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Each chapter in Human Physiology, tenth edition, offers students numerous pedagogical features in the text and illustrations; all are con-sistently organized within each chapter. The consistency and variety of tools are designed to help students from all learning backgroundsconceptually understand physiology.

ObjectivesStudents review the objectives before reading the chapter tobecome familiar with the material to be covered, and check offthe objectives as each major section is completed.

Chapter at a GlanceStudents use the chapter outline to get an overview of the chap-ter, or to find specific topics.

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ObjectivesAfter studying this chapter, you should be able to . . .

1. explain how sensory receptors are categorized, giveexamples of functional categories, and explain howtonic and phasic receptors differ.

2. explain the law of specific nerve energies.3. describe the characteristics of the generator

potential.4. give examples of different types of cutaneous

receptors and describe the neural pathways for thecutaneous senses.

5. explain the concepts of receptive fields and lateralinhibition.

6. Explain how taste cells are affected by salty, sour,sweet, bitter, and umami stimuli.

7. describe the structure and function of the olfactoryreceptors and explain how odor discriminationmight be accomplished.

8. describe the structure of the vestibular apparatusand explain how it provides information aboutacceleration of the body in different directions.

9. describe the functions of the outer and middle ear.10. describe the structure of the cochlea and explain

how movements of the stapes against the ovalwindow result in vibrations of the basilarmembrane.

11. explain how mechanical energy is converted intonerve impulses by the organ of Corti and howpitch perception is accomplished.

12. describe the structure of the eye and explain howimages are brought to a focus on the retina.

13. explain how visual accommodation is achieved anddescribe the defects associated with myopia,hyperopia, and astigmatism.

14. describe the architecture of the retina and tracethe pathways of light and nerve activity through theretina.

15. describe the function of rhodopsin in the rods andexplain how dark adaptation is achieved.

16. explain how light affects the electrical activity ofrods and their synaptic input to bipolar cells.

17. explain the trichromatic theory of color vision.18. compare rods and cones with respect to their

locations, synaptic connections, and functions.

19. describe the neural pathways from the retina,explaining the differences in pathways fromdifferent regions of the visual field.

Chapter at a GlanceCharacteristics of Sensory Receptors 000

Categories of Sensory Receptors 000Law of Specific Nerve Energies 000Generator (Receptor) Potential 000

Cutaneous Sensations 000Neural Pathways for Somatesthetic Sensations 000Receptive Fields and Sensory Acuity 000Lateral Inhibition 000

Taste and Smell 000Taste 000Smell 000

Vestibular Apparatus and Equilibrium 000Sensory Hair Cells of the Vestibular Apparatus 000Utricle and Saccule 000Semicircular Canals 000

The Ears and Hearing 000Outer Ear 000Middle Ear 000Cochlea 000Spiral Organ (Organ of Corti) 000

The Eyes and Vision 000Refraction 000Accommodation 000Visual Acuity 000

Retina 000Effect of Light on the Rods 000Electrical Activity of Retinal Cells 000Cones and Color Vision 000Visual Acuity and Sensitivity 000Neural Pathways from the Retina 000

Neural Processing of Visual Information 000Ganglion Cell Receptive Fields 000Lateral Geniculate Nuclei 000Cerebral Cortex 000

Interactions 000• • • •Summary 000Review Activities 000Related Websites 000

10 Sensory Physiology

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This chapter opener illustration is from theTenth Edition of Human Physiology, whichbecame the leading college physiology text.

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Refresh Your MemoryBefore you begin this chapter, you maywant to review these concepts fromprevious chapters:

• Cerebral Cortex 000• Ascending Tracts 000• Cranial and Spinal Nerves 000

Take Advantage of the TechnologyVisit the text website for these addi-tional study resources.

• Interactive quizzing• Online study guide• Current news feeds• Crossword puzzles and vocabulary

flashcards• Labeling activities

aris.mhhe.com

Refresh Your MemoryStudents are encouraged to review mate-rial from previous chapters in prepara-tion for their study of new concepts.

Take Advantage of theTechnologyStudents are invited to explore addition-al study resources on the Human Physi-ology website at aris.mhhe.com.

