Chapter 14: SENSORY SYSTEMS

14 sensory SYSTEMS Chapter Outline SENSORY receptors and pathways SOMATIC Sensations Receptors near the body surface sense touch, pressure, and more Pain is the perception of bodily injury Referred pain is a matter of perception Taste and Smell: Chemical senses Gustation is the sense of taste Olfaction is the sense of smell Tasty Science Hearing: detecting sound waves The ear gathers “sound signals” Sensory hair cells are the key to hearing Balance: sensing the body’s natural position DISORDERS OF the Ear vision: an overview The eye is built to detect light Eye muscle movements fine-tune the focus from visual signals to “sight” Rods and cones are the photoreceptors Visual pigments intercept light energy The retina begins processing visual signals Signals move on to the visual cortex disorders of The Eye Some eye disorders are inherited The eyes also are vulnerable to infections and cancer Aging increases the risk of some types of eye disorders Medical technologies can remedy some vision problems and treat eye injuries SUMMARY Review questions self-quiz critical thinking explore on your own your future Objectives Describe the characteristics of a receptor and list the various types of receptors. Contrast mechanisms by which the chemical and the somatic senses work. Understand how the senses of balance and hearing function. Describe how the sense of vision functions. Draw a medical section of the human eyeball through the optic nerve, identify each structure, and tell the function of each. Identify some common disorders of the eye. Key Terms stimulus sensation perception mechanoreceptors thermoreceptors nociceptors chemoreceptors osmoreceptors photoreceptors sensory adaptation somatic sensations somatosensory cortex free nerve endings encapsulated receptors chemical senses taste receptors olfactory receptors cochlea tympanic membrane organ of Corti hair cells tectorial membrane vestibular apparatus semicircular canals otitis media tinnitus deafness vision eyes cornea iris lens retina visual cortex accommodation rod cells cone cells fovea red-green color blindness astigmatism myopia (nearsightedness) hyperopia (farsightedness) conjunctivitis herpes infection malignant melanoma retinoblastoma cataracts macular degeneration glaucoma retinal detachment Lecture Outline Iris scanning is one of the newest security techniques. First, each person’s unique arrangement of smooth muscle fibers in the iris of the eye must be recorded in an electronic database. Each time the person passes through a check point, a small camera looks at the iris and compares it with the database. Usually, we use our eyes to see, but in this new technology, our eyes are seen. Sensory Receptors and Pathways In a sensory system, a stimulus activates a receptor, which transduces (converts) it to an action potential that travels to the brain where it triggers sensation or perception. A stimulus is any form of energy that activates receptor endings of a sensory neuron. Sensations are conscious responses to the stimuli. Perception is an understanding of what sensations mean. There are six major categories of sensory receptors. Mechanoreceptors detect changes in pressure, position, or acceleration. Thermoreceptors detect heat or cold. Nociceptors (pain receptors) detect tissue damage. Chemoreceptors detect ions or molecules. Osmoreceptors detect changes in water volume (solute concentration) in surrounding fluid. Photoreceptors detect the energy of visible light. All action potentials are the same; the brain determines the nature of a given stimulus based on which nerves are signaling, the frequency of the action potentials generated, and the number of axons responding. Specific sensory areas interpret action potentials in specific ways. Strong signals make receptors fire action potentials more often and for longer. Stronger stimuli recruit more sensory receptors. Sensory adaptation is the diminishing response to an ongoing stimulus. Somatic Sensations Somatic sensations occur when receptor signals from body surfaces reach the somatosensory cortex in the cerebrum. Receptors near the body surface sense touch, pressure, and more. Sensations of touch, pressure, cold, warmth, and pain are discerned near the body surface by receptors whose numbers vary by body region. Free nerve endings are the simplest receptors. These are thinly myelinated or unmyelinated dendrites of sensory neurons. One type coils around hair follicles to detect movement; another detects chemicals. Encapsulated receptors are surrounded by a capsule of epithelial or connective tissue. Merkel’s discs adapt slowly and are important for steady touch. Meissner’s corpuscles respond to light touching. Ruffini endings are sensitive to steady touch and pressure. The Pacinian corpuscles are sensitive to deep pressure and vibrations. Mechanoreceptors in skeletal muscle, joints, tendons, ligaments, and skin are responsible for awareness of the body’s position and of its limb movements. Pain is the perception of bodily injury. Pain is the perception of injury to some region of the body. Nociceptors are subpopulations of free nerve endings distributed