Chapter 16`

Ear sound (components)

(External Ear,Middle Ear,Internal Ear,)

FIlliform papillae

In addition, the entire surface of the tongue has filiform papillae (FIL-i-form = threadlike). These pointed, threadlike structures contain tactile receptors but no taste buds. They increase friction between the tongue and food, making it easier for the tongue to move food in the oral cavity.

How light passes in the rtetina (componetns)

Light passes through the front of the eye (cornea) to the lens. The cornea and the lens help to focus the light rays onto the back of the eye (retina). The cells in the retina absorb and convert the light to electrochemical impulses which are transferred along the optic nerve and then to the brain.`

How refraction abnormalities are fixed

Most errors of vision can be corrected by eyeglasses, contact lenses, or surgical procedures. A contact lens floats on a film of tears over the cornea. The anterior outer surface of the contact lens corrects the visual defect, and its posterior surface matches the curvature of the cornea. LASIK involves reshaping the cornea to permanently correct refraction abnormalities.

Color Blindless and Night Blindness

Most forms of color blindness, an inherited inability to distinguish between certain colors, result from the absence or deficiency of one of the three types of cones. The most common type is red-green color blindness, in which red cones or green cones are missing. As a result, the person cannot distinguish between the colors red and green. Prolonged vitamin A deficiency and the resulting below-normal amount of rhodopsin in rods may cause night blindness or nyctalopia (nik′-ta-LŌ-pē-a), an inability to see well at low light levels.

Tase bunds are found in

Papillae

Static equilibrium

Static equilibrium refers to the maintenance of the position of the body (mainly the head) relative to the force of gravity. Body movements that stimulate the receptors for static equilibrium include tilting the head forward or backward and linear (straight line) acceleration or deceleration, such as when the body is being moved in an elevator or in a plane that is speeding up or slowing down.

Middle ear (euschian tube)(

The anterior wall of the middle ear contains the opening of the auditory tube, commonly known as the eustachian tube (ū′-STĀ-kē-an). The auditory tube connects the middle ear with the nasopharynx (superior portion of the throat). It is normally closed at its medial (pharyngeal) end. During swallowing and yawning, it opens, allowing air to enter or leave the middle ear until the pressure in the middle ear equals the atmospheric pressure. Most of us have experienced our ears popping as the pressures equalize. When the pressures are balanced, the tympanic membrane vibrates freely as sound waves strike it. If the pressure is not equalized, intense pain, hearing impairment, ringing in the ears, and vertigo could develop. The auditory tube also is a route for pathogens to travel from the nose and throat to the middle ear, causing the most common type of ear infection (see the Clinical Connection entitled Otitis Media in your WileyPLUS resources).

What nerves supoort taste

The facial (VII) nerve serves taste buds in the anterior two-thirds of the tongue, the glossopharyngeal (IX) nerve serves taste buds in the posterior one-third of the tongue, and the vagus (X) nerve serves taste buds in the throat and epiglottis (

Refraction of light rays

-When light rays traveling through a transparent substance (such as air) pass into a second transparent substance with a different density (such as water), they bend at the junction between the two substances. This bending is called refraction (rē-FRAK-shun) (Figure 16.12a). As light rays enter the eye, they are refracted at the anterior and posterior surfaces of the cornea. Both surfaces of the lens of the eye further refract the light rays so they come into exact focus on the retina. -Images focused on the retina are inverted, or upside-down (Figure 16.12b, c). They also undergo right-to-left reversal; that is, light from the right side of an object strikes the left side of the retina, and vice versa. The reason the world does not look inverted and reversed is that the brain "learns" early in life to coordinate visual images with the orientations of objects. The brain stores the inverted and reversed images we acquired when we first reached for and touched objects and interprets those visual images as being correctly oriented in space. About 75 percent of the total refraction of light occurs at the cornea. The lens provides the remaining 25 percent of focusing power and also changes the focus to view near or distant objects. When an object is 6 m (20 ft) or more away from the viewer, the light rays reflected from the object are nearly parallel to one another (Figure 16.12b). The lens must bend these parallel rays just enough so that they fall exactly focused on the fovea centralis, where vision is sharpest. Because light rays that are reflected from objects closer than 6 m (20 ft) are divergent rather than parallel (Figure 16.12c), the rays must be refracted more if they are to be focused on the retina. This additional refraction is accomplished through the process called accommodation.

Cataracts

A cataract is a clouding of the lens of the eye which leads to a decrease in vision. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colors, blurry or double vision, halos around light, trouble with bright lights, and trouble seeing at night.

Deattached retina

A detached retina may occur due to trauma, such as a blow to the head, in various eye disorders, or as a result of age-related degeneration. The detachment occurs between the neural portion of the retina and the pigment epithelium. Fluid accumulates between these layers, forcing the thin, pliable retina to billow outward. The result is distorted vision and blindness in the corresponding field of vision. The retina may be reattached by laser surgery or cryosurgery (localized application of extreme cold). Reattachment must be accomplished quickly to avoid permanent damage to the retina

circumvallate papille

About 12 very large, circular vallate papillae (VAL-āt = wall-like) or circumvallate papillae form an inverted V-shaped row at the back of the tongue. Each of these papillae houses 100-300 taste buds.