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Stepped-Out FiguresBuilding on the extensive upgrades madeto some figures in the ninth edition manyfigures have now been broken out intosteps. Figures using this stepped-outprocess will aid student learning byfocusing attention on one step at a timeand helping students study the illustrationin correct sequence.

Consistent Use of ColorsAcross all figures the appearance of particular structures is presentedconsistently. This continuity makes it easier for students to interpreteach figure, thereby improving the clarity of the total presentation.This key shows a sampling of some of the structures that have beenstandardized.

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Na

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Anatomical Structures

Brain Heart Kidney

Lungs Liver

Bones

Muscles

Cell Organelles

Endoplasmic reticulum

Golgi complexMitochondria

Plasma membraneNucleus

Ribosomes

Elements and Molecular Models

Calcium

Carbon

Chlorine

DNA

RNA

Nitrogen

Potassium

Sodium

Neurons and Neural Pathways

Typical neuron

Sympatheticneurons

Parasympatheticneurons

Sensory neurons

Somatic motorneurons

Interneurons

1. Striking patellar ligament stretches tendon and quadriceps femoris muscle.

4. Alpha motoneuron stimulates extrafusal muscle fibers to contract.

3. Sensory neuron activates alpha motoneuron.

2. Spindle is stretched, activating sensory neuron.

Patellarligament

Spindle

Alphamotoneuron

Patella

Sensory neuron Spinal cord

Extrafusalmusclefibers

Tendon

1. Dendritic cell processes antigens

2. Dendritic cell activates T cell

4. Activated T cell extravasates to combat infection

3. Activated T cell travels through lymphatic system

Activated T cell

Attracted to

infected siteDendritic cell

Antigens

Antigen

Antigen

Lymph vessel

Dendritic cell

T cell

Activated T cell

Lymphvessel

Lymph node

Venule

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True learning is not complete until principles can be analyzed and applied, rather than merely comprehended. The many featurespresented in Human Physiology help students fine-tune their studies by encouraging them to establish connections between bodysystems, summarize the main concepts of the chapter, and then answer questions of varying degrees of difficulty.

InteractionsInteractions: HPer Links are resource pages that list the manyways a major concept applies to the study of different body sys-tems, and the ways a given system interacts with other body systems. Each application or interaction includes a page refer-ence to related material in the textbook. Students use the cross-references offered on the Interactions pages to find interrelatedtopics in the textbook.

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InteractionsHPer Links of the Endocrine System with Other Body Systems

Integumentary System• The skin helps to protect the body from

pathogens (p. 000)• The skin produces vitamin D3, which acts

as a prehormone (p. 000)

Skeletal System• Bones support and protect the pituitary

gland (p. 000)• Bones store calcium, which is needed for

the action of many hormones (p. 000)• Anabolic hormones, including growth

hormone, stimulate bone development(p. 000)

• Parathyroid hormone and calcitoninregulate calcium deposition andresorption in bones (p. 000)

• Sex hormones help to maintain bonemass in adults (p. 000)

Muscular System• Anabolic hormones promote muscle

growth (p. 000)• Insulin stimulates the uptake of blood

glucose into muscles (p. 000)• The catabolism of muscle glycogen and

proteins is promoted by severalhormones (p. 000)

Nervous System• The hypothalamus secretes hormones

that control the anterior pituitary (p. 000)• The hypothalamus produces the

hormones released by the posteriorpituitary (p. 000)

• Sympathetic nerves stimulate thesecretions of the adrenal medulla (p. 000)

• Parasympathetic nerves stimulate thesecretions of the pancreatic islets (p. 000)

• Neurons stimulate the secretion ofmelatonin from the pineal gland, which inturn regulates parts of the brain (p. 000)

• Sex hormones from the gonads regulatethe hypothalamus (p. 000)

Circulatory System• The blood transports oxygen, nutrients,

and regulatory molecules to endocrineglands and removes wastes (p. 000)

• The blood transports hormones fromendocrine glands to target cells (p. 000)

• Epinephrine and norepinephrine from theadrenal medulla stimulate the heart(p. 000)

Immune System• The immune system protects against

infections that could damage endocrineglands (p. 000)

• Autoimmune destruction of thepancreatic islets causes type I diabetesmellitus (p. 000)

