chapt 41 hbio nervous system · 2019-12-07 · a sodium-potassium pump is a carrier protein that...

Chapter 41 • Nervous System 943

Opening ActivityNervous System Function Askstudents to brainstorm about thefunctions of the nervous system.Write their responses on the chalk-board. Make three columns on thechalkboard with the followingheadings: Monitoring internal and external environments,Coordinating bodily activities, and Functioning of intellect andmemory. Ask students to place their responses into the appropriatecategories. Then ask them toexplain why they grouped theirresponses in this way. VerbalLS

GENERAL

• Vocabulary Worksheets

• Concept Mapping

Chapter Resource File

Answers

1. An ion channel is a transportprotein in a cell membranethrough which ions can pass.

2. A sodium-potassium pump is acarrier protein that transportssodium ions out of, and potas-sium ions into, a cell.

3. Endocytosis is the movementof a substance by a vesicle tothe inside of a cell. Exocytosisis the movement of a substanceby a vesicle to the outside of a cell.

4. Receptor proteins bind to specific signal molecules, causing the cell to respond,forming either open or closedion channels.

Answers

Students may agree or disagreewith the statements. They mayreassess their lists after studyingthe chapter.

Quick Review

Reading Activity

Looking AheadQuick ReviewAnswer the following without referring to

earlier sections of your book.

1. Describe the importance of ion channels in cell

transport. (Chapter 4, Section 1)

2. Identify the role of sodium-potassium pumps in

cells. (Chapter 4, Section 2)

3. Distinguish between endocytosis and

exocytosis. (Chapter 4, Section 2)

4. List three functions of receptor proteins.

(Chapter 4, Section 2)

Did you have difficulty? For help, review the

sections indicated.

Section 1

Neurons and Nerve ImpulsesNeurons

Communication Between Neurons

Section 2

Structures of the Nervous SystemCentral Nervous System

Peripheral Nervous System

Section 3

Sensory SystemsPerception of Stimuli

Eyes

Ears

Chemical Senses

Section 4

Drugs and the Nervous SystemPsychoactive Drugs

Drug Addiction and Neuron Function

Alcohol

Nicotine

Drugs of Abuse

www.scilinks.orgNational Science Teachers Association sciLINKS Internet

resources are located throughout this chapter.

Reading ActivityCopy the following statements in your notebook:

1. Addiction is a purely psychological response

to drug use.

2. Reflexes occur before the brain is aware

of danger.

3. Prolonged exposure to loud noise can cause

permanent hearing loss.

Before you read this chapter, write down

whether you agree or disagree with each

statement. After you have finished reading the

chapter, decide whether or not you still agree

with your first response.

Nerve-cell networks like this one transmit thought,

emotions, and sensations by conducting electro-

chemical signals from cell to cell.

NervousSystem

CHAPTER

41

943

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Overview

Before beginning this sectionreview with your students theobjectives listed in the StudentEdition. This section explains thestructure and function of neurons,the events that occur in synaptictransmission, and the steps of anerve impulse.

Ask students to write definitionsfor electrons and ions and to sug-gest a function of ions in nervecells. Ask them to check their defi-nitions by looking them up in theglossary. (Electrons are negativelycharged particles that move aroundthe nucleus of an atom; ions areatoms or molecules that have either apositive charge [resulting from theloss of one or more electrons] or anegative charge [resulting from thegain of one or more electrons]; nervecells have electrical charges that aredifferent from the electrical chargesin the fluid around them.)

Bellringer

FocusFocus

Section 1

944 Chapter 41 • Nervous System

• Directed Reading

• Active Reading

• Data Sheet for Data Lab GENERAL

GENERAL


Transparencies

TR Bellringer

TR J59 Structure of a Neuron

• Unit 1—Cell Transport and HomeostasisThis engaging tutorial reinforces cellmembrane structure and function.

BIOLOGYBIOLOGY

Section 1 Neurons and Nerve Impulses

NeuronsIf your body used only chemical signals to send messages, your

interaction with the environment would be slow. A quicker means

of communication is needed, especially if your brain has an urgent

message for the muscles in your legs, such as “Contract quickly! A

speeding car is headed this way!” In addition to chemical signals,

your nervous system uses electrical signals to send messages

rapidly throughout your body.

The nervous system contains a complex network of nerve cells, or

(NOO rahns). Neurons, such as those shown in Figure 1,

are specialized cells that transmit information throughout the body.

Neurons enable many important functions, such as movement,

perception, thought, emotion, and learning.

Structure of NeuronsA neuron’s unique structure enables it to conduct electrical signals

called nerve impulses. Neurons communicate by transmitting nerve

impulses to body tissues and organs, including muscles, glands, and

other neurons. Neurons vary greatly in form, but a typical neuron is

similar to the one shown in Figure 2. (DEHN driets),

which extend from the cell body of the neuron, are the “antennae”

of the neuron. Dendrites receive information from other cells. The

neuron’s cell body collects information from dendrites, relays this

information to other parts of the neuron, and maintains the general

functioning of the neuron. An is a long membrane-covered

extension of the cytoplasm that conducts nerve impulses. The ends

of an axon are called axon terminals. When a neuron communicates

with other cells, it does so at its axon terminals.

Nervous tissue consists mostly of neurons and their supporting

cells. Bundles of axons are called . The arrangement of axons

in a nerve is similar to a telephone cable with many different

communication channels, each carried by a separate wire.

Nerves appear as fine, white threads when viewed with

the unaided eye.

Insulated NeuronsMany neurons have a layer of insulation on their axon

called a myelin (MIE uh lihn) sheath, as shown in Figure

2. Myelin is produced by supporting cells that surround

the axon. The presence of myelin causes nerve impulses to

move faster down the axon. The myelin sheath is inter-

rupted at intervals, leaving gaps called nodes of Ranvier

nerves

axon

Dendrites

neurons

Objectives

● Analyze the structure and

function of neurons.

● Describe how the resting

potential is established in

a neuron.

● Sequence the steps of a

nerve impulse.

● List the events that occur in

synaptic transmission of a

nerve impulse.

Key Terms

neuron

dendrite

axon

nerve

membrane potential

resting potential

action potential

synapse

neurotransmitter

Figure 1 Two neurons.

An average adult human

brain contains about

100 billion neurons.

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Using the Figure Have students observe preparedslides or photographs of neuronswith simple morphology (such asstellate neurons). Remind studentsto look for cell bodies, axons, anddendrites. Have students draw andlabel what they see and comparetheir drawings with the neuronshown in Figure 2. Visual

ActivityInformation Transfer Ask stu-dents to recall the game “Telephone”in which one person whispers astatement to his or her neighbor,who whispers the statement to hisor her neighbor, and so on untilthe last person in the room hasheard the statement. Discuss thesimilarities and differences betweenthis game and nerve impulses asthey pass through the body.(Similarities: a nerve impulse, like awhispered statement, passes fromone cell [person] to the next in line.Differences: the whispered statementusually is garbled at the end of theline [the end of the game], but nerveimpulses retain their integrity.)

Interpersonal

Paired Summarizing Pair stu-dents with a partner. Pair Englishlanguage learners with proficientEnglish speakers. Have each stu-dent read silently about restingpotential and action potential onthe following page. Then have oneof the students summarize aloudwhat has been read without refer-ring to the text. The partner shouldlisten without interrupting and be prepared to point out any inaccuracies or omissions in thesummary. At this point studentscan refer to the text. Have students switch roles and repeat the activity.

InterpersonalEnglish Language

LearnersLS

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READINGREADING

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MotivateMotivate


• Reading Organizers

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Planner CD-ROMdid you know?

Phenylketonuria Phenylketonuria, or PKU, isa disease caused by an enzyme deficiency thatleads to a buildup of phenylpyruvic acid in thebody. This results in the destruction of themyelin sheaths on neurons. If undetected, PKUcan lead to severe mental retardation and evendeath. PKU is easy to detect, so all newbornsin the United States are tested for it. If found,a low phenylalanine diet is prescribed. Foodproducts such as diet sodas carry a message tophenylketonurotics warning them that theproduct contains phenylalanine.

www.scilinks.org

Topic: Neurons

Keyword: HX4129

(RAHN vee ay), where the axon membrane is exposed to the sur-

rounding fluid. Conduction of nerve impulses is faster in myeli-

nated axons because nerve impulses “jump” from node to node as

they move down the axon. Myelin is especially beneficial in neurons

that must transmit information very rapidly, such as those involved

with quick movement.

The speed of impulse conduction is also related to axon diameter.

A large-diameter axon conducts impulses faster than a small-

diameter axon, assuming both axons are either myelinated or

unmyelinated.

Neuron FunctionAll cells have an electrical charge on the inner surface of the cell

membrane that is different from the electrical charge of the fluid

outside the cell. The difference in electrical charge across the cell

membrane, called the , results from the move-

ment of ions into and out of the cell. This movement depends on

the relative concentration of ions inside and outside the cell, the

ability of the ions to diffuse across the cell membrane, and the elec-

trical charge of the ions. The membrane potential is expressed as

voltage, like that of a battery.