Cohlear sac (Inner ear)

Anterior to the vestibule is the cochlea (KOK-lē-a = snail-shaped), a bony spiral canal (Figure 16.21a) that resembles a snail's shell and makes almost three turns around a central bony hub. Sections through the cochlea reveal that it is divided into three channels: cochlear duct, scala vestibuli, and scala tympani (Figure 16.21a-c). The cochlear duct (KOK-lē-ar) is a continuation of the membranous labyrinth into the cochlea; it is filled with endolymph. The channel above the cochlear duct is the scala vestibuli, which ends at the oval window. The channel below is the scala tympani, which ends at the round window. Both the scala vestibuli and scala tympani are part of the bony labyrinth of the cochlea; therefore, these chambers are filled with perilymph. The scala vestibuli and scala tympani are completely separated by the cochlear duct, except for an opening at the apex of the cochlea, the helicotrema (hel-i-kō-TRĒ-ma; see Figure 16.22 ). The cochlea adjoins the wall of the vestibule, into which the scala vestibuli opens. The perilymph in the vestibule is continuous with that of the scala vestibuli.

Stapediues (middle ear)

Besides the ligaments, two tiny skeletal muscles also attach to the auditory ossicles. The tensor tympani (TIM-pan-ē) limits movement and increases tension on the tympanic membrane to prevent damage to the internal ear from loud noises. The stapedius (sta-PĒ-de-us) is the smallest skeletal muscle in the human body. By dampening large vibrations of the stapes due to loud noises, it protects the oval window. Because it takes a fraction of a second for the tensor tympani and stapedius muscles to contract, they help protect the internal ear from prolonged loud noises, but not from sudden, brief ones such as a gunshot.

Pysiology of gustation

Chemicals that stimulate gustatory receptor cells are known as tastants. Once a tastant is dissolved in saliva, it can make contact with the plasma membrane of the gustatory hairs, which are the sites on gustatory receptor cells that respond to tastants. The result is a receptor potential that stimulates release of neurotransmitter molecules from the gustatory receptor cell. In turn, the liberated neurotransmitter molecules trigger action potentials in the first-order neurons that synapse with gustatory receptor cells. The receptor potential arises differently for different tastants. The sodium ions (Na+) in a salty food enter gustatory receptor cells via Na+ channels in the plasma membrane. The accumulation of Na+ inside the receptor cell causes depolarization, which leads to release of neurotransmitter. The hydrogen ions (H+) in sour tastants may flow into gustatory receptor cells via H+ channels, and produce a depolarization that leads to release of neurotransmitter. Other tastants, responsible for stimulating sweet, bitter, and umami tastes, do not themselves enter gustatory receptor cells. Rather, they bind to receptor proteins on the plasma membrane that activate several different chemicals inside the gustatory receptor cell. Again, the result is the same—release of neurotransmitter. If all tastants cause release of neurotransmitter from many gustatory receptor cells, why do foods taste different? The answer to this question is thought to lie in the patterns of impulses in groups of first-order gustatory neurons that synapse with the gustatory receptor cells. Different tastes elicit activation of different groups of gustatory neurons. In addition, although each individual gustatory receptor cell responds to more than one of the five primary tastes, it may respond more strongly to some tastants than to others.

Conjucrtivis

Conjunctivitis, inflammation of the conjunctiva, is caused by bacteria such as pneumococci, staphylococci, or Hemophilus influenzae. It is very contagious and more common in children. Conjunctivitis may also be caused by irritants such as dust, smoke, or pollutants in the air, in which case it is not contagious.

Cornea transplant

During a corneal transplant, a defective cornea is removed and a donor cornea of similar diameter is sewn in. It is the most common and most successful transplant operation. Since the cornea is avascular, antibodies in the blood that might cause rejection do not enter the transplanted tissue, and rejection rarely occurs. The shortage of donor corneas has been partially overcome by the development of artificial corneas made of plastic.

Dynamic Equilibrium

Dynamic equilibrium is the maintenance of body position (mainly the head) in response to rotational (turning) acceleration or deceleration, such as when you turn your head or spin your body around while dancing. Collectively, the receptor organs for equilibrium are called the vestibular apparatus (ves-TIB-ū-lar); these include the saccule, utricle, and semicircular ducts

Gustatore decription

Each taste bud is an oval body consisting of three kinds of epithelial cells: supporting cells, gustatory receptor cells, and basal cells (Figure 16.4c, d). The supporting cells surround the gustatory receptor cells (GUS-ta-tō-rē). A single, long microvillus, called a gustatory hair, projects from each gustatory receptor cell to the external surface through the taste pore, an opening in the taste bud. Each gustatory receptor cell has a life span of about 10 days. Basal cells, stem cells found at the periphery of the taste bud near the connective tissue layer, produce supporting cells, which then develop into gustatory receptor cells. At their base, the gustatory receptor cells synapse with dendrites of the first-order neurons that begin the gustatory pathway.

