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Passport to Success The following terms and other boldface terms in the chapter are defined in the Glossary accommodation choroid After careful study of this chapter, you should be able to: cochlea conjunctiva 1. Describe the function of the sensory system convergence 2. Differentiate between the special and general senses and give examples of each cornea 3. Describe the structure of the eye gustation 4. List and describe the structures that protect the eye lacrimal apparatus 5. Define refraction and list the refractive parts of the eye lens (crystalline lens) 6. Differentiate between the rods and the cones of the eye olfaction 7. Compare the functions of the extrinsic and intrinsic muscles of organ of Corti the eye ossicle 8. Describe the nerve supply to the eye proprioceptor 9. Describe the three divisions of the ear refraction 10. Describe the receptor for hearing and explain how it functions retina 11. Compare static and dynamic equilibrium and describe the sclera location and function of these receptors semicircular canal 12. Explain the function of proprioceptors sensory adaptation 13. List several methods for treatment of pain sensory receptor 14. Describe sensory adaptation and explain its value tympanic membrane 15. Show how word parts are used to build words related to the vestibule sensory system (see Word Anatomy at the end of the chapter) vitreous body PASSport to Success Visit thePoint or see the Student Resource CD in the back of this book for definitions and pronun- ciations of key terms as well as a pretest for this chapter. ® Paul’s Second Case: Seeing More of the Sun’s Effects aul glanced once again at the postcard condition, and it does have a hereditary fac- sitting on his entranceway table as he ar- tor.” The doctor dilated Paul’s eyes with drops Prived home in the evening. He knew it and examined the fundus of each eye with an said he was due for a checkup with his oph- ophthalmoscope. In answer to Paul’s query, he thalmologist, but after his run-in with skin explained that in this way he could examine cancer 6 months earlier, he was in no hurry to the health of the retina and the optic nerve see the inside of a medical office. Well, it’s just and also look at the vessels at the back of the routine, and I may need a slight change in my pre- eye for any signs of diabetes or circulatory scription, so I’ll make the call, he thought. problems. In addition, he used a tonometer to At the office, Dr. Gilbert greeted Paul and test for glaucoma, a condition that can dam- asked how he was feeling in general and if he age the eye with high fluid pressure. “I need to thought there had been any change in his vi- check on another patient,” he told Paul. “Then sion. “My vision sometimes seems a little I’ll be back to explain what I’ve found. Just sit blurry, especially when my eyes are tired, but in the waiting room for a few minutes, please.” no major changes,” Paul replied. Dr. Gilbert proceeded with the eye exam, testing his vi- Dr. Gilbert uses his knowledge of the sual acuity and checking on his astigmatism. structure and function of the eye to diagnose “You’re right; not much change,” said the medical conditions. In this chapter, you will ophthalmologist. “But, we’ll give you a new pre- learn about the eye and other sensory system scription for your glasses. At 42, you’re a little organs. As well, we’ll examine the conse- young for presbyopia—far-sightedness that de- quences of Paul’s sun-loving youth on his velops with age—but we’ll do the routine check eyes and vision. for glaucoma. I know your father developed that 187 188 Unit III Coordination and Control The Senses > Hearing from receptors in the internal ear > Equilibrium from receptors in the internal ear The sensory system protects a person by detecting changes > Taste from receptors in the tongue in the environment. An environmental change becomes a stimulus when it initiates a nerve impulse, which then > Smell from receptors in the upper nasal cavities travels to the central nervous system (CNS) by way of a I General senses sensory (afferent) neuron. A stimulus becomes a sensa- > Pressure, temperature, pain, and touch from re- tion—something we experience—only when a specialized ceptors in the skin and internal organs area of the cerebral cortex interprets the nerve impulse re- ceived. Many stimuli arrive from the external environment > Sense of position from receptors in the muscles, and are detected at or near the body surface. Others, such tendons, and joints as stimuli from the viscera, originate internally and help to maintain homeostasis. The Eye and Vision Sensory Receptors In the embryo, the eye develops as an outpocketing