Ions diffuse across a neuron’s cell membrane by passing through

proteins that act as ion channels. Each type of channel allows

only specific ions to pass. Certain channels are voltage-gated—that

is, whether they are open or closed depends on the membrane

potential. Even a small change in the membrane potential can

affect the permeability of the cell membrane to certain ions. As

these ions diffuse into or out of the neuron, they in turn affect the

membrane potential.

membrane potential

Figure 2 Myelinated neuron.

A myelin sheath covers the

axons of many neurons. Myelin

increases the speed of

nerve impulses.

Nucleus

Cell body

Axon terminals

Myelin sheath

Axon

Dendrites

Nodes of

Ranvier

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Vocabulary The term restingpotential is misleading becausethere is constant activity in a neuron even when the neuron isnot conducting a nerve impulse.Ions are continuously beingtransported and diffusing acrossthe membrane.

BUILDERSKILL

Teach, continuedTeach, continued


Analyzing ChangesDuring a NerveImpulse

Skills AcquiredAnalyzing, interpreting,inferring, drawingconclusions

Teacher’s NotesEncourage students to studyFigure 3 on page 947 if theyneed help answering thesequestions.

Answers to Analysis1. The action potential lastsabout 1.5 milliseconds.

2. Voltage gated sodium channelsare open at point A.

3. Voltage gated potassium chan-nels are open at point B.

4. an influx of positively chargedsodium ions

5. an outflow of positivelycharged potassium ions

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The cause of multiple sclerosis (MS) isunknown, but many scientists think that itresults from the actions of the immune systemagainst the myelin sheaths of nerves of thecentral nervous system. The myelin sheaths inthe brain and spinal cord become sclerotic, orhard, causing poor impulse conductivity.Sometimes the symptoms of MS disappearaltogether, but they may return after periodsof remission. Current therapies for MSinclude corticosteroids and beta interferon.

REAL WORLDREAL WORLDCONNECTIONCONNECTION

Transparencies

TR J60 Conduction of a Nerve Impulse

TR J61 Synaptic Transmission

Resting PotentialWhen a neuron is not conducting a nerve impulse, the neuron is

said to be at rest. The membrane potential of a neuron at rest is

called the . In a typical neuron, the resting poten-

tial is negative, about –70 millivolts (mV). At the resting potential,

the inside of the cell is negatively charged with respect to the out-

side of the cell. Why is the resting potential negative? Recall that

sodium-potassium pumps actively transport sodium ions, Na+, out

of a cell and potassium ions, K+, into the cell. This results in a

greater concentration of sodium ions outside the cell than inside

the cell, and a greater concentration of potassium ions inside the

cell than outside the cell. In a neuron, voltage-gated sodium chan-

nels are closed at the resting potential. Thus, very few sodium ions

can diffuse into the cell, despite their strong concentration gradi-

ent. Some voltage-gated potassium channels are open at the resting

potential. Potassium ions can therefore diffuse out of the cell down

their concentration gradient, carrying their positive charge with

them. Neurons also contain negatively charged proteins that are

too large to exit the cell.

Action PotentialWhen a neuron is conducting a nerve impulse, changes occur in the

cell membrane of the neuron. A nerve impulse is also called an

action potential. An is a local reversal of polarity—

from a negative charge to a positive charge—inside the neuron. An

action potential moves down an axon like a flame burning down a

fuse. The events of an action potential are summarized in Figure 3.

action potential

resting potential

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Analysis

1. Determine about how

long an action potential

lasts.

2. State whether voltage-

gated sodium channels are

open or closed at point A.

3. State whether voltage-

gated potassium channels

are open or closed at

point B.

4. Critical Thinking Rec-

ognizing Relationships

What causes the membrane

potential to become less

negative at point A?

5. Critical Thinking Rec-

ognizing Relationships

What causes the membrane

potential to become more

negative at point B?

Background

The graph below illustrates changes that occur in the membrane

potential of a neuron during an action potential. Use the graph to

answer the following questions. Refer to Figure 3 as needed.

Analyzing Changes During a Nerve Impulse

Action Potential

Mem

bra

ne p

ote

nti

al

(mill

ivo

lts)

Time (milliseconds)

A B

+40

+20

0

–20

–40

–60

–80

0 1 2

Reading Effectively

The membrane potential

of a cell is expressed in

millivolts (mV). A millivolt is

equal to one-thousandth of

a volt (V). As shown in the

graph in the Data Lab

below, a neuron’s membrane

potential can be positive

or negative. The resting

potential of an average

neuron is about –70 mV.

During an action potential,

the membrane potential of

the neuron reaches

about +40 mV.

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Planner CD-ROM


Squid Neurons The squid (Loligo sp.) wasone of the first organisms used to measure theresting potential of a neuron. Because of itslarge, long axons that innervate musclesresponsible for expelling water from the bodycavity, the squid makes an ideal specimen forstudies in cellular neurophysiology. Some ofthese axons are as large as 1 mm in diameter,making them easy to work with.

Using the Figure Figure 3 illustrates action potentialconduction along an axon. Theaxon of this neuron has beenenlarged to show that the mem-brane potential becomes positiveduring an action potential. Tell stu-dents that the reversal of polarityduring an action potential is causedby the rapid influx of sodium ionsthrough voltage-gated sodiumchannels. After an action potentialhas passed, voltage-gated potassiumchannels remain open, causingpotassium ions to diffuse out of thecell. Thus, the membrane repolar-izes shortly after the actionpotential passes. Visual

Interactive Reading AssignChapter 41 of the Holt BiologyGuided Audio CD Program to helpstudents achieve greater success inreading the chapter.

Math Skills A single neuron mayhave synapses onto as many as1,000 other neurons. Have stu-dents calculate how many neuronscould be stimulated if each of these1,000 neurons forms synapses withanother 1,000 neurons. (one millionneurons) Students should readilysee how quickly and widely nerveimpulses can travel. In reality,some of these synapses are excita-tory, while others are inhibitory.Each neuron “evaluates” theimpulses it receives, averages theinput, and responds accordingly.For example, if the overall totalinput is excitatory, the receivingneuron initiates an impulse in aneighboring cell. LogicalLS

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Dr. Emmeline Edwards, an African-Americanscientist who grew up in Haiti, studies therole that neurochemicals play in coping withstress. Information gained from her researchcan be applied toward the development ofdrugs for treating human depression. Dr. Edwards works at the Department ofPsychiatry at the State University of NewYork, Stony Brook.

MEDICINEMEDICINECONNECTIONCONNECTION

Ed:Moved ELL logo per edits,but now not to spec. OK?

Step At the resting potential, the inside of the neuron is nega-

tively charged with respect to the outside of the neuron.

The neuron is ready to conduct an action potential.

Step An action potential begins when a stimulus, such as a sig-

nal molecule, causes a local change in the membrane

potential to a more positive value. This change causes

voltage-gated sodium channels to open, and sodium ions

rapidly flow into the axon. For a brief moment, the mem-

brane potential approaches about +40 mV as the inside of

the axon becomes positively charged. This sudden local

reversal of polarity begins a chain reaction that causes

voltage-gated sodium channels to open down the entire

length of the axon. As each sodium channel opens, sodium

ions flow into the axon. The action potential conducts

rapidly down the axon toward the axon terminals.

Voltage-gated sodium channels close immediately after the action

potential has passed. Then additional voltage-gated potassium chan-

nels open, allowing potassium ions to flow out of the axon. As a

result, the membrane potential becomes negative again immediately

after the action potential. The resting potential is fully restored as

sodium-potassium pumps reestablish the original concentrations of

sodium ions and potassium ions inside and outside the axon. The

neuron cannot conduct another action potential until that time.

BIOgraphic

Sodium channel

Sodium

channel

Potassium

channel

Axon

(enlarged)

Potassium

channel

– – – – – – + + + + – – –

– – – – – – + + + + – – –

+ + + + + + – – – – + + +

+ + + + + + – – – – + + +

At the resting potential,

sodium channels are closed

and some potassium

channels are open.

An action potential moves rapidly down an axon.

Conduction of a Nerve Impulse

1

During an action

potential, sodium

channels open,

allowing sodium

ions to move into

the axon.

2

Potassium

ion, K+

Sodium

ion, Na+

Figure 3

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Using the FigureHave students identify the den-drites and the cell body, whichreceive inputs, in Figure 4. Thenhave students identify the axon ter-minals, which allow the signal topass to the next neuron in a circuit.Tell students that neurotransmitter molecules are released at the axonterminals, and they then travel toadjacent dendrites and cell bodies.An impulse can travel in only onedirection—away from the cell body of the neuron and toward itsaxon terminals.

Visual

Teaching TipSummarizing an ActionPotential Have students summa-rize in writing the events involvedin the conversion of an actionpotential into a chemical signal atthe synapse. (Answers may vary butshould include depolarization, move-ment of the action potential downthe axon, release of neurotransmittersubstance from the axon terminal,diffusion across the synaptic cleft,and binding to the post-synaptic neu-ron’s membrane.) Verbal

Using the Figure Figure 5 illustrates chemical synap-tic transmission. Neurotransmittermolecules released from a pre-synaptic neuron bind to receptorproteins on a postsynaptic cell,opening ion channels in the post-synaptic cell. Emphasize to studentsthat neurotransmitters can be eitherexcitatory or inhibitory dependingon the nature of the neuron theystimulate and the mechanism oftheir action. VisualLS

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did you know?

