract causes only minor visual changes, no treatment may be necessary. When it causes severe vision problems, surgery is the only treatment option. Cataract surgery, in which the lens is removed and a replacement or artificial lens is inserted, is the most frequently performed surgery in the United States. It generally improves vision in over 90 percent of those who undergo the procedure.

Conjunctivitis

Conjunctivitis, commonly known as pinkeye, is an inflammation of the conjunctiva that is usually the result of an infection (viral or bacterial) or an allergic reaction. It is an extremely common eye problem because the conjunctiva is continually exposed to microorganisms and environmental agents that can cause infections or allergic reactions. If caused by an infection, conjunctivitis is extremely contagious and can be easily transmitted to others during close physical contact.

Conjunctivitis caused by a viral infection (such as a cold) is marked by mild to severe discomfort in one or both eyes; redness in the eye or eyes; swelling of the eyelids; and a watery, yellow, or green discharge. Symptoms of bacterial conjunctivitis include redness, swelling, a puslike discharge, and crusty eyelids upon awakening. Conjunctivitis caused by wind, smoke, dust, pollen, or grass has symptoms ranging from itching and redness to a mucus discharge.

In most cases, warm compresses applied to the affected eye several times a day may help to reduce discomfort. In cases caused by a bacterial infection, an antibiotic eye ointment or eye drops may be prescribed. For conjunctivitis caused by an allergic reaction, cool compresses may be applied to the affected eye. Antihistamine drugs and eye drops may also be prescribed.

If treated properly, viral or bacterial conjunctivitis usually clears in ten to fourteen days. Conjunctivitis caused by an allergic reaction should clear up once the allergen (substance causing the allergic reaction) is removed.

Farsightedness

Farsightedness, known formally as hyperopia, is a condition of the eye where incoming rays of light reach the retina before they converge to form a focused image. While objects far away may be seen clearly, objects close up cannot. Babies are generally born farsighted, but this normally decreases with age as the eye grows.

Light waves from close objects tend to scatter. In order to focus those light waves precisely on the retina, the lens must bulge or become convex. In farsightedness, the lens is flatter than needed for the length of the eyeball. In other words, the eyeball is too short for the curvature of the lens, and the image of a nearby object is focused behind the retina.

People who are farsighted can see far objects clearly because light waves from distant objects approach the eye as parallel rays. The lens does not change its relatively flat shape in order to focus those rays on the retina.

There is no way to prevent farsightedness. Individuals with low to moderate farsightedness can achieve full vision by wearing corrective convex lenses (either eyeglasses or contact lenses). At the beginning of the twenty-first century, surgery to correct farsightedness had not yet been perfected or approved.

Glaucoma

Glaucoma is a serious vision disorder caused by a buildup of aqueous humor, which is prevented for some reason from properly draining. The excessive amount of fluid causes pressure to build up. The high pressure distorts the shape of the optic nerve and destroys the nerve. Destroyed nerve cells result in blind spots in places where the image from the retina is not being transmitted to the brain.

Following cataracts, glaucoma is the second leading cause of blindness in the United States. Over 2 million people in the country have glaucoma. Some 80,000 of those are legally blind as a result. Glaucoma is the most frequent cause of blindness in African Americans. There are many underlying causes and forms of glaucoma, but most causes of the disorder are not known.

In the most common form of glaucoma, the field of vision is lost over time. Usually the condition starts with a loss of the peripheral (side) vision so that the person does not realize he or she is losing sight until it is too late for treatment.

If treated early, the condition can be controlled with drugs (typically given as eye drops) that either increase the outflow of aqueous humor or decrease its production. Laser surgery or microsurgery to open up drainage canals can be effective in increasing the outflow of aqueous humor. Although the surgeries are successful, the effects often last less than a year.

There is some evidence that marijuana lowers the pressure caused by excess aqueous humor. However, marijuana has serious side effects and contains carcinogens (cancer-causing substances). The U.S. Food and Drug Administration and the National Institutes of Health are currently supporting medical research into marijuana and its possible use as a treatment for glaucoma.

Meniere's disease

Meniere's disease is a condition characterized by recurring vertigo or dizziness, hearing loss, and tinnitus (a buzzing or ringing sound in the ears). The disease is named for French physician Prosper Meniere, who first described the illness in 1861. An estimated 3 to 5 million people in the United States are afflicted with the condition. Meniere's disease usually starts between the ages of twenty and fifty, and it affects men and women equally. In about 85 percent of the cases, only one ear is affected.

The disease is an abnormality in the inner ear, specifically within the fluid-filled semicircular canals. A change in the fluid volume within the canals or a swelling of the canals is thought to result in symptoms characteristic of the disease. The cause of Meniere's disease is unknown.

In addition to the symptoms listed above, a person suffering from the disease may feel pain or pressure in the affected ear. Symptoms can appear suddenly and last up to several hours. They can occur as often as daily or as infrequently as once a year. Attacks of severe vertigo can force the sufferer to have to sit or lie down. Headache, nausea, vomiting, or diarrhea may accompany the attack.

There is no cure for Meniere's disease. Certain symptoms of the disease, such as vertigo, nausea, and vomiting, can be controlled with a variety of medications that are either taken orally or injected by needle. If the vertigo attacks are frequent and severe, surgery may be required.

The most common surgical procedure is the insertion of a shunt or small tube to drain some of the fluid from the canal. Unfortunately, this is not a permanent cure in all cases. In another surgical procedure, the nerves responsible for transmitting nerve impulses related to balance are cut. The distorted impulses causing dizziness then no longer reach the brain. This procedure permanently cures the majority of cases, but there is a slight risk that hearing or facial muscle control will be affected.