• Hormones from the thymus help toregulate lymphocytes (p. 000)

• Adrenal corticosteroids have asuppressive effect on the immune system(p. 000)

Respiratory System• The lungs provide oxygen for transport

by the blood and eliminate carbon dioxide(p. 000)

• Thyroxine and epinephrine stimulate therate of cell respiration in the body(p. 000)

• Epinephrine promotes bronchodilation,reducing airway resistance (p. 000)

Urinary System• The kidneys eliminate metabolic wastes

produced by body organs, includingendocrine glands (p. 000)

• The kidneys release renin, whichparticipates in the reninangiotensin-aldosterone system (p. 000)

• The kidneys secrete erythropoietin, whichserves as a hormone that regulates redblood cell production (p. 000)

• Antidiuretic hormone, aldosterone, andatrial natriuretic hormone regulate kidneyfunctions (p. 000)

Digestive System• The GI tract provides nutrients to the

body organs, including those of theendocrine system (p. 000)

• Hormones of the stomach and smallintestine help to coordinate the activitiesof different regions of the GI tract(p. 000)

• Hormones from adipose tissue contributeto the sensation of hunger (p. 000)

Reproductive System• Gonadal hormones help to regulate the

secretions of the anterior pituitary(p. 000)

• Pituitary hormones regulate the ovariancycle (p. 000)

• Testicular androgens regulate the maleaccessory sex organs (p. 000)

• Ovarian hormones regulate the uterusduring the menstrual cycle (p. 000)

• Oxytocin plays an essential role in laborand delivery (p. 000)

• The placenta secretes several hormonesthat influence the course of pregnancy(p. 000)

• Several hormones are needed forlactation in a nursing mother (p. 000)

Section PreviewsEach major section begins with a concise statement of the sec-tion’s central concepts, or organizing themes, which will beillustrated and explained in detail in the text that follows. Thesebrief introductions give students a framework to work from asthey learn new material.

Test Yourself Before You ContinueEach major chapter section ends with a set of learning activitiesand essay questions that relate only to the material presented inthe section. Students are encouraged to answer the essay ques-tions, draw the outlines and flowcharts requested, and otherwiseactively participate in their learning of this material. Thus, thesesections serve as both a “reality check” for the student and amechanism for active learning.

Chemical Composition of the Body 43

proteins in cell membranes may serve as receptors for specificregulator molecules (such as hormones) and as carriers for trans-port of specific molecules across the membrane. Proteins pro-vide the diversity of shape and chemical properties required bythese functions.

Nucleic AcidsNucleic acids include the macromolecules DNA and RNA,

which are critically important in genetic regulation, and the

subunits from which these molecules are formed.These sub-

units are known as nucleotides.

Nucleotides are the subunits of nucleic acids, bondedtogether in dehydration synthesis reactions to form long polynu-cleotide chains. Each nucleotide, however, is itself composed ofthree smaller subunits: a five-carbon (pentose) sugar, a phosphategroup attached to one end of the sugar, and a nitrogenous baseattached to the other end of the sugar (fig. 2.29). The nitrogenousbases are nitrogen-containing molecules of two kinds: pyrim-idines and purines. The pyrimidines contain a single ring of car-bon and nitrogen, whereas the purines have two such rings.

Deoxyribonucleic AcidThe structure of DNA (deoxyribonucleic acid) serves as thebasis for the genetic code. For this reason, it might seem logicalthat DNA should have an extremely complex structure. DNA isindeed larger than any other molecule in the cell, but its structureis actually simpler than that of most proteins. This simplicity of

C

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Ionic bond

S

S

C

H

H2C CH3

H

C

CH2

CH3

H3C

H3C

O H

OC–O

+NH3

Hydrogenbond

Disulfide bond(covalent)

Van der Waalsforces

Figure 2.27 The bonds responsible for the tertiarystructure of a protein. The tertiary structure of a protein is held inplace by a variety of bonds.These include relatively weak bonds, such ashydrogen bonds, ionic bonds, and van der Waals (hydrophobic) forces, aswell as the strong covalent disulfide bonds.