Neurotransmitters and Emotional HealthExtremely small amounts of neurotransmittersunderlie all of our moods, from ecstasy todepression. The neurotransmitters norepi-nephrine and dopamine are involved in feelingsof pleasure. If the levels of these neurotrans-mitters are too low in the brain, we might feeldepressed. Endorphins and enkephalins aresubstances that block pain signals. Their production increases during physical stress,

and runners who experience the increase referto the sensation as a “runner’s high.” Anothercompound, substance P, is released in responseto certain painful stimuli, such as ingestion ofcapsaicin, a chemical irritant in hot peppers.Our neurotransmitter levels react to emo-tional and physical stressors, such as ourthoughts and environment. You literally canhave an “attitude” that can affect your health.

English Language Learners

Communication Between NeuronsA junction at which a neuron meets another cell is called a

(SIHN aps), shown in Figure 4. At synapses, neurons usually do not

touch the cells they communicate with. Between an axon terminal

and a receiving cell is a tiny gap called a synaptic cleft. At a synapse,

the transmitting neuron is called a presynaptic neuron, and the

receiving cell is called a postsynaptic cell.

When a nerve impulse arrives at an axon terminal of a pre-

synaptic neuron, the impulse cannot cross the synaptic cleft.

Instead, the impulse triggers the release of signal molecules called

into the synaptic cleft. Neurotransmitter mol-

ecules are produced by neurons and are stored inside vesicles.

There are many different neurotransmitters and several mecha-

nisms of neurotransmitter action. For example, in human muscles

the principal neurotransmitter is a chemical called acetylcholine

(as ee tihl KOH leen). The brain utilizes several neurotransmitters

such as glutamate (GLOO tuh mayt) and dopamine.

Release of NeurotransmitterA nerve impulse causes a presynaptic neuron to

release neurotransmitter molecules into the synaptic

cleft. When an action potential reaches an axon

terminal of the presynaptic neuron, vesicles that con-

tain neurotransmitter molecules fuse with the cell

membrane. This releases neurotransmitter molecules

into the synaptic cleft by exocytosis. Neurotransmitter

molecules diffuse across the synaptic cleft and interact

with the postsynaptic cell. As shown in Figure 5, neu-

rotransmitter molecules bind to receptor proteins on

the postsynaptic cell. In some cells, ion channels open

when a neurotransmitter binds to these receptor pro-

teins. Such channels are called chemical-gated ion

channels; whether these channels are open or closed

depends on the binding of a chemical—in this case a

neurotransmitter molecule.

A neurotransmitter may either excite or inhibit the

activity of the postsynaptic cell it binds to. For exam-

ple, when the neurotransmitter opens chemical-gated

ion channels, ions move across the cell membrane of

the postsynaptic cell. This causes the membrane

potential of the postsynaptic cell to change depending

on the charge of the ions that move into or out of the

cell. If positively charged ions enter a postsynaptic

neuron, an action potential may be produced (excita-

tion). On the other hand, if positively charged ions

flow out of the postsynaptic neuron, or if negatively

charged ions enter the neuron, an action potential

may be suppressed (inhibition).

neurotransmitters

synapse

Dendrite

Postsynaptic

neuron

Synapses

Axon

terminal

Axons

Presynaptic

neuron

Figure 4 Synapse.

A synapse is a junction at

which signals are transmitted

between a neuron and

another cell.

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Answers to Section Review

1. A typical neuron has a bulky cell body studdedwith dendrites, which are membrane-coveredprocesses. One axon, which is a long, mem-brane-covered process, extends from the cellbody to adjacent cells.

2. When ions move across the cell membrane, therelative concentration of ions changes insideand outside the cell. This results in a change ofcharge. The difference in electrical chargeacross a membrane is the membrane potential.

3. At the synaptic cleft, the presynaptic neuronreleases a neurotransmitter substance. Theneurotransmitter diffuses across the synaptic

cleft and binds to the postsynaptic cell, whichbegins a depolarization wave in the next cell.

4. The membrane potential determines whethervoltage-gated ion channels are opened orclosed.

5. A. Incorrect. The myelin sheath is the fattyouter layer found on many neurons. B. Correct. The synapse is the junction at which two neurons communicate. C. Incorrect. A nerve is a bundle of axons.D. Incorrect. A neurotransmitter is a signalmolecule that transmits nerve impulses acrossa synapse.

ReteachingHave students use clay or anothermoldable material to build a modelof a neuron and a synapse. Askthem to label the model, showingat what points the message is elec-trical and at what points the message is chemical. Have the students use arrows to show thedirection of the impulse. Visual

Quiz1. Compare action potential and

resting potential. (At restingpotential, sodium channels areclosed; during action potential,sodium channels are open.)

2. What is membrane potential?(the difference in electrical chargeacross the cell membrane)

3. What must happen to the cellmembrane of a neuron for anaction potential to occur? (Itspotential must become lessnegative.)

AlternativeAssessmentHave each student write a questionon one side of a 335 card for atrivia-type review game of thissection. Have them write theanswer to the question on thereverse side of the card. Collectthe cards and divide the class intotwo teams. Ask the questions andaward points to each team forcorrect responses.

GENERAL

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Neurotransmitter molecules do not remain in the synaptic cleft

indefinitely. Instead, most neurotransmitter molecules are cleared

from the synaptic cleft very shortly after they are released. Many

presynaptic neurons reabsorb neurotransmitter molecules and use

them again. At other synapses, neurotransmitter molecules are bro-

ken down by enzymes or other chemicals. This happens, for example,

at the synapses between neurons and skeletal muscle cells. The reup-

take or breakdown of the neurotransmitter molecules ensures that

their effect on postsynaptic cells is not prolonged.

Axon

Direction of

action potential

Synaptic

vesicles

Ion

channel

Neurotransmitter

molecule

Axon

terminal

Synaptic

cleft

Postsynaptic

cell

Presynaptic

neuron

Receptor

proteins

When neurotransmitter

molecules are released

from a presynaptic

neuron, they either

excite or inhibit a

postsynaptic cell.

Neurotransmitter molecules are released from a presynaptic neuron,

diffuse across the synaptic cleft, and interact with a postsynaptic cell.

Figure 5 Synaptic transmission

Section 1 Review

Describe the structure of a typical neuron.

Describe how the movement of ions across the cell membrane determines the membranepotential.

Summarize the events involved in the synaptictransmission of a nerve impulse.

Critical Thinking Inferring Relationships

How does the membrane potential affect thepermeability of a neuron’s cell membrane?

The junction at which aneuron communicates with another neuron or a muscle cell is called a

A myelin sheath. C nerve.

B synapse. D neurotransmitter.

Standardized Test PrepStandardized Test Prep

949


CareerCareerMedical Imaging Technician Have studentsprepare a report on the job of a medical imagingtechnician. They should include descriptions ofthe different medical procedures these workersperform, such as computerized axial tomogra-phy (CAT), magnetic resonance imaging (MRI),and positron emission tomography (PET). Theyshould also include a description of training nec-essary and starting salary. Have them write abrief report on what they found. Ask for volun-teers to read their reports to the class.

Overview

Before beginning this sectionreview with your students theobjectives listed in the StudentEdition. This section explores thecentral nervous system, the periph-eral nervous system, the autonomicnervous system, and the somaticnervous system. The major partsand functions of the brain are dis-cussed, as well as the spinal cordand spinal reflex.

Ask students to make a list of reflexbehaviors. (Examples include blink-ing, swallowing, kneejerk, recoilingfrom a hot surface.) Have studentsindicate which, if any, of thesebehaviors require consciousthought.

Demonstration

Have a volunteer stand at the frontof the room. While talking to theclass, suddenly snap your fingersdirectly in front of the volunteer’sface. He or she will blink reflex-ively when you snap your fingers.Ask students if a person can over-come a reflex such as blinkingwhen something comes towardtheir eyes unexpectedly. (Answerswill vary.) Try snapping again a fewmore times. Ask the student volun-teer if he or she was able to controlthe blinking reflex. (Yes, if the stu-dent is expecting the noise, he or shewill probably be able to consciouslycontrol their blinking response overtime.) KinestheticLS

MotivateMotivate

Bellringer

FocusFocus

Section 2



• Active Reading GENERAL


Transparencies

TR Bellringer

TR J63 Structure of the Human Brain

Central Nervous SystemNeurons are the most important cells of the nervous system. The func-

tions of the nervous system depend on the complex interaction

between billions of neurons. Networks of neurons constantly gather,

integrate, interpret, and respond to information about the body’s inter-

nal state and environmental conditions. How are neurons organized in

the nervous system? As shown in Figure 6, there are two main divi-

sions of the nervous system—the central nervous system, shown in

orange, and the peripheral nervous system, shown in purple. The

(CNS) consists of the brain and the spinal

cord. The CNS is the control center of the body. The CNS interprets

and responds to information from

the environment and from with-

in the body. The

(PNS) contains sensory neu-

rons and motor neurons.

send information from

sense organs, such as the skin, to

the CNS. send com-

mands from the CNS to muscles and

other organs.