Loud sound and hair cell damage

Exposure to loud music and the engine roar of jet planes, revved-up motorcycles, lawn mowers, and vacuum cleaners damages hair cells of the cochlea. Because prolonged noise exposure causes hearing loss, employers in the United States must require workers to use hearing protectors when occupational noise levels exceed 90 dB. Rock concerts and even inexpensive headphones can easily produce sounds over 110 dB. Continued exposure to high-intensity sounds is one cause of deafness, a significant or total hearing loss. The louder the sounds, the more rapid is the hearing loss. Deafness usually begins with loss of sensitivity for high-pitched sounds. If you are listening to music through headphones and bystanders can hear it, the dB level is in the damaging range. Most people fail to notice their progressive hearing loss until destruction is extensive and they begin having difficulty understanding speech. Wearing earplugs with a noise-reduction rating of 30 dB while engaging in noisy activities can protect the sensitivity of your ears.

Types of papillae

Filiform, Vallate, Fungiform, Foliate

Foliate papillae

Foliate papillae (FŌ-lē-āt = leaflike) are located in small trenches on the lateral margins of the tongue, but most of their taste buds degenerate in early childhood.

Describing flow of visual impulses from photoreceptors to optic nerve

From photoreceptors, visual information flows to bipolar cells through the outer synaptic layer, to bipolar cells, and then through the inner synaptic layer to ganglion cells. The axons of ganglion cells extend posteriorly to the optic disc and exit the eyeball as the optic (II) nerve. The optic disc is also called the blind spot. Because it contains no rods or cones, we cannot see an image that strikes the blind spot. Normally, you are not aware of having a blind spot, but you can easily demonstrate its presence. Hold this page about 20 in. from your face with the cross shown below directly in front of your right eye. You should be able to see the cross and the square when you close your left eye. Now, keeping the left eye closed, slowly bring the page closer to your face while keeping the right eye on the cross. At a certain distance the square will disappear because its image falls on the blind spot.

Eyelid 2

From superficial to deep, each eyelid consists of skin overlying the orbicularis oculi, tarsal glands, and conjunctiva (Figure 16.7a). The orbicularis oculi muscle closes the eyelid. Embedded in each eyelid is a row of modified sebaceous glands, known as tarsal glands or meibomian glands (mī-BŌ-mē-an), which secrete a fluid that helps keep the eyelids from adhering to each other. The conjunctiva (kon′-junk-TĪ-va) is a thin, protective mucous membrane. The palpebral conjunctiva lines the inner aspect of each eyelid. The bulbar conjunctiva passes from the eyelids onto the anterior surface of the eyeball, where it covers the sclera (the "white" of the eye) but not the cornea, which is a transparent region that forms the outer anterior surface of the eyeball. Both the sclera and the cornea are discussed in more detail shortly. Dilation and congestion of the blood vessels of the bulbar conjunctiva due to local irritation or infection are the cause of bloodshot eyes.

fungiform papillae

Fungiform papillae (FUN-ji-form = mushroomlike) are mushroom-shaped elevations scattered over the entire surface of the tongue that contain about five taste buds each.

Glaucoma

Glaucoma is a group of eye conditions that damage the optic nerve, the health of which is vital for good vision. This damage is often caused by an abnormally high pressure in your eye. Glaucoma is one of the leading causes of blindness for people over the age of 60

Accomodation

Images focused on the retina are inverted, or upside-down (Figure 16.12b, c). They also undergo right-to-left reversal; that is, light from the right side of an object strikes the left side of the retina, and vice versa. The reason the world does not look inverted and reversed is that the brain "learns" early in life to coordinate visual images with the orientations of objects. The brain stores the inverted and reversed images we acquired when we first reached for and touched objects and interprets those visual images as being correctly oriented in space. About 75 percent of the total refraction of light occurs at the cornea. The lens provides the remaining 25 percent of focusing power and also changes the focus to view near or distant objects. When an object is 6 m (20 ft) or more away from the viewer, the light rays reflected from the object are nearly parallel to one another (Figure 16.12b). The lens must bend these parallel rays just enough so that they fall exactly focused on the fovea centralis, where vision is sharpest. Because light rays that are reflected from objects closer than 6 m (20 ft) are divergent rather than parallel (Figure 16.12c), the rays must be refracted more if they are to be focused on the retina. This additional refraction is accomplished through the process called accommodation.

Photopigment process

In darkness, retinal has a bent shape, called cis-retinal, which fits snugly into the opsin portion of the photopigment. When cis-retinal absorbs a photon of light, it straightens out to a shape called trans-retinal. This cis-to-trans conversion is called isomerization (ī-som′-er-i-ZĀ-shun) and is the first step in visual transduction. After retinal isomerizes, several chemical changes occur that generate a receptor potential. In about a minute, trans-retinal completely separates from opsin. The final products look colorless, so this part of the cycle is termed bleaching of photopigment. An enzyme called retinal isomerase converts trans-retinal back to cis-retinal. The cis-retinal then can bind to opsin, reforming a functional photopigment. This part of the cycle—resynthesis of a photopigment—is called regeneration.