throughout the skin (somatic pain) and internal tissues (visceral pain). When cells are damaged, they release chemicals (bradykinins, histamine, and prostaglandins) to activate neighboring pain receptors. Pain receptors signal interneurons, which release substance P. Substance P allows for natural opiates called endorphins and enkephalins to be released to reduce pain perception. Referred pain is a matter of perception. Much visceral pain is referred pain; that is, it is felt at some distance from the real stimulation point. Phantom pain is the sensation that amputees feel when they sense the missing part as if it were still there. Taste and Smell: Chemical Senses Taste and smell are chemical senses; they begin at chemoreceptors, the signals traveling to the brain where they are perceived, transmitted to the limbic system, and remembered. Gustation is the sense of taste. Sensory organs called taste buds hold the taste receptors. Receptors are located on the tongue, roof of the mouth, and throat. The five general taste categories are sweet, sour, salty, bitter, and umami. The flavors of most foods are a combination of the five basic tastes plus sensory input from olfactory receptors in the nose. Olfaction is the sense of smell. Olfactory receptors in the olfactory epithelium of the nose detect water-soluble or volatile substances—odors. The interpretation of smell is done by the olfactory bulbs located in the frontal area of the brain. Olfaction is one of the most ancient senses, useful in survival as the receptors respond to molecules from food, mates, and predators, though this is less true for modern humans. Humans also have a vomeronasal organ whose receptors can detect pheromones, which are signaling molecules with roles in sexual attraction. Tasty Science Receptors in taste buds associate the five main taste categories with particular “tastant” molecules that the brain interprets depending on the action potentials that come its way. Each taste bud has receptors that can respond to tastants of at least two, if not all five, of the taste classes. Not all taste receptors, however, are equally sensitive; bitter receptors tend to be the most sensitive. This is valuable as many poisons have a bitter taste Various tastants commingle together with odors into what we perceive as flavors. Hearing: Detecting Sound Waves Sounds are waves of compressed air; the amplitude (loudness) and frequency (pitch) of sounds are detected by vibration-sensitive mechanoreceptors deep in the ear. The ear gathers “sound signals.” The outer ear collects sound waves and turns them into vibrations, which are amplified in the middle ear; vibrations are distinguished in the inner ear. Inner ear structures include semicircular canals, for balance, and the cochlea, where hearing takes place. Sensory hair cells are the key to hearing. Vibrations are passed from the tympanic membrane to the middle ear bones (malleus, incus, stapes) and on to the oval window, stretched across the entrance to the cochlea. Sound is amplified because the oval window is smaller than the tympanic membrane. The cochlea has two compartments in its outer chamber (the scala vestibuli and scala tympani), which curl around an inner cochlear duct; all are fluid filled. Vibrations of the oval window send pressure waves through the fluid to the basilar membrane on the floor of the cochlear duct; resting on the membrane is the organ of Corti, which includes sensory hair cells. The tips of the hair cells rest against the jellylike tectorial membrane; vibrations cause the hair cells to bend. Bending causes the release of neurotransmitters, triggering action potentials that travel to the brain. Loudness is determined by the total number of cells that become stimulated; tone or “pitch” depends on the frequency of vibration. The round window at the far end of the cochlea serves as a release valve for the pressure waves in the middle ear. The eustachian tube extending from the middle ear to the throat permits equalization of pressures. Balance: Sensing the Body’s Natural Position The sense of balance depends on messages from receptors in the eyes, skin, and joints, as well as organs of equilibrium in the inner ear. The vestibular apparatus is a closed system of fluid-filled sacs and semicircular canals inside the ear; the canals are arranged to represent the three planes of space. Rotational receptors are located at the base of each semicircular canal; sensory hair cells project into a jellylike cupula. Movement of the head causes the hairs to bend within the jelly, generating action potentials. Rotation of the head determines dynamic equilibrium. Static equilibrium, the head’s position in space, is monitored by two sacs in the vestibular apparatus, the utricle and saccule. The sacs contain the otolith organs (hair cells) and otoliths (ear stones), which detect changes in orientation as well as acceleration and deceleration. Action potentials from different parts of the vestibular apparatus travel to reflex centers in the brainstem. As signals are integrated, the brain orders compensatory movements necessary to maintain postural balance. Extreme motion