Table 2.4 Composition of Selected Proteins Found in the BodyProtein Number of Polypeptide Chains Nonprotein Component Function

Hemoglobin 4 Heme pigment Carries oxygen in the bloodMyoglobin 1 Heme pigment Stores oxygen in muscleInsulin 2 None Hormonal regulation of metabolismBlood group proteins 1 Carbohydrate Produces blood typesLipoproteins 1 Lipids Transports lipids in blood

Elastic fibers

Collagenousfibers

Figure 2.28 A photomicrograph of collagenous fiberswithin connective tissue. Collagen proteins strengthen theconnective tissues.

Test Yourself Before You Continue1. Write the general formula for an amino acid and describe

how amino acids differ from one another.2. Describe and account for the different levels of protein

structure.3. Describe the different categories of protein function in the

body and explain why proteins can serve functions that are sodiverse.

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SummariesAt the end of each chapter, the material is summarized in outlineform. This outline summary is organized by major section head-ings with page references, followed by the key points in the sec-tion. Students may read the summary after studying the chapterto be sure that they haven’t missed any points, and can use thechapter summaries to help review for examinations.

Review ActivitiesA battery of questions collectively titled Review Activitiesfollows each chapter summary. These self-examinations areorganized into three increasingly difficult learning levels tohelp students progress from simple memorization to higherlevels of understanding.• Test Your Knowledge of Terms and Facts is a series of

multiple-choice questions that prompt students to recallkey terms and facts presented in the chapter. Answers tothese questions are found in Appendix B.

• Test Your Understanding of Concepts and Principlesconsists of brief essay questions that require students todemonstrate their understanding of chapter material.

• Test Your Ability to Analyze and Apply Your Knowledgepresents questions that stimulate critical thinking bychallenging students to utilize chapter concepts to solve aproblem.

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SummaryAtoms, Ions, and ChemicalBonds 26

I. Covalent bonds are formed by atomsthat share electrons. They are thestrongest type of chemical bond.A. Electrons are equally shared in

nonpolar covalent bonds andunequally shared in polarcovalent bonds.

B. Atoms of oxygen, nitrogen, andphosphorus strongly attractelectrons and become electricallynegative compared to the otheratoms sharing electrons withthem.

II. Ionic bonds are formed by atoms thattransfer electrons. These weak bondsjoin atoms together in an ioniccompound.A. If one atom in this compound

takes an electron from anotheratom, it gains a net negativecharge and the other atombecomes positively charged.

B. Ionic bonds easily break when theionic compound is dissolved inwater. Dissociation of the ioniccompound yields charged atomscalled ions.

III. When hydrogen bonds with anelectronegative atom, it gains a slightpositive charge and is weakly attractedto another electronegative atom. Thisweak attraction is a hydrogen bond.

IV. Acids donate hydrogen ions tosolution, whereas bases lower thehydrogen ion concentration of asolution.A. The pH scale is a negative

function of the logarithm of thehydrogen ion concentration.

B. In a neutral solution, theconcentration of H� is equal tothe concentration of OH�, and thepH is 7.

C. Acids raise the H� concentrationand thus lower the pH below 7;bases lower the H� concentrationand thus raise the pH above 7.

V. Organic molecules contain atoms ofcarbon and hydrogen joined together

by covalent bonds. Atoms of nitrogen,oxygen, phosphorus, or sulfur may bepresent as specific functional groupsin the organic molecule.

Carbohydrates and Lipids 33I. Carbohydrates contain carbon,

hydrogen, and oxygen, usually in aratio of 1:2:1.A. Carbohydrates consist of simple

sugars (monosaccharides),disaccharides, andpolysaccharides (such asglycogen).

B. Covalent bonds betweenmonosaccharides are formed bydehydration synthesis, orcondensation. Bonds are brokenby hydrolysis reactions.

II. Lipids are organic molecules that areinsoluble in polar solvents such aswater.A. Triglycerides (fat and oil) consist

of three fatty acid moleculesjoined to a molecule of glycerol.

B. Ketone bodies are smallerderivatives of fatty acids.

C. Phospholipids (such as lecithin)are phosphate-containing lipidsthat have a hydrophilic polargroup. The rest of the molecule ishydrophobic.