Nature of sound waves

In order to understand the physiology of hearing, it is necessary to learn something about its input, which occurs in the form of sound waves. Sound waves are alternating high- and low-pressure regions traveling in the same direction through some medium (such as air). They originate from a vibrating object in much the same way that ripples arise and travel over the surface of a pond when you toss a stone into it. The number of sound waves that arrive in a given time is defined as frequency of a sound and is measured in hertz (Hz; 1 Hz = 1 cycle per second). We interpret different sound frequencies as differences in pitch. The higher the frequency of vibration, the higher is the pitch. The entire audible range of the human ear extends from 20 to 20,000 Hz. Sounds of speech primarily contain frequencies between 100 and 3000 Hz, and the "high C" sung by a soprano has a dominant frequency at 1048 Hz. The sounds from a jet plane several miles away range from 20 to 100 Hz. The larger the intensity (size or amplitude) of the vibration, the louder is the sound. Sound intensity is measured in units called decibels (dB). The hearing threshold—the point at which an average young adult can just distinguish sound from silence—is defined as 0 dB. Rustling leaves have a decibel level of 15; whispered speech, 30; normal conversation, 60; a vacuum cleaner, 75; shouting, 80; and a nearby motorcycle or jackhammer, 90. Sound becomes uncomfortable to a normal ear at about 120 dB, and painful above 140 dB.

Lacrimal (2)

Lacrimal fluid is a watery solution containing salts, some mucus, and lysozyme, a protective bactericidal enzyme. The fluid protects, cleans, lubricates, and moistens the eyeball. After being secreted by the lacrimal gland, lacrimal fluid is spread medially over the surface of the eyeball by the blinking of the eyelids. Each gland produces about 1 mL of lacrimal fluid per day. Normally, tears are cleared away as fast as they are produced, either by evaporation or by passing into the lacrimal canals and then into the nasal cavity. If an irritating substance makes contact with the conjunctiva, however, the lacrimal glands are stimulated to oversecrete, and tears accumulate (watery eyes) as the tears dilute and wash away the irritating substance. Watery eyes also occur when an inflammation of the nasal mucosa, such as occurs with a cold, obstructs the nasolacrimal ducts and blocks drainage of tears. Only humans express emotions, both happiness and sadness, by crying. In response to parasympathetic stimulation, the lacrimal glands produce excessive lacrimal fluid that may spill over the edges of the eyelids and even fill the nasal cavity with fluid. This is how crying produces a runny nose.

Color Blindness and NIght blindness

Most forms of color blindness, an inherited inability to distinguish between certain colors, result from the absence or deficiency of one of the three types of cones. The most common type is red-green color blindness, in which red cones or green cones are missing. As a result, the person cannot distinguish between the colors red and green. Prolonged vitamin A deficiency and the resulting below-normal amount of rhodopsin in rods may cause night blindness or nyctalopia (nik′-ta-LŌ-pē-a), an inability to see well at low light levels.

Nerves responsible for eye

Oculomotor nerve (III),Trochlear (IV) nerve,(Abducens IV) nerve

Olfactory receptor cells

Olfactory receptor cells are the first-order neurons of the olfactory pathway. They are bipolar neurons with an exposed dendrite and an axon projecting through a foramen of the cribriform plate of the ethmoid bone and ending in the olfactory bulb. The parts of the olfactory receptor cell that respond to inhaled chemicals are the olfactory hairs, nonmotile cilia that project from the dendrite. Odorants are chemicals that can stimulate the olfactory hairs and, therefore, be detected as odors. Olfactory receptors respond to the chemical stimulation of an odorant molecule by producing a generator potential, thus initiating the olfactory response.

photoreceptors

Rods and cones were named for differences in the appearance of the outer segment, the distal end of the photoreceptor next to the pigmented layer. The outer segments of rods are cylindrical or rodshaped; those of cones are tapered or cone-shaped (Figure 16.14). Transduction of light energy into a receptor potential occurs in the outer segment of the photoreceptor. The photopigments are proteins in the plasma membrane of the outer segment. In cones the plasma membrane is folded back and forth in a pleated fashion, but in rods the pleats pinch off from the plasma membrane to form discs. The outer segment of each rod contains a stack of about 1000 discs, piled up like coins inside a wrapper. Photoreceptor outer segments are renewed at an astonishingly rapid pace. In rods, one to three new discs are added to the base of the outer segment every hour while old discs slough off at the tip and are phagocytized by epithelial cells of the pigmented layer of the retina.

Supporting cells

Supporting cells are columnar epithelial cells of the mucous membrane lining the nose. They provide physical support, nourishment, and electrical insulation for the olfactory receptor cells, and they help detoxify chemicals that come in contact with the olfactory epithelium. Basal cells are stem cells that lie between the bases of the supporting cells and continually undergo cell division to produce new olfactory receptor cells, which live for only a month or so before being replaced. This process is remarkable considering that olfactory receptors are neurons, and, as you have already learned, mature neurons are generally not replaced.

Gustatation

Taste or gustation (gus-TĀ-shun; gust- = taste), like olfaction, is a chemical sense. However, taste is much simpler than olfaction in that only five primary tastes are considered distinguishable: sour, sweet, bitter, salty, and umami (ū-MAM-ē). The umami taste, more recently discovered than the other tastes, was first reported by Japanese scientists and is described as "meaty" or "savory." Umami is believed to arise from taste receptors that are stimulated by monosodium glutamate (MSG), a substance naturally present in many foods and added to others as a flavor enhancer. All other flavors, such as chocolate, pepper, and coffee, are combinations of the five primary tastes, plus accompanying olfactory and tactile (touch) sensations. Odors from food can pass upward from the mouth into the nasal cavity, where they stimulate olfactory receptor cells. Because olfaction is much more sensitive than taste, a given concentration of a food substance may stimulate the olfactory system thousands of times more strongly than it stimulates the gustatory system. When you have a cold or are suffering from allergies and cannot taste your food, it is actually olfaction that is blocked, not taste.