or continuous overstimulation of the hair cells of the vestibular apparatus can result in motion sickness. Disorders of the Ear The hearing apparatus of the ears is sturdy, but it can be damaged by various illnesses and injuries. Otitis media, painful inflammation of the middle ear, often occurs in children following spread of a respiratory infection; pus and/or fluid buildup as a result can cause the eardrum to rupture. Tinnitus, or ringing or buzzing in the ears, can be triggered by infection, aspirin consumption, or other, unknown causes. Deafness is the partial or complete loss of hearing; deafness may be congenital or due to aging, disease, or environmental causation. The loudness of sounds is measured in decibels. Quiet conversation occurs at about 50 decibels. Damage begins when exposed to sounds between 75–85 decibels over extended periods. Rock concerts and shotgun blasts easily reach 130 decibels. Vision: An Overview Vision is an awareness of the position, shape, brightness, distance, and movement of visual stimuli as detected by the sensory organs, the eyes. The eye is built to detect light. The eye has three layers, sometimes called “tunics.” The outer layer consists of the sclera and transparent cornea. The middle layer consists of a choroid, ciliary body, and iris. The inner layer is the retina. The sclera (“white” of the eye) protects the eye; the dark-pigmented choroid underlies the sclera and prevents light from scattering. Most of the blood vessels lie in the choroid. Behind the cornea is the pigmented iris; the hole at the center of the iris is the pupil, the entrance for light, which can be adjusted depending on the level of light present. The lens is found behind the iris; the lens is attached to the ciliary body, a muscle functioning in the focusing of light. The lens focuses light onto a layer of photoreceptor cells in the retina. A clear fluid (aqueous humor) bathes both sides of the lens; vitreous humor fills the chamber behind the lens. The retina is a thin layer of neural tissue at the back of the eyeball; axons from some of the neurons converge to form the optic nerve, which sends signals to the visual cortex in the thalamus. The curved surface of the cornea bends incoming light so that light rays converge at the back of the eyeball; images appear “upside down and backwards” on the retina but are corrected in the brain. Eye muscle movements fine-tune the focus. Because of the bending of the light rays by the cornea, accommodation must be made by the lens so that the image is in focus on the retina. Accommodation is performed by the ciliary muscles attached to the lens. From Visual Signals to “Sight” Rods and cones are the photoreceptors. The retina’s basement layer is pigmented and is covered by photoreceptors called rod cells and cone cells. Rod cells are sensitive to dim light and detect changes in light intensity; cone cells respond to high-intensity light and contribute to sharp daytime vision. Visual pigments intercept light energy. Each rod contains more than a billion molecules of rhodopsin; this pigment can detect and respond to even a few photons of light, allowing us to see in dim light. Rhodopsin consists of a protein (opsin) and a signal molecule (retinal) that is derived from vitamin A. Photons of blue-green light stimulate rhodopsin to change shape; shape changes alter the distribution of ions across the rod cell membrane and slow down the release of an inhibitory neurotransmitter. Without the inhibitor, neurons send visual signals to the brain. Cone cells have different visual pigments (red, green, or blue); absorption of photons also prevents release of neurotransmitters, thus allowing signaling to the brain. Visual acuity is greatest in the fovea, a depression located at the center of the retina that is densely packed with photoreceptors. The retina begins processing visual signals. Signals flow from rods and cones to bipolar interneurons, and then to ganglion cells, the axons of which form the optic nerves. Before leaving the retina, signals are dampened or enhanced by horizontal cells and amacrine cells. Signals move on to the visual cortex. The brain converts the signals to an image. The visual field represents the part of the outside world a person actually sees. The right side of each retina gathers light from the left half of the visual field, and the left side gathers light from the right half of the field. The optic nerve from each eye sends signals from the left visual field to the right cerebral hemisphere, and signals from the right visual field to the left hemisphere. Axons of the optic nerves end in the lateral geniculate nucleus, from which they proceed to the brain’s visual cortex, which has several visual fields sensitive to direction, movement, color, and so on; here is where final interpretation of the signals is made to produce an organized sense of sight. Disorders of the Eye Normal eye function can be disrupted by disease, injury, inherited abnormalities, and aging. Some eye disorders are inherited. Red-green color blindness is the inability to distinguish red and green colors in dim light (and sometimes bright light) due