D. Steroids (including the hormonesof the adrenal cortex and gonads)are lipids with a characteristicfour-ring structure.

E. Prostaglandins are a family ofcyclic fatty acids that serve avariety of regulatory functions.

Proteins 40I. Proteins are composed of long chains

of amino acids bound together bycovalent peptide bonds.A. Each amino acid contains an

amino group, a carboxyl group,and a functional group.Differences in the functionalgroups give each of the more thantwenty different amino acids anindividual identity.

B. The polypeptide chain may betwisted into a helix (secondarystructure) and bent and folded toform the tertiary structure of theprotein.

C. Proteins that are composed of twoor more polypeptide chains aresaid to have a quaternarystructure.

D. Proteins may be combined withcarbohydrates, lipids, or othermolecules.

E. Because they are so diversestructurally, proteins serve awider variety of specificfunctions than any other type ofmolecule.

Nucleic Acids 43I. DNA is composed of four nucleotides,

each of which contains the sugardeoxyribose.A. Two of the bases contain the

purines adenine and guanine; twocontain the pyrimidines cytosineand thymine.

B. DNA consists of twopolynucleotide chains joinedtogether by hydrogen bondsbetween their bases.

C. Hydrogen bonds can only formbetween the bases adenine andthymine, and between the basesguanine and cytosine.

D. This complementary base pairingis critical for DNA synthesis andfor genetic expression.

II. RNA consists of four nucleotides,each of which contains the sugarribose.A. The nucleotide bases are adenine,

guanine, cytosine, and uracil (inplace of the DNA base thymine).

B. RNA consists of only a singlepolynucleotide chain.

C. There are different types of RNA,which have different functions ingenetic expression.

Chemical Composition of the Body 47

Review ActivitiesTest Your Knowledge of Terms and Facts

1. Which of these statements aboutatoms is true?a. They have more protons than

electrons.b. They have more electrons than

protons.c. They are electrically neutral.d. They have as many neutrons as

they have electrons.2. The bond between oxygen and

hydrogen in a water molecule isa. a hydrogen bond.b. a polar covalent bond.c. a nonpolar covalent bond.d. an ionic bond.

3. Which of these is a nonpolar covalentbond?a. bond between two carbonsb. bond between sodium and

chloridec. bond between two water

moleculesd. bond between nitrogen and

hydrogen4. Solution A has a pH of 2, and solution

B has a pH of 10. Which of thesestatements about these solutions istrue?a. Solution A has a higher H�

concentration than solution B.b. Solution B is basic.c. Solution A is acidic.d. All of these are true.

5. Glucose isa. a disaccharide.b. a polysaccharide.c. a monosaccharide.d. a phospholipid.

6. Digestion reactions occur by means ofa. dehydration synthesis.b. hydrolysis.

7. Carbohydrates are stored in the liverand muscles in the form ofa. glucose.b. triglycerides.c. glycogen.d. cholesterol.

8. Lecithin isa. a carbohydrate.b. a protein.c. a steroid.d. a phospholipid.

9. Which of these lipids have regulatoryroles in the body?a. steroidsb. prostaglandinsc. triglyceridesd. both a and be. both b and c

10. The tertiary structure of a protein isdirectly determined bya. genes.b. the primary structure of the

protein.

c. enzymes that “mold” the shape ofthe protein.

d. the position of peptide bonds.11. The type of bond formed between two

molecules of water isa. a hydrolytic bond.b. a polar covalent bond.c. a nonpolar covalent bond.d. a hydrogen bond.

12. The carbon-to-nitrogen bond that joinsamino acids together is calleda. a glycosidic bond.b. a peptide bond.c. a hydrogen bond.d. a double bond.

13. The RNA nucleotide base that pairswith adenine in DNA isa. thymine.b. uracil.c. guanine.d. cytosine.

14. If four bases in one DNA strand are A(adenine), G (guanine), C (cytosine),and T (thymine), the complementarybases in the RNA strand made fromthis region area. T,C,G,A.b. C,G,A,U.c. A,G,C,U.d. U,C,G,A.

Test Your Understanding of Concepts and Principles1. Compare and contrast nonpolar

covalent bonds, polar covalent bonds,and ionic bonds.1

2. Define acid and base and explain howacids and bases influence the pH of asolution.

3. Using dehydration synthesis andhydrolysis reactions, explain therelationships between starch in aningested potato, liver glycogen, andblood glucose.