BrainThe is the body’s main pro-

cessing center. Encased entirely

within the skull, the brain contains

about 100 billion neurons. An aver-

age adult brain weighs about 1.5 kg

(3 lb). Thoughts, feelings, emo-

tions, behavior, perception, and

memories are controlled by your

brain. Your brain also enables you

to learn and process information,

such as the text in this book. Scien-

tists have determined the location

of various functions in the brain.

The brain consists of three major

parts, shown in Figure 7—the cer-

ebrum, the cerebellum, and the

brain stem.

brain

Motor neurons

neurons

Sensory

system

peripheral nervous

central nervous system

Section 2 Structures of theNervous System

Objectives

● Distinguish between the

central nervous system

and the peripheral nervous

system.

● Identify the major parts

of the brain and their

functions.

● Describe the structure

of the spinal cord.

● Sequence the events

of a spinal reflex.

● Compare the somatic

nervous system with

the autonomic nervous

system.

Key Terms

central nervous

system

peripheral nervous

system

sensory neuron

motor neuron

brain

cerebrum

cerebellum

brain stem

thalamus

hypothalamus

spinal cord

reflex

interneuron

Nervous System

Brain

Spinal

cord

Figure 6 Nervous system.

The central nervous system

(orange) consists of the brain

and the spinal cord. The

peripheral nervous system

(purple) branches throughout

the body.

950


Teaching TipNervous System As studentsread about the nervous system,have them construct a GraphicOrganizer similar to the one at thebottom of this page that describesthe main structures of the nervoussystem. Have them use the follow-ing terms: autonomic nervoussystem, brain, central nervous sys-tem, nervous system, peripheralnervous system, spinal cord, andsomatic nervous system.

Teaching TipMeningitis Remind students thatthe brain is encased in the skull. Inaddition, tough protective mem-branes known as meninges coverthe brain. A disease known asmeningitis results when bacteria orviruses cause inflammation of themeninges. Meningitis can be fatal ifnot treated quickly.

Using the Figure Have students review the parts ofthe brain as labeled in Figure 7.Point out that the brain is dividedinto a right and a left half, whichare connected by the corpus callo-sum. Ask students if they thinkthey are right-brained or left-brained. Although in most people,both parts of the brain worktogether, some people rely moreon one side or the other. Peoplewho rely more on their rightbrain tend to be more imagina-tive and intuitive, whereas people who rely more on their left brain tend to be more logicaland analytical.

VisualLS

TeachTeach


Answer

Answers will vary. Proponents ofhelmet laws may cite the largecost, usually borne by the public,of caring for head-injury victims.

Real Life

Ed:Moved ELL logo per edits,but now not to spec. OK?

Ed:Leveling on Real Life OK?

Cerebrum The (seh REE bruhm) is the largest part of the

brain. The capacity for learning, memory, perception, and intellec-

tual function resides in the cerebrum. The cerebrum has a folded

outer layer with many bumps and grooves. A long, deep groove

down the center divides the cerebrum into right and left halves, or

hemispheres. The cerebral hemispheres communicate through a

connecting band of axons called the corpus callosum (KOR puhs

kuh LOH suhm). In general, the left cerebral hemisphere receives

sensations from and controls movements of the right side of the

body. The right cerebral hemisphere receives sensations from and

controls movements of the left side of the body.

Most sensory and motor processing occurs in the cerebral cortex

(KOHR teks), the folded, thin (2–4 mm) outer layer of the cerebrum.

The cerebral cortex contains about 10 percent of the brain’s neu-

rons. The folded outer surface of the cerebrum is the cerebral cor-

tex, which has a large surface area. The cerebral cortex is primarily

involved with the functioning of sensory systems.

Cerebellum The (ser uh BEL uhm), which is located at

the posterior base of the brain, regulates balance, posture, and

movement. The cerebellum smooths and coordinates ongoing

movements, such as walking, by timing the contraction of skeletal

muscles. The cerebellum integrates and responds to information

about body position from the cerebrum and the spinal cord to con-

trol balance and posture.

Brain Stem At the base of the brain is the stalklike . The

brain stem is a collection of structures leading down to the spinal

cord and connecting the cerebral hemispheres with the cerebellum.

The lower brain stem consists of the midbrain, the pons, and the

brain stem

cerebellum

cerebrum Real Life

Each year, nearly 250

bicyclists die because

of brain injuries.

Wearing a bicycle helmet

reduces the risk of head

trauma by more than

70 percent.

Evaluating Viewpoints

Should there be helmet

laws for bicyclists just as

there are seatbelt laws for

automobile

drivers?

CerebrumThalamus

Corpus callosumHypothalamus

Cerebellum

Spinal cord

Midbrain

Pons

Medulla

oblongata

Upper brain

stem

Lower brain

stem

Figure 7 Brain. The

cerebrum is divided into two

hemispheres. This view shows

the right hemisphere.

951

English Language Learners

Use this graphic organizer with

Teaching Tip on this page.

Graphic Organizer

Brain

Spinal cord

Somatic

nervous system

Autonomic

nervous system

Nervous system

Central

nervous system

Peripheral

nervous system


Discussion Ask students if theythink there are structural or physicaldifferences in the brains of peoplewho are considered to be geniuses.Ask students if they think AlbertEinstein’s brain, for example, mightshow differences that indicate hisgenius. Dr. Einstein was a brilliantscientist who won a Nobel Prize inphysics in 1921. He permitted hisbrain to be preserved after his deathfor study. Scientists at McMasterUniversity, Ontario, Canada com-pared the size and shape of his brainto those of 91 men and women ofaverage intelligence and found thatEinstein’s brain was 15% wider inone region. Other studies have beendone to determine if his brain wasstructurally different from the brainsof average human brains. To date,no conclusive findings have beenmade to relate any differences inbrain structure to intelligence.

Demonstration Have students work in pairs todemonstrate reaction time. Haveone student of each pair hold aruler vertically. Have the other student position his or her fingers1 inch below the ruler, in a pinch-ing mode. (This works best if theforearm rests on a table.) Have thefirst student drop the ruler withoutwarning. The other student shouldtry to catch it between the thumband index finger. The shorter thesegment allowed to fall through thefingers, the quicker the reactiontime. KinestheticLS

GENERAL

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READINGREADING



Medical scientists think that some psychoso-matic illnesses may be mediated by thehypothalamus through its connections to theautonomic nervous system and the endocrinesystem. Many stress-related illnesses, such asulcers, asthma, and high blood pressure, areclearly connected to hypothalamic functions.


MISCONCEPTION

ALERT

Brain Size Students may believe that mod-ern Homo sapiens have the largest brains ofany hominid in history. Remind students thatNeanderthals had larger brains than those ofsome modern humans. The brains of modernhumans are much more convoluted thanthose of Neanderthals. This suggests thatintelligence may be related more to theextent of the convolutions (which is relatedto the area of the cerebral cortex) than tooverall brain size.

medulla oblongata (mi DUHL uh ahb lahn GAHT uh). These struc-

tures relay information throughout the CNS and play an important

role in homeostasis by regulating vital functions, such as heart rate,

breathing rate, body temperature, and sleep.

The upper brain stem contains important relay centers that

direct information to and from different parts of the brain. The

(THAL uh muhs) is a critical site for sensory processing.

Sensory information from all parts of the body converges on the

thalamus, which relays the information to appropriate areas of the

cerebral cortex. Below the thalamus, at the base of the brain, is the

hypothalamus. The , along with the medulla oblon-

gata, helps regulate many vital homeostatic functions, such as

breathing and heart rate. The hypothalamus is responsible for feel-

ings of hunger and thirst. It also regulates many functions of the

endocrine system by controlling the secretion of many hormones.

The thalamus and hypothalamus are linked to some areas of the

cerebral cortex by an extensive network of neurons called the limbic

system. The limbic system includes structures of both the brain stem

and the cerebrum. The limbic system has an important role in mem-

ory, learning, and various emotions, such as pleasure and anger.

Spinal CordThe , shown in Figure 8, is a dense cable of nervous tis-

sue that runs through the vertebral column. The spinal cord extends

from the medulla oblongata through the vertebrae to a level just

below the ribs. The spinal cord links the brain to the PNS. The brain

receives information that travels upward through the spinal cord.

Through the spinal cord, the brain also sends commands that con-

trol the rest of the body. In addition to relaying messages, the spinal

cord functions in reflexes. A is a sudden, involuntary con-

traction of muscles in response to a stimulus.

reflex

spinal cord

hypothalamus

thalamus

Interpreting Graphics

As you look at Figure 8,

think of the spinal cord as

a busy two-way highway

with sensory traffic going

north and motor traffic

going south.

Sensory

input

Back of body

Front of body

Cross section of spinal cord

Motor

outputGray matterNerve

fibers

Ventral

root

Dorsal

root

ganglion

White matter

Spinal

nerve

Spinal

nerve

Spinal nerves have a dorsal root and a ventral root that diverge as they enter the spinal cord.

Figure 8 Spinal cord

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Teaching Tip Encourage interested students touse the Internet Connect box onthis page to find out more aboutcurrent research into the cure forspinal cord injury. Have studentsgive a brief oral report to the classpresenting what they find.