to a lack of red and green cone cells. Total color blindness results when an individual has only one of the three kinds of cones. In astigmatism, one or both corneas have uneven curvature and cannot bend light to the same focal point. Nearsightedness (myopia) results when the image is focused in front of the retina. Farsightedness (hyperopia) is due to an image focused behind the retina. The eyes are also vulnerable to infections and cancer. Conjunctivitis, inflammation of the membrane lining the inside of the eyelids and covering the sclera, is among the most common reasons for doctor visits in the U.S. Herpes infection of the cornea results from infection with various herpes simplex viruses and can also lead to blindness. Malignant melanoma is eye cancer that develops in the choroid; retinoblastoma is cancer of the retina that occurs in infants. Retinoblastoma is a cancer of the retina that occurs in 1 in 20,000 babies. Aging increases the risk of some types of eye disorders. Cataracts, the gradual clouding of the lens associated with aging and diabetes, can completely block light from entering the eye. Macular degeneration is an age-related degeneration of the retina. Glaucoma results from excess of fluid in the eyeball, causing pressure on the retina. Medical technologies can remedy some vision problems and treat eye injuries. Many procedures can be used to correct eye disorders. Corneal transplant surgery can replace defective corneas with artificial plastic corneas or donor corneas; cataracts may be corrected in a similar fashion by replacing the lens. “Lasik” (laser-assisted in situ keratomileusis) or “lasek” (laser-assisted subepithelial keratectomy) surgeries can be used to correct severe nearsightedness. Conductive keratoplasty (CK) uses radio waves to reshape the cornea. Retinal detachment can result from a physical blow to the head; laser coagulation can be used to “reattach” the retina to the underlying choroid. Suggestions for Presenting the Material This chapter presents information that students will find more familiar. Most likely they have probably heard about sensory organs from a previous health or biology class. Assuming students do possess basic knowledge of the senses, there are two areas to emphasize. The first is to relate the sense receptor and its interpretation within the brain. This is the subject of the initial portion of the chapter. The second is to provide some depth to the students’ understanding of sensory receptor mechanisms. Two of the more difficult questions beginning students pose are: “How do I distinguish, say, sight from sound?” and “How do I perceive varying intensities of a stimulus?” Emphasize the role of the brain as an interpreter of impulses directed to specific regions by specialized receptors. Also point out that the frequency of action potentials and the number of axons that “fire” provide the quality we call “intensity” of stimulus. Each of the senses provides unique input. Try to draw distinctions between those that operate rather independently (for example, sight) and cooperatively (for example, taste and smell). As each sense is described, identify the structure of the sense organ, the mechanism of stimulus reception, and the interpretation of that stimulus. For example, the eye perceives light by reaction with chemicals on the retina to give the sensation of degrees of light and color. Students frequently have difficulty understanding how contraction of the ciliary muscles causes the suspensory ligaments to relax and the lens of the eye to thicken. It is the opposite of what they expect. Explain how the contraction of these muscles essentially lessens the circumference of the circle of processes to which the suspensory ligaments are attached. As a result the ligaments relax. Classroom and Laboratory Enrichment The simple detection of taste by a blindfolded person is still a student favorite. Ask volunteers to hold their nose and close their eyes while drops of various liquids (vinegar, onion, lemon, applesauce, and so on) are placed on the tongue (ask students ahead of time if they have any food allergies). Ask students to identify the substances, but not to respond until the experiment with the students has been repeated using both smell and taste receptors. If the lecture room or laboratory can be sufficiently darkened, reveal the abilities of rods and cones by a simple demonstration. Pull the shades and turn out the lights, quickly pull a red cloth from your pocket, and ask students to identify the color. Substitute other colors, change the light intensity, and wait for iris accommodation as variations in the protocol. One of the usual practices in optometrists’ offices is to take instant photos of the retina. Ask for permission to make copies of several photos, perhaps some exhibiting defects, and show them to the class. If the doctor will speak to the class, even better! A model of a cross-section through the organ of Corti is an excellent aid to comprehending this rather complex structure. Use dissectible models of the ear and eye to illustrate their structure. Use a Snellen chart to demonstrate the visual acuity test. If