4. “All fats are lipids, but not all lipidsare fats.” Explain why this is anaccurate statement.

5. What are the similarities anddifferences between a fat and an oil?Comment on the physiological and

clinical significance of the degree ofsaturation of fatty acid chains.

6. Explain how one DNA moleculeserves as a template for the formationof another DNA molecule and whyDNA synthesis is said to besemiconservative.

1Note: This question is answered in the chapter 2 Study Guide found on the Human Physiology website at aris.mhhe.com.

48 Chapter 2

Test Your Ability to Analyze and Apply Your Knowledge1. Explain the relationship between the

primary structure of a protein and itssecondary and tertiary structures.What do you think would happen tothe tertiary structure if some aminoacids were substituted for others in theprimary structure? What physiologicalsignificance might this have?

2. Suppose you try to discover ahormone by homogenizing an organ ina fluid, filtering the fluid to eliminatethe solid material, and then injecting

the extract into an animal to see theeffect. If an aqueous (water) extractdoes not work but one using benzeneas the solvent does have an effect,what might you conclude about thechemical nature of the hormone?Explain.

3. From the ingredients listed on a foodwrapper, it would appear that the foodcontains high amounts of fat. Yet onthe front of the package is the largeslogan, “Cholesterol Free!” In what

sense is this slogan chemicallycorrect? In what way is it misleading?

4. A butter substitute says“Nonhydrogenated, zero trans fats” onthe label. Explain the meaning of theseterms and their relationship to health.

5. When you cook a pot roast, you don’tend up with an amino acid soup.Explain why this is true, in terms ofthe strengths of the different types ofbonds in a protein.

Related WebsitesCheck out the Links Library on the Human Physiology website ataris.mhhe.com for links to sites containing resources related to thechemical composition of the body.

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True conceptual understanding often occurs in the context of real-life situations. Therefore, clinical information is presentedthroughout the text to provide concrete examples that demonstrate the real-life application of complex physiological concepts andfurther students’ comprehension.

Clinical InvestigationClinical Investigations are diagnostic case studies provided at thevery beginning of each chapter. These thought-provoking casesare designed to engage students’ interest and motivate them todelve into the content of each chapter. Students must read thechapter, understand the concepts, and look for clues in order toarrive at the correct diagnosis.

Clinical Investigation CluesScattered within each chapter, these short boxes remind studentsof the ongoing clinical investigation case study and provide cluesto the solution. Clues are carefully placed so they always relateto the information presented in the preceding text. These clueshelp reinforce comprehension of the text material and spur stu-dents to continue reading so they can gather all of the pertinentinformation needed to solve the case study. After attempting todiagnose the case, students can find the solution to each ClinicalInvestigation in Appendix A.

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260 Chapter 10

Clinical InvestigationEd is a 45-year-old man who goes to the doctor complaining of severeear pain and reduced hearing immediately after disembarking from aninternational flight. It is apparent that Ed has a bad head cold, and thedoctor recommends that he take a decongestant. He further recom-mends that Ed come back after the cold is better for an audiology test,if his hearing has not improved by then.While talking to the doctor, Edcomplains that he can’t see print very clearly anymore, even though he’snever worn glasses. However, he tells the doctor that his distance vision,and ability to drive, are still fine.

What may have caused Ed’s ear pain and reduced hearing? Whatmay be responsible for his impaired ability to see print?

Characteristics of SensoryReceptors

Each type of sensory receptor responds to a particular

modality of environmental stimulus by causing the produc-

tion of action potentials in a sensory neuron.These impulses

are conducted to parts of the brain that provide the proper

interpretation of the sensory information when that partic-

ular neural pathway is activated.

endings may be free—such as those that respond to pain and tem-perature—or encapsulated within nonneural structures—such asthose that respond to pressure (see fig. 10.4). The photoreceptorsin the retina of the eyes (rods and cones) are highly specializedneurons that synapse with other neurons in the retina. In the caseof taste buds and of hair cells in the inner ears, modified epithe-lial cells respond to an environmental stimulus and activate sen-sory neurons.