Stimulation of gnaglion cells and photoreceptors

The absorption of light and isomerization of retinal initiates chemical changes in the photoreceptor that lead to production of a receptor potential. To understand how the receptor potential arises, however, we first need to examine the operation of photoreceptors in the absence of light. In darkness, sodium ions (Na+) flow into photoreceptor outer segments through Na+ channels that are held open by cyclic GMP (guanosine monophosphate) or cGMP (Figure 16.16a). The inflow of Na+ partially depolarizes the photoreceptor. As a result, in darkness the membrane potential of a photoreceptor is about −30 mV. This is much closer to zero than a typical neuron's resting membrane potential of −70 mV. The depolarization during darkness triggers continual release of the neurotransmitter glutamate at the synaptic terminals. At synapses between photoreceptors and bipolar cells, glutamate is an inhibitory neurotransmitter. Glutamate triggers inhibitory postsynaptic potentials (IPSPs) that hyperpolarize the bipolar cells and prevent them from sending signals to the ganglion cells.

Pysiology of hearing

The auricle directs sound waves into the external auditory canal. When sound waves strike the tympanic membrane, the alternating high and low pressure of the air causes the tympanic membrane to vibrate back and forth. The tympanic membrane vibrates slowly in response to low-frequency (low-pitched) sounds and rapidly in response to high-frequency (high-pitched) sounds. It vibrates more forcefully in response to higher intensity (louder) sounds, more gently in response to lower intensity (quieter) sounds. The central area of the tympanic membrane connects to the malleus, which also starts to vibrate. The vibration is transmitted from the malleus to the incus, and then to the stapes. As the stapes moves back and forth, its oval-shaped footplate vibrates in the oval window. The vibrations at the oval window are about 20 times more vigorous than the vibrations at the tympanic membrane because the auditory ossicles efficiently transmit small vibrations, spread over a large surface area (tympanic membrane), into larger vibrations of a smaller surface (oval window). The vibrations of the stapes at the oval window sets up fluid pressure waves in the cochlea by pushing on the perilymph of the scala vestibuli. Pressure waves are transmitted from the scala vestibuli to the scala tympani and eventually to the round window, where the pressure waves are finally absorbed as the flexible membrane of the round window bulges outward into the middle ear. (See in the figure.) The pressure waves travel through the perilymph of the scala vestibuli, then the vestibular membrane, and then move into the endolymph inside the cochlear duct. The pressure waves in the endolymph cause the basilar membrane to vibrate, which moves the hair cells of the spiral organ against the tectorial membrane. This leads to bending of the hair cell microvilli, which produces receptor potentials that ultimately lead to the generation of impulses. Sound waves of various frequencies cause certain regions of the basilar membrane to vibrate more intensely than other regions. Each segment of the basilar membrane is "tuned" for a particular pitch. Because the membrane is narrower and stiffer at the base of the cochlea (portion closer to the oval window), high-frequency (high-pitched) sounds near 20,000 Hz induce maximal vibrations in this region. Toward the apex of the cochlea near the helicotrema, the basilar membrane is wider and more flexible; low-frequency (lowpitched) sounds near 20 Hz cause maximal vibration of the basilar membrane there. As noted previously, loudness is determined by the intensity of sound waves. High-intensity sound waves cause larger vibrations of the basilar membrane, which leads to a higher frequency of impulses reaching the brain. Louder sounds also may stimulate a larger number of hair cells.

External Ear

The external ear consists of the auricle, external auditory canal, and tympanic membrane. The auricle (AW-ri-kul) is a flap of elastic cartilage shaped like the flared end of a trumpet and covered by skin. Ligaments and muscles attach the auricle to the head. The external auditory canal (audit- = hearing) is a curved tube about 2.5 cm (1 in.) long that lies in the temporal bone and leads from the auricle to the tympanic membrane. The tympanic membrane (tim-PAN-ik; tympan- = a drum), or eardrum, is a thin, semitransparent partition between the external auditory canal and middle ear. The tympanic membrane is covered by skin on the side facing the external auditory canal and mucus membrane on the internal surface. Tearing of the tympanic membrane is called a perforated eardrum. It may be due to pressure from a cotton swab, trauma, or a middle ear infection. A perforated eardrum usually heals within a month. Near the exterior opening, the external auditory canal contains a few hairs and specialized sweat glands called ceruminous glands (se-ROO-mi-nus) that secrete earwax or cerumen (se-ROO-men). The combination of hairs and cerumen helps prevent dust and foreign objects from entering the ear. Cerumen also prevents damage to the delicate skin of the external ear canal by water and insects. Cerumen usually dries up and falls out of the ear canal. However, some people produce a large amount of cerumen, which can become impacted and can muffle incoming sounds. The treatment for impacted cerumen is usually periodic ear irrigation or removal of earwax by trained medical personnel.

Eyelashes and eyebrows

The eyelashes project from the border of each eyelid, and the eyebrows arch transversely above the upper eyelids. Eyelashes and eyebrows help protect the eyeballs from foreign objects, perspiration, and the direct rays of the sun.