Nearsightedness

Nearsightedness, known formally as myopia, is the condition of the eye where incoming rays of light are bent too much and converge to form a focused image in front of the retina. While objects close to the eye may be seen clearly, distant objects appear blurred or fuzzy. Nearsightedness affects about 30 percent of the population in the United States.

Light waves from distant objects approach the eye as parallel rays. Normally, the lens does not change its relatively flat shape in order to focus those rays on the retina. In nearsightedness, the lens is thicker and more convex than needed for the length of the eyeball. In other words, the eyeball is too long (oblong instead of the normal almost spherical shape) for the curvature of the lens, and the image of a distant object is focused in front of the retina.

People who are nearsighted can see close objects clearly because light waves from those objects tend to scatter or spread out. The already convex shape of the lens helps to focus those rays on the retina.

Nearsightedness is considered to be primarily a hereditary disorder, meaning that it runs in families. However, some medical researchers believe that it results from a combination of genetic and environmental factors. The tendency toward being nearsighted may be inherited, but it may actually be brought about by factors such as close work, stress, and eye strain.

People who are nearsighted can (but not always) achieve full vision by wearing corrective concave lenses (either eyeglasses or contact lenses). Another possible treatment for nearsightedness is refractive eye surgery. In most surgical procedures, either a laser is used to vaporize small amounts of tissue from the cornea's surface (thereby flattening it) or a knife is used to cut a circular flap on the cornea, then a laser is used to change the shape of the inner layers of the cornea underneath. Depending on a person's degree of nearsightedness and other factors, refractive surgery can make permanent improvements.

Otitis media

Otitis media is an infection of the middle ear space behind the eardrum. By the age of three, almost 85 percent of all children will have had otitis media at least once. Babies and children between the ages of six months and six years are most likely to develop this type of infection. The most usual times of the year for otitis media to strike are in winter and early spring.

Otitis media is an important problem because it often results in fluid accumulation within the middle ear. The fluid can last for weeks or months, and it can cause significant hearing impairment. When hearing impairment occurs in a young child, it may interfere with the development of normal speech.

Because the middle ear is connected to the throat by the eustachian tube, an infection in the throat may easily reach the middle ear. In fact, most cases of otitis media occur during a bacterial infection of the upper respiratory tract.

Symptoms of otitis media include fever, ear pain, and problems with hearing. When significant fluid is present in the middle ear, pain may increase when a person lies down. Pressure from increased fluid buildup may also perforate or rupture the eardrum, causing bloody fluid or greenish-yellow pus to drip from the ear.

Antibiotics are the treatment of choice for ear infections. The type of drug depends on the type of bacteria causing the infection. Special nose drops, decongestants, or antihistamines may also be prescribed to improve the functioning of the eustachian tube. In rare cases, a procedure to drain the middle ear of pus may be performed.

TAKING CARE: KEEPING THE SPECIAL SENSES HEALTHY

As stated earlier, aging brings about a decline in the functioning of the special senses. Older people often do not see, hear, smell, and taste as well as they once did. This decline is often gradual and, for the most part, affects the quality of an individual's life to a modest degree.

However, neglect of the senses during life can cause a premature decline in their ability to function. Excessive exposure to loud sounds can damage hair cells in the ears that cannot be replaced. Operating modern machinery without wearing protective ear devices or listening to loud music for long periods of time both contribute to hearing loss.

Wearing protective devices over the eyes in most work situations is equally important to prevent injury and possible permanent damage. Eye strain brought on by staring at a computer screen for hours is an increasingly common problem in modern life. To minimize the strain placed on the eyes, an individual should regularly take breaks and walk away from the computer or look away from the screen and focus on some distant object for a while.

Since the special senses are often affected by problems in other parts or systems of the body, it is important to take care of the body as a whole. By getting adequate rest, reducing stress, drinking healthy amounts of good-quality drinking water, not smoking, drinking moderate amounts of alcohol (or not drinking at all), following a proper diet, and exercising regularly, a person can help his or her body to operate at peak efficiency.

FOR MORE INFORMATION

Books

Green, Patrick. Seeing Is Believing. Englewood Cliffs, NJ: Silver Burdett Press, 1996.

Kittredge, Mary, and Mary Talbot. The Senses. New York: Chelsea House, 1990.

Llamas, Andreu. Taste. New York: Chelsea House, 1996.

Parker, Steve. Senses. Brookfield, CT: Copper Beech Books, 1997.

Ripoll, Jaime. How Our Senses Work. New York: Chelsea House, 1994.

Suzuki, David, and Barbara Hehner. Looking at Senses. New York: John Wiley, 1991.

WWW Sites

Mystery of the Senses Activity Guide
http://www.weta.org/weta/education/teachers/itv/mos/mos_guide.html
Site provides information on the five traditional senses and an activity/experiment for the student related to each of those senses. Information based on the NOVA special mini-series Mystery of the Senses.

Neuroscience for Kids—The Senses
http://faculty.washington.edu/chudler/chsense.html
Highly recommended interactive site provides an extensive amount of information about the five traditional senses. A link to each sense provides further information and experiments/activities to learn more about that particular sense.

Seeing, Hearing, and Smelling the World
http://www.hhmi.org/senses/
Site developed by the Howard Hughes Medical Institute is an extensive educational resource with information about brain function and the senses. Also contains a useful glossary of terms relating to sight, hearing, and smell.

The Way the Eye Works
http://www.eyeinfo.com/wayworks.html
Site provides a straightforward discussion of the way the eye works and what happens when it does not.

Your Gross and Cool Body—Sense of Sight
http://www.yucky.com/body/index.ssf?/systems/sight/
Site presents facts and answers questions about the sense of sight.

Your Gross and Cool Body—Sense of Smell
http://www.yucky.com/body/index.ssf?/systems/smell/
Site presents facts and answers questions about the sense of smell.