VerbalLS


Spinal Cord Injury

Teaching StrategiesBring in pictures of ChristopherReeve and other people whohave paralysis as a result ofspinal cord injuries. If the infor-mation is available, talk abouthow they were injured. Wereany of the injuries preventable?

Discussion

• Why does paralysis occur?(Nerve impulses cannot cross the site of injury in thespinal cord.)

• Is it possible for a person tohave arms but not legs para-lyzed as the result of a spinalcord injury? Explain youranswer. (No, because every-thing below the spinal cordinjury is affected. If the injuryis above the spinal cord areathat controls the arms, all fourlimbs are affected.)

• Would you volunteer to be atest case for the methodsdescribed if you had a spinalcord injury? Why or whynot? (Answers will vary.)

MISCONCEPTION

ALERT

Brain Tumors Because most neurons donot undergo mitosis in adult humans,students may think that a brain cannotdevelop a tumor. Brain tumors, however,are not formed from neurons. They arisefrom the supportive glial cells in the nerv-ous system, which do undergo mitosis.

Transparencies

TR J64 Structure of the Human Spinal Cord

TR J66 Knee-Jerk Reaction

TR J67 Physiological Effects of theAutonomic Nervous System

The spinal cord is linked to the PNS through 31 pairs of spinal

nerves. The spinal nerves, which branch from the spinal cord, carry

information to and from the CNS. Spinal nerves in the upper part

of the spinal cord branch into the arms and upper body, and spinal

nerves in the lower part of the spinal cord branch into the legs and

lower body. Each spinal nerve has a dorsal root and a ventral root.

Dorsal roots contain sensory neurons, which carry information

from areas of sensory input to the CNS. Ventral roots contain motor

neurons, which carry motor responses from the CNS to muscles,

glands, and other organs. As shown in Figure 8, dorsal and ventral

roots come together to form the spinal nerves near the spinal cord.

The spinal cord contains a core of gray matter covered by a

sheath of white matter, as shown in Figure 8. Gray matter contains

the cell bodies of neurons, whereas white matter contains the axons

of neurons. Included in the gray matter are , which

link neurons to each other.

interneurons

Spinal Cord Injury

Unlike most other parts of the

body, the spinal cord does

not heal after an injury. Damaged

neurons stop conducting nerve

impulses at the site of injury, per-

manently paralyzing the legs or,

in some cases, all four limbs.

Every year, spinal cord

injuries—whether incurred in

athletics or automobile acci-

dents—leave nearly 15,000

Americans partially or totally par-

alyzed. In 1995, actor Christo-

pher Reeve, shown in the photo

at right, injured his spinal cord

after falling headfirst from a

horse. The fall broke vertebrae in

Reeve’s neck, paralyzing him

from the neck down.

A treatment currently available

for people with spinal cord

injuries is an anti-inflammatory

drug called methylprednisolone.

If given within 8 hours after the

spinal cord is injured, the drug

can improve chances of recov-

ery. Even with this drug, how-

ever, recovery is usually far from

complete.

Stopping Cell Death

Cells continue to die near the

site of a spinal cord injury for

several weeks after injury

occurs. Myelin-producing cells

die, leaving neurons in the spinal

cord unable to function. Some

scientists think that stopping the

death of these cells could help

avoid paralysis. In experiments

on rats, researchers have found

that a cell-death inhibitor

improves the rats’ ability to use

their hind legs after a spinal cord

injury. Researchers are investi-

gating other cell-death inhibitors

that could be used on humans.

Bridging the Gap

After the spinal cord is injured,

damaged axons begin to regrow.

However, their growth is inhib-

ited by substances in the spinal

cord. Peripheral nerves lack

these substances, so the axons

in these nerves can regrow quite

well. To stimulate the growth of

axons in the injured spinal cord,

researchers have grafted pieces

of peripheral nerves into the

spinal cord. The nerve grafts

provide tunnels for regrowing

axons. Rats with such nerve

grafts begin to show signs of

recovery within 3 weeks. Within

a year, they can support their

own weight. Similar grafts have

not yet been tried on humans.

www.scilinks.org

Topic: Spinal Cord

Keyword: HX4168

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Paired Summarizing Have stu-dents choose a partner. Have bothpartners read silently about theperipheral nervous system on thispage and the facing page. Thenhave one student summarize aloudwhat has been read without refer-ring to the textbook. The partnershould listen without interruptingand be prepared to point out anyinaccuracies or omissions in thesummary. Students may refer tothe text to verify the facts. Thenhave students switch roles.

InterpersonalLS

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READINGREADING


Referred Pain Referred pain is pain thatoriginates in a location different from where itis felt. Referred pain is important in clinicaldiagnoses. For example, inadequate oxygena-tion of the heart muscle often results in painbeing referred to the chest wall and shoulder.Inflammation of the appendix, in the lowerright quadrant of the abdomen, is sometimesfelt as pain in the navel area.


Relate membrane, resting, and action potentials topotential difference. Potential difference is the changein the electrical potential energy of a charged particledivided by its mass and is measured in volts (V). Inaddition, remind students that electrical potential ener-gy is increased if a force pushes electrical charges inthe direction opposite the electric force. Check theirunderstanding by asking what biological mechanismpushes electrical charges opposite the electric force.(the sodium-potassium pumps.)

Integrating Physics and Chemistry

StrategiesStrategiesINCLUSIONINCLUSION

Have students work with a partner to make observations ofhow the body keeps a sense ofbalance. With the partner stand-ing perfectly still, feet together,and his or her eyes closed for oneminute, have the student observefrom the side to see how muchthe subject wavers back andforth. Next, observe the subjectfrom behind to see if there is anywavering from side to side. Writethese observations down. Repeatthe test after switching roles.Students may report to the classtheir findings.

• Attention Deficit Disorder• Learning Disability

Peripheral Nervous SystemThe peripheral nervous system connects the brain and the spinal cord

to the rest of the body. In addition to the 31 pairs of spinal nerves, 12

pairs of cranial nerves connect the brain with areas in the head and

neck. The PNS contains two principal divisions—the sensory division

and the motor division. The sensory division directs sensory informa-

tion to the central nervous system. The motor division carries out

responses to sensory information. The motor division of the PNS con-

sists of two independent systems—the somatic nervous system and

the autonomic nervous system.

Somatic Nervous SystemMost motor neurons that stimulate skeletal muscles are under our

conscious control. These neurons are part of the somatic nervous

system. Some activity in the somatic nervous system, such as spinal

reflexes, is involuntary. A spinal reflex is a self-protective motor

response. Spinal reflexes are extremely rapid because they usually

involve the spinal cord but do not involve the brain.

The knee-jerk reflex, shown in Figure 9, is an example of a spinal

reflex. When the ligament below your kneecap is tapped, your lower

leg suddenly kicks forward. Tapping the ligament stimulates a sensory

neuron, shown in red. The sensory neuron sends a nerve impulse to

the spinal cord and excites a motor neuron, shown in green, which

causes the quadriceps to contract. This causes the leg to extend

rapidly. The sensory neuron also stimulates an interneuron, shown in

blue. The interneuron inhibits a motor neuron that would normally

cause the hamstrings to contract, allowing the hamstrings to relax.

Quadriceps

Patellar

ligament

Hamstrings

Sensory

neuron

Dorsal

root

Spinal

cord

Ventral root

Motor

neuron to

quadriceps

Motor

neuron to

hamstrings

Interneuron

Patella

(kneecap)

When the ligament below the patella is tapped, the quadriceps contracts,

the hamstrings relax, and the leg rapidly extends.

Figure 9 Knee-jerk reflex

Interpreting Graphics

As you look at Figure 9,

notice that in a spinal reflex,

motor neurons stimulate

muscles in the same region

in which the stimulus that

caused the reflex originated.

954



1. The two main divisions of the nervous systemare the central nervous system and the periph-eral nervous system. The central nervous system,consisting of the brain and spinal cord, is thecontrol center of the body. The peripheralnervous system connects the brain and spinalcord to the rest of the body.

2. The cerebellum regulates balance and the timing of movement, while the brain stem regulates vital functions such as breathing,heart rate, body temperature, and sleep.

3. Dorsal roots contain sensory neurons; ventralroots contain motor neurons.

4. The parasympathetic division is more active.

5. A spinal reflex travels only as far as the spinalcord, whereas a voluntary movement involvesthe brain and therefore takes longer than aspinal reflex.

6. A. Incorrect. The patella does not change. B. Incorrect. The hamstrings will relax if thequadriceps contract. C. Incorrect. The quadri-ceps will elongate if the hamstrings contract. D. Correct. For the knee to jerk, the quadricepsmust contract.

ReteachingOn the chalkboard or overheadprojector, list the major conceptspresented in this section. Have theclass review the text to identifyexamples that can be used to illus-trate each of these concepts.

Quiz1. What might be the result from

damage to the cerebellum? (lossof balance)

2.What controls the “fight orflight” response? (sympatheticdivision of the autonomic nervoussystem)

3. What do the hypothalamus andthe medulla oblongata regulate?(vital homeostatic functions)

AlternativeAssessmentHave pairs of students take turnslightly tapping each other’s legslightly below the knee. If possible,have them use a rubber-headedhammer borrowed from the schoolnurse. Ask students to each write abrief summary of their observationsand the cause of the effect produced.Tell them that their summary mustinclude the following terms: reflex,axon, synapse, motor neuron, sensory neuron, spinal cord.