Photo pigment

The inner segment of a rod or cone contains the cell nucleus, Golgi complex, and mitochondria. At its proximal end, the photoreceptor expands into bulblike synaptic terminals filled with synaptic vesicles. The first step in visual transduction is absorption of light by a photopigment, a colored protein that undergoes structural changes when it absorbs light, in the outer segment of a photoreceptor. Light absorption initiates the events that lead to the production of a receptor potential. The photopigment in rods is rhodopsin (rō-DOP-sin; rhod- = rose; -opsin = related to vision). Three different cone photopigments are present in the retina, one in each of the three types of cones. Color vision results from different colors of light selectively activating the different cone photopigments. Photopigments contain two parts: a glycoprotein known as opsin and a derivative of vitamin A called retinal. Vitamin A derivatives are formed from carotene, the plant pigment that gives carrots their orange color. Good vision depends on adequate dietary intake of carotene-rich vegetables such as carrots, spinach, broccoli, and yellow squash, or other foods that contain vitamin A, such as liver. Retinal is the light-absorbing part of photopigments. In the human retina, there are four different opsins, one in each type of cone and one in the rods (rhodopsin). Small molecular variations of the different opsins permit the rods and cones to absorb different colors (wavelengths) of incoming light.

Internal Ear

The internal ear is a complicated series of canals (Figure 16.20). Structurally, it consists of two main divisions: an outer bony labyrinth that encloses an inner membranous labyrinth. The bony labyrinth is a series of cavities in the petrous portion of the temporal bone divided into three areas: (1) the semicircular canals and (2) the vestibule, both of which contain receptors for equilibrium, and (3) the cochlea, which contains receptors for hearing. The bony labyrinth contains perilymph. This fluid, which is chemically similar to cerebrospinal fluid, surrounds the membranous labyrinth, a series of sacs and tubes inside the bony labyrinth and having the same general form as the bony labyrinth. The membranous labyrinth contains endolymph.

How iris regulatesp upil size

The iris regulates the amount of light entering the eyeball through the pupil, the hole in the center of the iris. The pupil appears black because, as you look through the lens, you see the heavily pigmented back of the eye (the choroid). However, if bright light is directed into the pupil, the reflected light is red because of the blood vessels on the surface of the retina. It is for this reason that a person's eyes sometimes appear red in a photograph when the flash is directed into the pupil. Autonomic reflexes regulate pupil diameter in response to light levels (Figure 16.9). When bright light stimulates the eye, parasympathetic neurons stimulate the circular muscle of the iris to contract, causing a decrease in the size of the pupil (constriction). In dim light, sympathetic neurons stimulate the radial muscle of the iris to contract, causing an increase in the pupil's size (dilation).

Maccule

The maculae contain hair cells, sensory receptors that are surrounded by columnar supporting cells. Hair cells have on their surface hair bundles that consist of 40-80 microvilli. The hair bundles of the hair cells project into a thick, gelatinous layer called the otolithic membrane (ō-tō-LITH-ik). Calcium carbonate crystals, called otoliths (Ō-tō-liths; oto- = ear; -liths = stones), extend over the surface of the otolithic membrane. The densely packed otoliths add weight to the otolithic membrane, amplifying the pull of gravity during movements. Because the otolithic membrane sits on top of the macula, when you tilt your head forward, the otolithic membrane and the otoliths are pulled by gravity and slide "downhill" over the hair cells in the direction of the tilt, bending the hair bundles. The movement of the hair bundles produces receptor potentials, releasing neurotransmitter that stimulates impulses in sensory neurons in the vestibular branch of the vestibulocochlear (VIII) nerve (see Figure 16.21b

Equilibirum Pathway

The medulla oblongata and pons integrate information arriving from the utricle, saccule, semicircular ducts, eyes, and somatic receptors, especially proprioceptors in the neck muscles that indicate the position of the head, and then send commands to the following: The oculomotor (III), trochlear (IV), and abducens (VI) nerves. Control movements of the eyes coupled with those of the head to help maintain focus on the visual field. The accessory (XI) nerves. Help control head and neck movements to assist in maintaining equilibrium. The vestibulospinal tract. Conveys impulses down the spinal cord to maintain muscle tone in skeletal muscles to help maintain equilibrium. The thalamus and the vestibular area. The thalamus conveys impulses to the vestibular area, part of the primary somatosensory area in the parietal lobe of the cerebral cortex (see Figure 13.10 ), which provides us with the conscious awareness of the position and movements of the head

Retina

The neural layer is a multilayered outgrowth of the brain that processes visual data before sending impulses into axons that form the optic (II) nerve. The neural layer contains three distinct layers of retinal neurons—photoreceptors, bipolar cells, and ganglion cells—separated by two zones, the outer and inner synaptic layers, where synaptic contacts are made (Figure 16.10a, c). Note that light passes through the ganglion and bipolar cell layers before it reaches the photoreceptors. Photoreceptors are specialized cells that begin the process of converting light rays to impulses. There are two types of photoreceptors: rods and cones. Each retina has about 6 million cones and 120 million rods. Rods allow us to see in dim light, such as moonlight. Because rods do not provide color vision, in dim light we see only black, white, and shades of gray. Brighter lights stimulate the cones, which produce color vision. Three types of cones are present in the retina: (1) blue cones, which are sensitive to blue light, (2) green cones, which are sensitive to green light, and (3) red cones, which are sensitive to red light. Color vision results from the stimulation of various combinations of these three types of cones. Most of our visual experiences are mediated by the cone system, the loss of which produces legal blindness. In contrast, a person who loses rod vision mainly has difficulty seeing in dim light and thus should not drive at night.