Functional CategoriesSensory receptors can be grouped according to the type of stimulusenergy they transduce. These categories include (1) chemorecep-tors, which sense chemical stimuli in the environment or the blood(e.g., the taste buds, olfactory epithelium, and the aortic and carotidbodies); (2) photoreceptors—the rods and cones in the retina ofthe eye; (3) thermoreceptors, which respond to heat and cold; and(4) mechanoreceptors, which are stimulated by mechanical defor-mation of the receptor cell membrane (e.g., touch and pressurereceptors in the skin and hair cells within the inner ear).

Nociceptors—or pain receptors—have a higher threshold foractivation than do the other cutaneous receptors; thus, a moreintense stimulus is required for their activation. Their firing rate thenincreases with stimulus intensity. Receptors that subserve other sen-sations may also become involved in pain transmission when thestimulus is prolonged, particularly when tissue damage occurs.

Receptors also can be grouped according to the type of sen-sory information they deliver to the brain. Proprioceptors

outer ear. The external auditory meatus channels the soundwaves (while increasing their intensity) to the eardrum, or tym-panic membrane. Sound waves in the external auditory meatusproduce extremely small vibrations of the tympanic membrane;movements of the eardrum during speech (with an average soundintensity of 60 dB) are estimated to be about the diameter of amolecule of hydrogen!

Middle EarThe middle ear is the cavity between the tympanic membrane onthe outer side and the cochlea on the inner side (fig. 10.19).Within this cavity are three middle-ear ossicles—the malleus(hammer), incus (anvil), and stapes (stirrup). The malleus isattached to the tympanic membrane, so that vibrations of thismembrane are transmitted via the malleus and incus to the stapes.The stapes, in turn, is attached to a membrane in the cochleacalled the oval window, which thus vibrates in response to vibra-tions of the tympanic membrane.

The fact that vibrations of the tympanic membrane are trans-ferred through three bones instead of just one affords protection. Ifthe sound is too intense, the ossicles may buckle. This protectionis increased by the action of the stapedius muscle, which attachesto the neck of the stapes (fig. 10.19). When sound becomes tooloud, the stapedius muscle contracts and dampens the movementsof the stapes against the oval window. This action helps to preventnerve damage within the cochlea. If sounds reach high amplitudesvery quickly, however—as in gunshots—the stapedius musclemay not respond soon enough to prevent nerve damage.

The auditory (eustachian) tube is apassageway leading from the middle ear tothe nasopharynx (a cavity positionedbehind the nasal cavity and extending downto the soft palate).The auditory tube isusually collapsed, so that debris and infec-tious agents are prevented from traveling from the oral cavityto the middle ear. In order to open the auditory tube, the ten-sor tympani muscle, attaching to the auditory tube and themalleus (fig. 10.19), must contract.This occurs during swallow-ing, yawning, and sneezing. People sense a “popping” sensationin their ears as they swallow when driving up a mountainbecause the opening of the auditory canal permits air to movefrom the region of higher pressure in the middle ear to theregion of lower pressure in the nasopharynx.

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Clinical Investigation CluesRemember that Ed experienced severe ear pain and reduced hear-ing immediately after disembarking from an international flight.Remember also that he had a bad head cold.

What may have caused his pain and hearing impairment? How could this be helped by taking a decongestant?

370 Chapter 12

producing myostatin “knocked out” have greatly increased mus-cle mass. The functions of myostatin, and the mechanisms thatregulate satellite cell proliferation and formation of myotubes,have many potential health applications and are currently activeareas of research.

Motor neurons in the spinal cord, or lower motor neurons(often shortened to motoneurons), are those previously describedthat have cell bodies in the spinal cord and axons within nervesthat stimulate muscle contraction (table 12.5). The activity ofthese neurons is influenced by (1) sensory feedback from themuscles and tendons and (2) facilitory and inhibitory effects fromupper motor neurons, which are interneurons in the brain thatcontribute axons to descending motor tracts. Lower motor neu-rons are thus said to be the final common pathway by which sen-sory stimuli and higher brain centers exert control over skeletalmovements.