GENERAL

GENERAL

CloseClose


Autonomic Nervous SystemPeripheral motor neurons that regulate smooth muscles do not

require our conscious control. These neurons are part of the auto-

nomic nervous system, which regulates heart rate and blood flow

by controlling contractions of cardiac muscle in the heart and

smooth muscle lining the walls of blood vessels. It also controls

muscles in the digestive, urinary, respiratory, and reproductive sys-

tems, as well as the secretions of many glands.

Two divisions of the autonomic nervous system—the parasympa-

thetic division and the sympathetic division—maintain stability in the

body by counterbalancing each other’s effects. The parasympathetic

division is most active under normal conditions. It keeps your body

functioning even when you are not active. For example, you continue

to breathe when you fall asleep.

The sympathetic division dominates in times of physical or emo-

tional stress. It controls the “fight-or-flight” response that you

experience during a stressful situation, such as “nervousness” when

taking a pop quiz. The sympathetic division increases blood pres-

sure, heart rate, and breathing rate. It also directs blood flow toward

your heart and skeletal muscles. Effects of the autonomic nervous

system are summarized in Table 1.

www.scilinks.org

Topic: AutonomicNervous System

Keyword: HX4017

Table 1 Physiological Effects of the Autonomic Nervous System

Organ Effect of sympathetic division Effect of parasympathetic division

Eyes Pupils dilate Pupils constrict

Heart Heart rate increases Heart rate decreases

Lungs Bronchioles dilate Bronchioles constrict

Intestines Gastric secretions decrease Gastric secretions increase

Blood vessels Blood vessels dilate Little or none

Name the two main divisions of the nervoussystem, and state their general functions.

Compare the functions of the cerebellum andthe brain stem.

Distinguish between dorsal roots and ventralroots of the spinal cord.

Name the division of the autonomic nervous system that is more active under normal conditions.

Critical Thinking Comparing Functions

Why is a spinal reflex more rapid than a voluntarymovement?

A sudden stretch of thequadriceps muscle triggers the knee-jerk reflex,which maintains homeostasis by causing the

A patella to elongate.

B hamstrings to contract.

C quadriceps to elongate.

D quadriceps to contract.


Section 2 Review

955


OverviewBefore beginning this sectionreview with your students theobjectives listed in the StudentEdition. This section explores thesenses. The structures and mecha-nisms of touch, vision, hearing,taste, and smell are described.

Ask students each to select anobject in the room. Ask them topoint to a specific place on theobject with their index finger.Direct them to focus on that place,first with both eyes open, thenwith just the left eye closed, andfinally with just the right eyeclosed. Have students write downwhat happens to the image andoffer an explanation. (Each eyeregisters an image that is slightlydifferent from the other. When botheyes are used, the brain combines theimages to create a new image.)

ActivityPhosphenes Have students closetheir eyes and press lightly on theireyelids. Ask what they see. (patternsof colors) These patterns are calledphosphenes. The nerves in the eyessend messages to the brain, whichinterprets these messages as colors.Then ask what this phenomenontells them about the sense of sight.(The eyes respond to visual stimuli.The brain is interpreting pressurestimuli as visual stimuli.) VisualLS

GENERAL

MotivateMotivate

Bellringer

FocusFocus

Section 3



• Active Reading

• Data Sheet for Quick Lab GENERAL

GENERAL


Transparencies

TR Bellringer

TR J68 Types of Sensory Receptors

TR J62 Cortex

Remind students that electric circuits—likenerve pathways—usually have a switch, whichopens and closes the path in order to stop orpermit the flow of charges through the electriccurrent. Whereas the switch is often amechanical device in an electric circuit, it isthe synapse for nerve pathways.


Section 3 Sensory Systems

Perception of StimuliThe perception of everything you respond to in the environment,

such as the horn of a passing car or cold rain on your face, is made

possible by sensory systems. Sensory systems are essential to sur-

vival, and they enable us to experience both pleasurable and painful

stimuli. Sensory systems help maintain homeostasis by constantly

adjusting body conditions to respond to changes in the environ-

ment. This requires the integration of the peripheral nervous system

and the central nervous system. The sensory division of the PNS col-

lects information about sensory stimuli in and around the body. The

sensory information is sent to the brain, which processes the infor-

mation and, if necessary, generates a motor response to the stimuli.

How does the nervous system detect sensory stimuli? Specialized

neurons called detect sensory stimuli and then

convert the stimuli to electrical signals, in the form of nerve impulses,

that can be interpreted by the brain. Although sensory receptors are

located throughout the body, they are most concentrated in the sense

organs—the eyes, ears, nose, mouth, and skin. Table 2 lists several

types of sensory receptors and some of their locations.

Sensory ReceptorsMechanoreceptors throughout the body respond to physical stimuli—

such as pressure and tension—that cause distortion or bending of

tissue. These stimuli alter the electrical activity of mechanoreceptors.

Many mechanoreceptors are found in the skin, and they are concen-

trated in very sensitive areas, including the face, hands, fingertips,

and neck. Pain receptors, which respond to potentially harmful

stimuli—such as intense heat or cold and tissue damage—are respon-

sible for painful sensations. Pain is a very important sensation

sensory receptors

Objectives

● List five types of sensory

receptors and the stimuli to

which they respond.

● Identify sites of sensory

processing in the brain.

● Analyze the structure of

the eye and its role in the

visual system.

● Describe how the ear

detects sound and helps

maintain balance.

● Compare the senses of

taste and smell.

Key Terms

sensory receptor

retina

rod

cone

optic nerve

cochlea

semicircular canal

www.scilinks.org

Topic: Sensory Receptors

Keyword: HX4162

Table 2 Types of Sensory Receptors

Receptor type Stimuli Locations

Thermoreceptors Temperature change Skin, hypothalamus

Pain receptors Tissue damageAll tissues and organs

except the brain

MechanoreceptorsMovement, pressure,

Skin, ears, musclestension

Photoreceptors Light Eyes

Chemoreceptors Chemical Tongue, nose

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Phineas Gage On September 13, 1848, anaccidental explosion on a Vermont railroadcaused an extremely unusual injury. A railwayforeman named Phineas Gage was settingexplosive charges with a tamping iron, aheavy iron rod. One of the charges exploded,sending the tamping iron through the left sideof his skull. It entered under his left cheek-bone, exited through the top of his head, andlanded 25 yards away. Most of the left frontallobe of his brain was destroyed, but it is said

that Gage never even lost consciousness. After seven months of recuperation, Gage waswell enough to return to work. His physicaland intellectual functions were unchanged,but his personality was completely different.Once polite and patient, he became profane,impatient, and rude, and was unable toresume his work as a foreman. His horrificinjury, however, greatly influenced physiciansand scientists who were attempting to deter-mine how function is localized in the brain.

Teaching TipTouch Receptors Tell studentsthat although touch receptors arespread throughout the skin, someareas have a larger concentrationof touch receptors than others. For example, lips and hands havelarge numbers of touch receptorssituated close together. It is easy todistinguish two points at very closedistances on skin covering the lipsand hands. However, on the backthere are areas where touch recep-tors are several millimeters apart.Tell students that they may not beable to distinguish two differentpoints that are touched on theirbacks because of this.

Activity Visual Feedback and TouchHave students work in pairs. Onestudent should hold his or herarms out and crossed so that thepalms meet, and the fingers areinterlocked. Have students bendtheir elbows and move the handsdown and in toward the body.Keep the elbows bent, and bringthe hands close to the body, twist-ing the hands upward. Have thepartner point to a finger withouttouching it. The student should tryto move that finger quickly. Askstudents to share what happens.(The student will have difficultyidentifying and moving the fingerindicated by his or her partner. Thisis because the eyes confirm bodypositions that are monitored by positional receptors, called proprio-ceptors. If the hands are in anunusual position, proprioceptors and the eyes send conflicting mes-sages to the brain.) Ask students ifit is easier to identify the finger ifthe partner first touches that finger.(Yes, because touch receptors giveadditional information to the brain.)

KinestheticLS

GENERAL

TeachTeach




Planner CD-ROM

because it informs you that something is wrong in your body. Many

self-protective responses, such as reflexes, are initiated by pain

receptors. Thermoreceptors, located in the skin and hypothalamus,

detect changes in temperature. Thermoreceptors play an important

role in homeostasis, helping to keep the body temperature within

its normal range.

Sensory receptors are located throughout the body, and sensory

input from these receptors enters the central nervous system in an

organized fashion. Sensory stimuli that originate in the lower body

enter the lower part of the spinal cord. Sensory stimuli that origi-

nate in the upper body enter the upper part of the spinal cord and

the brain.

Processing of Sensory InformationRecall that the cerebral cortex contains a large percentage of the

brain’s neurons. Many of the neurons in the cerebral cortex are

responsible for processing incoming sensory information from the

sense organs. The thalamus relays information from the sense

organs to certain regions of the cerebral cortex. As shown in

Figure 10, deep grooves divide the cerebral hemispheres into four

general areas, or lobes: the occipital (ahk SIP ih tuhl) lobe, the pari-

etal (puh RIE uh tuhl) lobe, the temporal lobe, and the frontal lobe.