available, have the students use the Ishihara color charts for colorblindness. Demonstrate the use of the ophthalmoscope for viewing the retina. Demonstrate the use of the otoscope for viewing the tympanic membrane. Classroom Discussion Ideas With all the current security devices in use, why are technologists looking for new ones, such as iris scanning? Does the act of placing your eyes close to a scanning device seem more of an invasion of privacy than using your fingerprint or swiping your ID card? How could the iris scanning device be “fooled” by a person trying to impersonate another? Will blind persons have any difficulty using this device? Why is it that the tastiest foods seem bland and flat when eaten by a person with a bad head cold? It’s trivia quiz time; name the sense (or senses) that: Is most easily fatigued? Can be dulled by smoking? Cannot be shut out easily? Has more receptors in more places than any other? Uses small bones? Operates like a camera? Sometimes musicians are said to have a “trained ear.” What does this expression really mean? Why do many persons in their mid-forties and beyond need to use bifocal lenses? Is there any scientific basis to the slogan “carrots are good for your eyes”? What is “motion sickness”? How can it be controlled? In our “civilized” world, many people experience hearing loss as a result of aging, a condition called presbycusis. A study revealed that this condition did not exist in a primitive Sudanese tribe, the Mebans. This study suggests that some environmental factor of the civilized world is responsible for this type of deafness. What is that factor? Term Paper Topics, Library Activities, and Special Projects Scientists are gathering increasing amounts of evidence from laboratory studies and human testing that show gradual hearing loss caused by exposure to highly amplified music. Report on the dangers—are they real or imagined? One of the most intriguing subjects of sensory study is the phenomenon of “phantom” pain. Explore its manifestations. Based on library research, prepare an “awards” list for the animal group that exhibits the keenest of each of the five major senses (sight, hearing, smell, taste, touch). How do local anesthetics block the sensation of pain? Describe the various problems associated with vision that are correctable with lenses, surgery, drugs, or other means. How does a detector device that checks blood vessel patterns in the eye and compares them to known records provide a better security system for military installations than do fingerprints? Radial keratotomy, a surgical procedure for correcting myopia, is controversial. What does this procedure involve, and what are the pros and cons of its use? Investigate the differences between sensorineural deafness and conductive deafness. Videos, Animations, and Websites Videos Films for the Humanities and Sciences Human Senses _http://ffh.films.com/id/6826/Human_Senses.htm_ BBC – Human Senses Series Smell and Taste: _http://video.google.com/videoplay?docid=-4692469533667111036_ Hearing and Balance: _http://www.youtube.com/watch?v=6UOmVSNGTV8_ Touch and Vision: _http://www.cornel1801.com/bbc/HUMAN-SENSES/Touch-And-Vision.html_ University of Toronto: Accommodation of the Eye _http://library.med.utah.edu/kw/hyperbrain/movies/ch7/accommodation_sml.html_ Animations BBC Animation: Balance _http://www.bbc.co.uk/science/humanbody/body/factfiles/balance/balance_ani_f5.swf_ BBC Animation: Taste _http://www.bbc.co.uk/science/humanbody/body/factfiles/taste/taste_ani_f5.swf_ BBC Animation: Olfaction _http://www.bbc.co.uk/science/humanbody/body/factfiles/smell/smell_ani_f5.swf_ WISC-Online: Hearing _http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP14204_ Vision 101 _http://www.1800contacts.com/StaticContent/vision101/frames.html_ WISC-Online: Vision _http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP14304_ Websites HHMI – Seeing, Hearing, and Smelling the World Overview of the senses and how the brain processes information. _http://www.hhmi.org/senses/_ Human Senses – BBC Interactive games and information on human senses _http://www.bbc.co.uk/science/humanbody/tv/humansenses/_ Possible Responses to Review Questions When a receptor cell detects a specific stimulus, the energy from the stimulus is converted to an action potential that is sent to the brain; the brain responds with sensation and perception. The six categories of sensory receptors are listed in nice detail in Table 14.1. Briefly: mechanoreceptors detect mechanical energy (e.g., changes in pressure); thermoreceptors detect hot and cold; nociceptors detect pain; chemoreceptors detect chemical energy; osmoreceptors detect changes in water volume; and photoreceptors detect visible light. Somatic sensations come from the body surfaces (touch, pressure, heat, cold, pain); other senses are more complex, giving degrees of feeling that are not part of general somatic sensations. Free nerve endings are what allow for somatic sensations and are located most densely in those areas of the body, such as fingers, that need to be most sensitive to stimulation. One type of free nerve ending coils around hair follicles