Refraction and abnormalites

The normal eye, known as an emmetropic eye (em′-e-TROP-ik), can sufficiently refract light rays from an object 6 m (20 ft) away so that a clear image is focused on the retina (Figure 16.13a). Many people, however, lack this ability because of refraction abnormalities. Among these abnormalities are myopia (mī-Ō-pē-a), or nearsightedness, which occurs when the eyeball is too long relative to the focusing power of the cornea and lens, or when the lens is thicker than normal, so an image converges in front of the retina. Myopic individuals can see close objects clearly, but not distant objects. In hyperopia (hī-per-Ō-pē-a), or farsightedness, the eyeball length is short relative to the focusing power of the cornea and lens, or the lens is thinner than normal, so an image converges behind the retina. Hyperopic individuals can see distant objects clearly, but not close ones. Figure 16.13 illustrates these conditions and explains how they are corrected. Another refraction abnormality is astigmatism (a-STIG-ma-tizm), in which either the cornea or the lens has an irregular curvature. As a result, parts of the image are out of focus and vision is blurred or distorted.

Semicircular ducts

The three semicircular ducts function in dynamic equilibrium. The ducts lie at right angles to one another in three planes of space (Figure 16.25): The two vertical ducts are the anterior and posterior semicircular ducts, and the horizontal one is the lateral semicircular duct. This positioning permits detection of rotational acceleration or deceleration. In the ampulla, the dilated portion of each duct, is a small elevation called the crista (KRIS-ta = crest; plural is cristae). Each crista contains a group of hair cells and supporting cells. Covering the crista is a mass of gelatinous material called the cupula (KŪ-pū-la). Hair bundles project from the hair cells into the cupula. -When your head moves along the plane of one of the semicircular ducts, the enclosed endolymph moves through the semicircular duct, pushing on the cupula and bending the hair bundles of the hair cells. The three different planes of the three semicircular ducts in each ear are able to detect virtually any rotational movement of the head. Bending of the hair bundles produces receptor potentials. In turn, the receptor potentials lead to impulses that pass along the vestibular branch of the vestibulocochlear (VIII) nerve (see Figure 16.21b ).

Eyelids

The upper and lower eyelids, or palpebrae (PAL-pe-brē), shade the eyes during sleep, protect the eyes from excessive light and foreign objects, and spread lubricating secretions over the eyeballs (Figure 16.6). The upper eyelid is more movable than the lower and contains in its superior region the levator palpebrae superioris, which raises the upper eyelid (Figure 16.7a). The space between the upper and lower eyelids that exposes the eyeball is the palpebral fissure (PAL-pe-bral). Its angles are known as the lateral commissure (KOM-i-shur), which is closer to the temporal bone, and the medial commissure, which is more proximal to the nasal bone. In the medial commissure is a small, reddish elevation, the lacrimal caruncle (KAR-ung-kul), which contains sebaceous (oil) glands and sudoriferous (sweat) glands. The whitish material that sometimes collects in the medial commissure comes from these glands.

Vestiublar membrane (inner ear)

The vestibular membrane separates the cochlear duct from the scala vestibuli, and the basilar membrane (BĀS-i-lar) separates the cochlear duct from the scala tympani. Resting on the basilar membrane is the spiral organ (Figure 16.21c, d). The spiral organ is a coiled sheet of epithelial cells, including supporting cells and about 16,000 hair cells, which are the receptors for hearing. At the apical tip of each hair cell is a hair bundle consisting of 40-80 long, hairlike microvilli that extend into the endolymph of the cochlear duct and are arranged in several rows of graded height. At their basal ends, hair cells synapse with sensory neurons from the cochlear branch of the vestibulocochlear (VIII) nerve. Cell bodies of the sensory neurons are located in the spiral ganglion (Figure 16.21b, c). The tectorial membrane (tek-TŌ-rē-al; tector- = covering), a flexible gelatinous membrane, projects over and comes in contact with the hair cells of the spiral organ (Figure 16.21d).

Vestibule (inner ear)

The vestibule (VES-ti-būl) is the oval central portion of the bony labyrinth. The membranous labyrinth in the vestibule consists of two sacs called the utricle (Ū-tri-kul = little bag) and the saccule (SAK-ūl = little sac), which are connected by a small duct. Projecting superiorly and posteriorly from the vestibule are the three bony semicircular canals, each of which lies at approximately right angles to the other two. Based on their positions, they are named the anterior, posterior, and lateral semicircular canals. The anterior and posterior semicircular canals are vertically oriented; the lateral canal is horizontally oriented. At one end of each canal is a swollen enlargement called the ampulla (am-PUL-la = saclike duct). The portions of the membranous labyrinth that lie inside the bony semicircular canals are called the semicircular ducts. These structures connect with the utricle of the vestibule. The vestibular branch (ves-TIB-ū-lar) of the vestibulocochlear (VIII) nerve transmits impulses for equilibrium from the vestibule and semicircular canals. Cell bodies of the sensory neurons are located in the vestibular ganglia (see Figure 16.21b).