The cell bodies of lower motor neurons are located in theventral horn of the gray matter of the spinal cord (chapter 8).Axons from these cell bodies leave the ventral side of the spinalcord to form the ventral roots of spinal nerves (see fig. 8.29). Thedorsal roots of spinal nerves contain sensory fibers whose cellbodies are located in the dorsal root ganglia. Both sensory (affer-ent) and motor (efferent) fibers join in a common connective tis-sue sheath to form the spinal nerves at each segment of the spinalcord. In the lumbar region there are about 12,000 sensory and6,000 motor fibers per spinal nerve.

About 375,000 cell bodies have been counted in a lumbarsegment—a number far larger than can be accounted for by thenumber of motor neurons. Most of these neurons do not con-tribute fibers to the spinal nerve. Rather, they serve as interneu-rons, whose fibers conduct impulses up, down, and across thecentral nervous system. Those fibers that conduct impulses tohigher spinal cord segments and the brain form ascending tracts,and those that conduct to lower spinal segments contribute todescending tracts. Those fibers that cross the midline of the CNSto synapse on the opposite side are part of commissural tracts.Interneurons can thus conduct impulses up and down on the same,or ipsilateral, side, and can affect neurons on the opposite, or con-tralateral, side of the central nervous system.

Muscle atrophy (reduction in size) andaccompanying declines in muscle strengthoccur in the weight-bearing muscles of thelegs when astronauts experience micrograv-ity (weightlessness) for long periods. Forexample, reductions in muscle volume andperformance were measured in the United States Skylab mis-sions. However, in Skylab 4 (which lasted 84 days), adjustmentsin the diet and the exercise program were able to significantlycompensate for the effects of microgravity on tested muscles.Like the effects of weightlessness in astronauts, weight-bearingmuscles are similarly “unloaded” in bedridden people and inpeople with a leg immobilized by a cast. In prolonged bed restof two to three weeks, the calf and leg muscles experiencedeclines in size and strength comparable to those seen in spaceflights. Perhaps surprisingly, immobilization of the leg in a castresults in more rapid declines in muscle performance and sizethan those observed for similar time periods in bed rest or themicrogravity of space.

Test Yourself Before You Continue1. Draw a figure illustrating the relationship between ATP and

creatine phosphate, and explain the physiological significanceof this relationship.

2. Describe the characteristics of slow- and fast-twitch fibers(including intermediate fibers). Explain how the fiber types aredetermined and list the functions of different fiber types.

3. Explain the different causes of muscle fatigue with referenceto the various fiber types.

4. Describe the effects of endurance training and resistancetraining on the fiber characteristics of muscles.

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Neural Control of Skeletal Muscles

Skeletal muscles contain stretch receptors called muscle

spindles that stimulate the production of impulses in sensory

neurons when a muscle is stretched.These sensory neurons

can synapse with alpha motoneurons, which stimulate the

muscle to contract in response to the stretch. Other motor

neurons, called gamma motoneurons, stimulate the tighten-

ing of the spindles and thus increase their sensitivity.

The disease known as amyotrophic lat-eral sclerosis (ALS) involves degenera-tion of the lower motor neurons, leadingto skeletal muscle atrophy and paralysis.This disease is sometimes called LouGehrig’s disease, after the baseball playerwho suffered from it, and also includes the famous physicistSteven Hawking among its victims. Scientists have recentlylearned that the inherited form of this disease is caused by adefect in the gene for a specific enzyme—superoxide dismutase.This enzyme is responsible for eliminating superoxide free rad-icals, which are highly toxic products that can damage themotor neurons.The mutant gene produces an enzyme that hasa different, and in fact destructive, action. Scientists haverecently demonstrated that the enzyme must be mutated inboth astrocytes and neurons in order for degeneration ofmotor neurons to occur.Mice that were genetically engineeredto express the mutant enzyme in only neurons or only astro-cytes did not develop ALS.

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Boxed Clinical and Fitness ApplicationsApplications—in clinical medicine, general health, and physicalfitness—of basic physiological principles are found intermittent-ly throughout the body of the text. Placement of these applica-tions is precise—they always relate to concepts that have beenpresented immediately preceding the application. As such, theyprovide immediate reinforcement for students learning the fun-damental principles on which the applications are based. This ispreferable to longer but fewer applications in the style of maga-zine articles that are separated from the text information. Theimmediate reinforcement allows students to see the practicalimportance of learning the material they have just studied.

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