to detect movement; movement of hairs can alert you to an insect, for example, walking across your skin. Pain is perceived injury to a body region as detected by nociceptors. Somatic pain is one type of pain and could describe the sensation that comes with a needle prick to the finger. The “damage” the needle causes stimulates the release of bradykinins, histamine, prostaglandins, and other substances to trigger inflammation and ensure the “message” of pain gets transmitted to the brain. The brain, in turn, will release natural opiates to “dim” our perception of the pain. The stimuli for taste receptors consists of fluids carrying one of five primary tastes—sweet, sour, salty, bitter, and umami—reaching the receptors in the taste buds as well as the “smell” input to the olfactory receptors. Water-soluble and volatile odor molecules stimulate the olfactory receptors in the nose, sending signals directly to the olfactory bulbs in the frontal area of the brain for interpretation. The figures to be labeled for the ear correspond to Figure 14.8 of the text. For the top figure, left side, top to bottom: outer ear; outer ear including the auditory canal. Top figure, right side, top to bottom: inner ear (vestibular apparatus, cochlea); middle ear (ear drum, ear bones). For the bottom figure, left side, top to bottom: stirrup (stapes); anvil (incus); hammer (malleus); auditory canal; eardrum. Bottom figure, right side, top to bottom: oval window; round window; auditory nerve; cochlea; eustachian tube. Vibrations are passed from the tympanic membrane to the middle ear bones and on to the oval window, stretched across the entrance to the cochlea. Vibrations of the oval window send pressure waves through the fluid-filled cochlea to the basilar membrane on the floor of the cochlear duct; resting on the membrane is the organ of Corti, which includes sensory hair cells. The tips of the hair cells rest against the jellylike tectorial membrane; vibrations cause the hair cells to bend. Bending of the hair cells causes the release of neurotransmitters, triggering action potentials that travel to the brain. The diagram of the eye to be labeled corresponds to Figure 14.13. Left side, top to bottom: sclera, choroid, iris, lens, pupil, cornea, aqueous humor, ciliary muscle, retina. Right side, top to bottom: retina, fovea, optic disk, optic nerve. The cornea bends light so the rays converge on the back of the eyeball; adjustments are made when the ciliary muscle adjusts the shape of the lens. This is called accommodation and is designed to make sure that all rays focus at the same point on the retina. In nearsightedness, the focal point ends up being in front of the retina; in farsightedness, the focal point is behind the retina. Possible Responses to Critical Thinking Questions If the dizziness that Juanita experienced were a rather temporary feeling of lightheadedness, which subsided when she repositioned her body, then the doctor would conclude that it most likely was caused by a momentary drop in blood pressure to the brain. It happens to everyone at some time in their lives. However, if she reported the sensation of things “spinning around,” then she is probably suffering from vertigo, which indicates a problem with the inner ear. This could result from an inflammation, which will necessitate chemical treatment. The infection in Michael’s middle ear is in a rather closed-in place. If antibiotics fail to halt the multiplication of the bacteria, there is really no way to get the bacteria out of that space but through the ear canal to the outer ear. In this case, nature took its course and ruptured the tympanic membrane. Sometimes, the physician will recommend inserting tubes in the ears to provide a drainage port for children with chronic ear problems. Jill’s hearing difficulties are associated with the cochlea of the inner ear. Here there are ducts containing fluid and sensory hairs. When the vibrations of sound are received in this region, the fluid vibrates and stimulates the hairs, which transmit the events to the auditory nerve and on to the brain. The auditory nerve is not damaged, so we can suspect that it is the fluid and hairs that are not responding to the vibrations as they should. Larry’s lack of vision in the right side of the visual field is due to the inability of the left side of the retina to intercept the incoming light and images. While rock climbing, the semicircular canals of the vestibular apparatus are activated. Possible Responses to Explore on Your Own Questions Most students will have been given a color blindness test at some point in their lives if they have ever had their eyes formally examined; such tests are standard practice at most physician offices. Students should examine the Ishihara plates under different lighting to see if it “alters” their perception of the colors. Color blindness should be confirmed through eye examination, but poses few risks to overall health. 170 Chapter Fourteen Sensory Systems 159 170 Chapter Fourteen Sensory Systems 159 170 Chapter Fourteen Sensory Systems 169 170 Chapter Fourteen Sensory Systems 169 170 Chapter Fourteen Sensory Systems 169