Light and Dark adaptation

When you emerge from dark surroundings (say, a tunnel) into the sunshine, light adaptation occurs—your visual system adjusts in seconds to the brighter environment by decreasing its sensitivity. On the other hand, when you enter a darkened room such as a movie theater, your visual system undergoes dark adaptation—its sensitivity increases slowly over many minutes. As the light level increases, more and more photopigment is bleached, assisting light adaptation. While light is bleaching some photopigment molecules, however, others are being regenerated. In daylight, regeneration of rhodopsin cannot keep up with the bleaching process, so rods contribute little to daylight vision. In contrast, cone photopigments regenerate rapidly enough that some of the cis form is always present, even in very bright light. If the light level decreases abruptly, sensitivity increases. In complete darkness, full regeneration of cone photopigments occurs during the first 8 minutes of dark adaptation. Rhodopsin regenerates more slowly, and our visual sensitivity increases until even a single photon (the smallest unit of light) can be detected. In that situation, barely perceptible light appears gray-white, regardless of its color. At very low light levels, such as starlight, objects appear as shades of gray because only the rods are functioning.

Olfactory glands

Within the connective tissue that supports the olfactory epithelium are olfactory glands, which produce mucus that is carried to the surface of the epithelium by ducts. The secretion moistens the surface of the olfactory epithelium and dissolves odorants, allowing them to interact with olfactory hairs. Both supporting cells of the nasal epithelium and olfactory glands are innervated by branches of the facial (VII) nerve, which can be stimulated by certain chemicals. Impulses in the facial (VII) nerve in turn stimulate the lacrimal glands in the eyes and nasal mucous glands. The result is tears and a runny nose after inhaling substances such as pepper, onion, or the vapors of household ammonia.

Physiology of olfaction

any attempts have been made to distinguish among and classify "primary" sensations of smell. Genetic evidence now suggests the existence of hundreds of primary odors. Our ability to recognize about 10,000 different odors probably depends on patterns of activity in the brain that arise from activation of many different combinations of olfactory receptor cells. Olfactory receptor cells react to odorant molecules in the same way that most sensory receptors react to their specific stimuli: A depolarizing generator potential develops and triggers one or more action potentials. How the generator potential arises in olfactory receptors is known in some cases. Many odorants bind to receptor proteins, called odorant binding proteins, on the plasma membrane and activate the enzyme adenylate cyclase, which synthesizes cyclic adenosine monophosphate (cAMP).

Olfactory transduction

cBinding of the odorant molecule to the plasma protein receptor activates adenylate cyclase, resulting in the production of cyclic AMP. Cyclic AMP (cAMP) opens sodium channels and Na+ enters the olfactory receptor cell. The resulting depolarization may generate an action potential, which is transmitted to the axon terminals. Odorants can produce depolarizing generator potentials, which can lead to action potentials.

Lacrimal appartus

he lacrimal apparatus (LAK-ri-mal; lacrim- = tears) is a group of structures that produces and drains lacrimal fluid, or tears. The lacrimal glands, each about the size and shape of an almond, secrete lacrimal fluid, which drains into lacrimal ducts that empty tears onto the surface of the conjunctiva of the upper lid (Figure 16.7b). From there, the tears pass medially over the anterior surface of the eye to enter two small openings called lacrimal puncta (singular is lacrimal punctum). Tears then pass into two ducts, the lacrimal canals, which lead into the lacrimal sac. As tears fill the lacrimal sac, they are pumped into the nasolacrimal duct by the blinking muscular action of the orbicularis oculi (see Figure 11.3 ). The nasolacrimal duct carries tears into the nasal cavity.

MIddle ear (1)

he middle ear is a small, air-filled cavity in the petrous portion of the temporal bone (Figure 16.19). It is separated from the external ear by the tympanic membrane and from the internal ear by a thin bony partition that contains two small membrane-covered openings: the oval window and the round window. Extending across the middle ear and attached to it by ligaments are the three smallest bones in the body, the auditory ossicles (OS-si-kuls), which are connected by synovial joints. The bones, named for their shapes, are the malleus, incus, and stapes—commonly called the hammer, anvil, and stirrup, respectively. The auditory ossicles are connected to one another by synovial joints. The "handle" of the malleus (MAL-ē-us) attaches to the internal surface of the tympanic membrane. The head of the malleus articulates with the body of the incus. The incus (ING-kus), the middle bone in the series, articulates with the head of the stapes. The base or footplate of the stapes (STĀ-pēz) fits into the oval window. Directly below the oval window is another membrane-covered opening, the round window.

Sacuule

he walls of both the utricle and the saccule contain a small, thickened region called a macula (MAK-ū-la; Figure 16.24). The two maculae (plural; MAK-ū-lē), which are perpendicular to one another, are the receptors for static equilibrium. They provide sensory information on the position of the head in space, essential to maintaining appropriate posture and balance. The maculae also detect linear acceleration and deceleration—for example, the sensations you feel while in a car that is speeding up or slowing down.