Sensory neurons from the different sense organs come together at

certain regions in the cerebral cortex. For example, most visual proc-

essing takes place in the occipital lobe, located at the back of the

head. Similarly, processing of sound is carried out within the

temporal lobe.

Frontallobe

Frontal lobe Parietallobe

Parietal lobe

Vision

Occipital lobe

Cerebellum

Side view of brain(left hemisphere)

Righthemisphere

Lefthemisphere

Top view of brain

Smell

Speech

Taste

Hearing

Occipitallobe

Motor function

Intellectual function

Sensory function

Temporallobe

Specific areas of the cerebral cortex control different functions of the body.

Figure 10 Processing sites and lobes of the cerebral cortex

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Demonstration

You will need a cardboard tubefrom a roll of paper towels for thisdemonstration. Tell a student vol-unteer to keep both eyes open, andto place the tube over his or herleft eye. Have the student hold hisor her right hand next to the endof the tube, and move it slowlyalong the tube toward his or herface. Ask the student what he orshe observes. (It should appear asthough the tube passes through thepalm of the hand. The brain auto-matically combines the images seenwith both eyes into one picture.)

VisualLS

GENERAL


Tell students that if they stare at a picture longenough and then look at a white wall, theywill see the same image shapes, but with dif-ferent colors. Give students pieces of blackconstruction paper with large blue spots in themiddle. After they have stared at the blackpaper for a minute, have them look at a whitewall. Ask students what color spot they see onthe white wall. (Students should see a red spot.)


DemonstratingYour Blind Spot

Skills AcquiredSummarizing, drawingconclusions, applyinginformation

Teacher’s NotesMake sure students are closingtheir right eyes and are lookingat the O. It should disappearabout 1 ft from the end of thenose. Try the lab with the Xand the left eye as well.

Answers to Analysis1. rods and cones

2. There are no photoreceptors(rods and cones) at that spot.

3. The X disappears when it is inthe part of the visual fieldmediated by the part of theretina where there are no photoreceptors.

did you know?

Glaucoma Glaucoma is an eye disease inwhich increased pressure inside the eye causesdamage to the retina and the optic nerve. Thecause of the increased pressure is a buildup ofthe aqueous humor, the fluid interior of theeye. Glaucoma is the second most commoncause of blindness.

Transparencies

TR J72 Structure of the Eye

TR J69 Anatomy of the Ear

Eyes

Humans have very good eyesight. Our eyes enable us to see in color

and to distinguish fine details and movement. The structure of the eye

is shown in Figure 11. Light enters the eye through the pupil. Light

then passes through the lens, a thick, transparent disk that focuses

light on the retina. The is a lining on the back inner surface of

the eye that consists of photoreceptors and neurons. The retina con-

tains two types of photoreceptors—rods

and cones—which convert light energy to

electrical signals that can be interpreted

by the brain. respond best to dim

light. respond best to bright light

and enable color vision. The retina also

contains many other neurons that process

visual information. The axons of some of

these neurons make up the .

The optic nerve exits through the back of

the eye and runs along the base of the

brain to the thalamus. The thalamus then

relays visual information to the occipital

lobe of the cerebral cortex, where the

information is processed.

optic nerve

Cones

Rods

retina

Optic nerve

Lens

Cornea

Pupil

Iris

Retina

Figure 11 Structure of

eye. Light enters the eye

through the pupil and is

focused on the retina, which

contains photoreceptors.

The blind spot in your visual field corresponds to the site

where the optic nerve exits the back of the eye. There are

no photoreceptors at the site where the optic nerve exits.

Use the procedure below to demonstrate your blind spot.

Demonstrating Your Blind Spot

Procedure

1. On the index card, draw an X

about 1 in. from the left side

of the card. Draw an O about

the same size 3 in. to the

right of the X.

2. Hold your index card in front

of you at arm’s length. Close

your right eye and stare at the

O with your left eye. Slowly

move the card toward you

while continuing to stare at

the O until the X disappears

from view.

Analysis

1. Name the two kinds of

photoreceptors found in

the retina.

2. Propose why you cannot

see images that fall on the

site where the optic nerve

exits the eye.

3. Critical Thinking

Relating Concepts What

is the relationship between

the structure of the retina and

the disappearance of the X on

the index card?

Materials

unlined 3 � 5 index card, pencil

Retina

958


Demonstration Show the class a chart used to testpeople for colorblindness. Determineif any students are colorblind.Point out that colorblindness iscaused by a chemical disorder inthe cones of the eye. Complete colorblindness is extremely rare. A deficiency in the red or greencones is most common, affectingabout 5 percent of the population.

Visual

Teaching TipMeasuring Sound Play recordedmusic at a volume high enough tobe disconcerting to an average listener. Tell students that the loudness of sound is measured indecibels, ranging from a sound of 0 decibels (dB), which is just audi-ble, to a sound of 140 dB, which ispainful. A jet taking off can pro-duce a sound of 150 dB and causehearing loss. This is why peoplewho work on the tarmac at air-ports wear protective earplugs and headsets.

LS


did you know?

The “Five” Senses Students have heard ofthe five senses: hearing, smell, taste, sight, andtouch. But these are misleading categories.Touch actually consists of four distinct typesof sensory receptors: hot, cold, pressure, andpain. Another sense, equilibrium or balance, isequally important but not usually discussed interms of the senses. Equilibrium is maintainedthrough fluids in the semicircular canals of theear. Complex sensory organs are responsiblefor some of the senses, such as hearing andsight. Other senses are the result of small clus-ters of receptors, such as taste buds.

When light passes through the lens of an eye, lightwaves are bent and focused on the retina. Review thefollowing properties of light: polarization, reflection,and refraction. Ask students to identify and describewhich property of light occurs as light passes throughthe lens.


StrategiesStrategiesINCLUSIONINCLUSION

Working with a partner, have students make observations onsound detection. With the subjectseated and eyes closed, have theother student use a clicker forthe sound test. The studentshould “click” the clicker at dif-ferent points around the studentand record where the subjectdetected the sound origination.After recording observations andswitching roles, students shouldcompare the observation data.They may report their findingsto the class.

• Attention Deficit Disorder• Learning Disability

EarsHow do your ears enable you to hear? Your ears convert the energy

in sound waves to electrical signals that can be interpreted by your

brain. Figure 12 shows the structure of the ear. Sound waves enter

the ear through the ear canal and strike the tympanic (tim PAN ik)

membrane, or eardrum, causing it to vibrate. Behind the eardrum,

three small bones of the middle ear—the hammer, anvil, and

stirrup—transfer the vibrations to a fluid-filled chamber within the

inner ear. This chamber, called the (KAHK lee uh), is coiled

like a snail’s shell, and it contains mechanoreceptors called hair

cells. Hair cells rest on a membrane that vibrates when waves enter

the cochlea. Waves of different frequencies cause different parts of

the membrane to vibrate and thus stimulate different hair cells.

When hair cells are stimulated, they generate nerve impulses in

the auditory nerve. The impulses travel to the brain stem through the

auditory nerve. The thalamus then relays the information to the

temporal lobe of the cerebral cortex, where the auditory information

is processed.

Keeping Your BalanceThe ears not only enable you to hear but also help you maintain equi-

librium. The are fluid-filled chambers in the

inner ear that contain hair cells. Clusters of these hair cells respond

to changes in head position with respect to gravity. When your head

moves, the hair cells are stimulated according to the magnitude and

direction of the fluid’s movement, and they send electrical signals to

the brain. Signals generated by the hair cells enable the brain to

determine the orientation and position of the head.

semicircular canals

cochlea

Figure 12 Structure of ear.

Sound waves are transmitted to

the inner ear and are detected

by mechanoreceptors. The

semicircular canals detect the

position of the head.

Ear

canal

Semicircular

canals

Cochlea

Inner earMiddle ear

Tympanic

membrane

(eardrum)

HammerAnvil

Auditory nerve

Stirrup

Outer ear

The word cochlea is from

the Greek kochlias, meaning

“snail shell.”

959


ReteachingAssign students to cooperativegroups of four. Have each groupchoose the sensory system theywould least like to lose. Instructeach group to defend its choice.

Quiz1. List the four lobes of the brain.

(occipital, parietal, temporal,frontal)

2.What structures in the ear areresponsible for equilbrium?(semicircular canals)

3. What is a sensory receptor? (aneuron that sends informationfrom the sense organ to the centralnervous system)

AlternativeAssessmentAsk students to prepare a diagramthat traces a sound from its originto the brain. Make sure that stu-dents use the following key termsin their diagram: source, tympanicmembrane, auditory nerve, anvil,stirrup, cochlea, ear canal, tempo-ral region of the cerebral cortex,mechanoreceptors, and hammer.

GENERAL

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1. Answers may vary. Sample answers: painreceptors respond to potentially harmful stim-uli; thermo-receptors respond to temperaturechanges; and mechano-receptors respond topressure and tension.

2. Light enters the eye through the pupil. The lensfocuses light on the retina. Rods and cones onthe retina convert light into electrical signalssent along the axons of the optic nerve, whichruns to the thalamus, and finally to the occipi-tal lobe of the cerebrum where the informationis processed.