of the brain. It is a delicate organ, protected by a number of The part of the nervous system that detects a stimulus is structures: the sensory receptor. In structure, a sensory receptor may be one of the following: I The skull bones form the walls of the eye orbit (cavity) and protect the posterior part of the eyeball. I The free dendrite of a sensory neuron, such as the re- ceptors for pain and temperature I The upper and lower eyelids aid in protecting the eye’s anterior portion (Fig. 10-1). The eyelids can be I A modified ending, or end-organ, on the dendrite of closed to keep harmful materials out of the eye, and an afferent neuron, such as those for touch blinking helps to lubricate the eye. A muscle, the le- I A specialized cell associated with an afferent neuron, vator palpebrae, is attached to the upper eyelid. such as the rods and cones of the eye’s retina and the When this muscle contracts, it keeps the eye open. If receptors in the other special sense organs the muscle becomes weaker with age, the eyelids may droop and interfere with vision, a condition Receptors can be classified according to the type of called ptosis. stimulus to which they respond: I Chemoreceptors, such as receptors for taste and smell, detect chemicals in solution. PASSport Visit thePoint or see the Student Resource CD in to Success the back of this book for an image of ptosis. I Photoreceptors, located in the retina of the eye, re- spond to light. I Thermoreceptors detect change in temperature. Many of these receptors are located in the skin. I Mechanoreceptors respond to movement, such as Eyelashes Eyebrow stretch, pressure, or vibration. These include pressure receptors in the skin, receptors that monitor body po- sition, and the receptors of hearing and equilibrium in the ear, which are activated by the movement of cilia on specialized receptor cells. Upper eyelid Any receptor must receive a stimulus of adequate in- (superior tensity, that is, at least a threshold stimulus, in order to re- palpebra) spond and generate a nerve impulse. Lower eyelid (inferior Special and General Senses palpebra) Another way of classifying the senses is according to the Iris Pupil Sclera distribution of their receptors. A special sense is localized (covered with in a special sense organ; a general sense is widely distrib- conjunctiva) uted throughout the body. Figure 10-1 The eye’s protective structures. (Reprinted with I Special senses permission from Bickley LS. Bates’ Guide to Physical Examination and > Vision from receptors in the eye History Taking, 8th ed. Philadelphia: Lippincott Williams & Wilkins, 2003.) Chapter 10 The Sensory System 189 Lacrimal gland the eye from the lacrimal gland, located in the orbit’s upper lateral part, they carry away small particles that Superior canal may have entered the eye. The tears then flow into ducts near the eye’s nasal corner where they drain into the nose by way of the nasolacrimal (na-zo-LAK-rih- Lacrimal sac mal) duct (see Fig. 10-2). An excess of tears causes a “runny nose”; a greater overproduction of them re- sults in the tears spilling onto the cheeks. With age, Ducts of Nasolacrimal lacrimal duct the lacrimal glands produce less secretion, but tears gland still may overflow onto the cheek if the nasolacrimal Opening of ducts become plugged. Inferior canal duct (in nose) CHECKPOINT 10-1 ® What are some structures that protect the eye? Figure 10-2 The lacrimal apparatus. The lacrimal (tear) gland and its associated ducts are shown. Coats of the Eyeball I The eyelashes and eyebrow help to keep foreign mat- The eyeball has three separate coats, or tunics (Fig. 10-3). ter out of the eye. 1. The outermost tunic, called the sclera (SKLE-rah), is I A thin membrane, the conjunctiva (kon-junk-TI-vah), made of tough connective tissue. It is commonly re- lines the inner surface of the eyelids and covers the vis- ferred to as the white of the eye. It appears white be- 10 ible portion of the white of the eye (sclera). Cells cause of the collagen it contains and because it has no within the conjunctiva produce mucus that aids in lu- blood vessels to add color. (Reddened or “bloodshot” bricating the eye. Where the conjunctiva folds back eyes result from inflammation and swelling of blood from the eyelid to the eye’s anterior surface, a sac is vessels in the conjunctiva). formed. The lower portion of the conjunctival sac can 2. The second tunic of the eyeball is the choroid (KO- be used to instill medication drops. With age, the con- royd). This coat is composed of a delicate network of junctiva often thins and dries, resulting in inflamma- connective tissue interlaced with many blood vessels.
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