3. Sound waves enter the ear and strike thetympanic membrane, making it vibrate. The

vibrations pass through the hammer, anvil, andstirrup to the cochlea where mechanoreceptorssend impulses via the auditory nerve to thetemporal lobe of the cerebrum.

4. Chemicals in food are detected by one or moreof four kinds of taste receptors on the tongue.Olfactory receptors bind chemicals in the air.

5. A. Incorrect. Odors are detected by theolfactory receptors. B. Incorrect. The cone cellsdetect colors. C. Correct. The hair cells in thecochlea transfer sound waves into nerveimpulses. D. Incorrect. Tastes are detected bythe taste buds.


Chemical SensesEmbedded within the surface of the tongue are 2,000–5,000 taste

buds. Most taste buds are located within small projections on the

surface of the tongue. A taste bud, shown in Figure 13, is a cluster

of 50–100 taste cells. Taste cells are chemoreceptors that detect at

least four basic chemical substances: sugars (sweet), acids (sour),

alkaloids (bitter), and salts (salty). Each taste cell is generally sen-

sitive to all tastes but is most sensitive to only one of them. A taste

bud is stimulated when food molecules dissolved in saliva bind to

taste cells. Taste cells generate electrical signals that can be inter-

preted by the brain.

Chemoreceptors that detect odors, called olfactory (ahl FAK tuh

ree) receptors, are located in the roof of the nasal passage. Chemicals

in the air stimulate olfactory receptors, which generate electrical sig-

nals that can be interpreted by the brain. Your sense of smell affects

your enjoyment of food. When you have a bad cold and your nose is

stuffed up, your food may seem to have little taste.

www.scilinks.org

Topic: Chemical Senses

Keyword: HX4041

Surface of tongue

Taste buds

Sensory

neurons

When food molecules dissolve in

saliva, they enter taste pores and bind

to taste cells.

Taste

poreTaste

cells

A taste bud is a cluster of taste cells surrounding a taste pore.

Figure 13 Location and structure of taste buds

List two different types of sensory receptors andthe kinds of stimuli to which they respond.

Sequence the events that occur when lightenters the eye.

Describe how sound waves are transmittedthrough the ear.

Critical Thinking Comparing Structures

Distinguish between taste cells and olfactoryreceptors.

A person who had defectsin both cochleas likely would be unable to detect

A odors. C sounds.

B colors. D tastes.


Section 3 Review

960


Section 4

Overview

Before beginning this sectionreview with your students theobjectives listed in the StudentEdition. This section exploresexamples of psychoactive drugs,addiction, and some commonlyabused drugs.

Ask students to list on paper asmany physical and social effects ofsmoking cigarettes as they can.Have them compare their list tothe text on page 965, Effects ofTobacco.

Demonstration

Demonstrate the effect of second-hand smoke. Put several smokingcigarettes in an ashtray. Caution:Burn cigarettes only under a venthood. Place a piece of glassapproximately 5 cm (2 in.) awayfrom the cigarettes. After the ciga-rettes have burned completely,wipe the glass with a white clothand show the cloth to the class.

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GENERAL

MotivateMotivate

Bellringer

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• Active Reading GENERAL




• Occupational Applications WorksheetPharmacistForensic Toxicologist GENERAL

Planner CD-ROM

Transparencies

TR Bellringer

Psychoactive DrugsMany different kinds of drugs are available to the public. Adver-

tisements tell you about pain relievers, antacids, cough syrups, and

other medications that can help you feel better. Drugs can prevent,

treat, or cure many different illnesses. However, drugs, whether

legal or illegal, can also be misused or abused.

In the broadest sense, a drug is a chemical that alters body struc-

tures or biological functions. Drugs that alter the functioning of the

central nervous system are known as . Many

medications, such as those prescribed by doctors to treat mental

disorders, contain psychoactive drugs. Caffeine, found in coffee and

soft drinks, is also a psychoactive drug. Alcohol, marijuana, and

cocaine are examples of commonly abused psychoactive drugs. Psy-

choactive drugs also include many other substances, such as

inhalants. Many psychoactive drugs produce physiological depen-

dence and addiction. Abuse of psychoactive drugs can damage the

body, and in some cases, can result in death. Table 3 lists several

classes of commonly abused psychoactive drugs.

psychoactive drugs

Drugs and theNervous System

Section 4

Objectives

● Identify types of psychoac-

tive drugs, and describe their

effects.

● Describe how drug

addiction develops.

● Describe effects of com-

monly abused drugs on the

nervous system.

Key Terms

psychoactive drug

addiction

tolerance

withdrawal

stimulant

depressant

Drug Examples Psychoactive effects Risks associated with use

DepressantsBarbiturates (sedatives), Decreased activity of the Drowsiness, depression, brain or nerve

tranquilizers, alcohol central nervous system damage, coma, respiratory failure

StimulantsCocaine, crack, Increased activity of the Aggressive behavior, paranoia, cardiac

nicotine, amphetamines central nervous system arrest, high blood pressure, brain damage

Nitrous oxide, ether, paint Disorientation, confusion, Brain damage, kidney and liver

Inhalants thinner, glue, cleaning memory loss damage, respiratory failure

fluid, aerosols

LSD, PCP, MDMA (ecstasy), Sensory distortion, anxiety, Depression, paranoia,

Hallucinogens peyote (mescaline),hallucinations, numbness aggressive behavior

psilocybe mushroom

THC Marijuana, hashishShort-term memory loss, Lung damage, loss

impaired judgment of motivation

Heroin, morphine,Feeling of well-being, seda-

Narcoticscodeine, opium

tion, impaired sensory per- Coma, respiratory failure

ception, impaired reflexes

Table 3 Psychoactive Drugs of Abuse

961


Using the Figure Emphasize that addiction is psy-chological as well as physiological.Ask students to study Figure 15.Have them explain the relationshipbetween the craving for an addic-tive drug and the changes in thenumber of receptor proteins onpostsynaptic cells. Visual

Writing Skills A number ofpotentially addictive substances,including opiates and cocaine, havenow been linked to specific recep-tors in the brain. Have studentsresearch how the work of neuro-chemist Solomon H. Snyder led toa greater understanding of howpotentially addictive substancesaffect the operation of receptors inthe brain. Have students writereports that describe the evidencethat led Snyder to infer the exis-tence of endorphins, the naturallyoccurring opiate-like substances inbrain tissue. VerbalLS

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did you know?

Dangerous Street Drugs A street drugcalled “croak” results from the mixture ofmethamphetamines and crack cocaine.Methamphetamines stimulate the release ofnorepinephrine and dopamine. Crack cocaineprevents the reabsorption of dopamine.Extremely high heart rates and blood pressurecan result when large amounts of dopamineenter and remain in synapses. Drug dealersoften mix deadly combinations of drugs tomake more money, to keep the users“hooked,” and in some cases to fool the usersinto thinking they are using a “pure” drug.

Drug Addiction and Neuron Functionis a physiological response caused by use of a drug that

alters the normal functioning of neurons and synapses. Once a

neuron or synapse has been altered by a drug, it cannot function

normally unless the drug is present. With repeated exposure to a

drug, a person addicted to the drug develops tolerance to the drug.

is a characteristic of drug addiction in which increasing

amounts of the drug are needed to achieve the desired sensation.

is a set of emotional and physical symptoms caused by

removal of the drug from the body. The severity of drug addiction is

evident in recovering addicts who experience withdrawal when they

stop taking an addictive drug. Symptoms of withdrawal may include

vomiting, headache, depression, and seizures. Withdrawal from bar-

bituates, and withdrawal in cases of severe alcohol addiction, can

cause death and should be supervised by a doctor.

A Model of Drug AddictionCocaine is a highly addictive stimulant found in the leaves of the

coca plant, Erythroxylon coca, shown in Figure 14. A is a

drug that generally increases the activity of the central nervous sys-

tem. Despite being illegal, cocaine is still used by many people.

Recall that in synaptic transmission, neurotransmitter molecules

are released from a presynaptic neuron and bind to receptor pro-

teins on a postsynaptic cell. Some neurotransmitter molecules are

stimulant

Withdrawal

Tolerance

Addiction

962

BIOgraphic

Action of Cocaine

Cocaine alters the function of dopamine-producing neurons in the limbic system.

Normal synapse

Dopamine is reabsorbed by

the presynaptic neuron.

1 Synapse with cocaine

Cocaine blocks the reabsorption

of dopamine.

2

Dopamine

molecules

Reuptake

receptor

Receptor

protein

Postsynaptic

cell

Presynaptic

neuron

Synaptic

cleft

Ion

channel

Cocaine

molecules

Postsynaptic

membrane

Presynaptic

membrane

Synaptic

vesicles

Figure 14 Coca plant.

Cocaine is derived from the

coca plant, Erythroxylon coca.

Figure 15

962


Teaching TipEffects of Drugs Have studentschoose a class of frequently abuseddrugs (such as opiates, depressants,or stimulants) or a specific drug(such as cocaine, marijuana, oralcohol). Ask them to make a pam-phlet about the effects of thedrug(s), the dangers of using thedrug(s), and where to seek help fordrug addiction. Verbal

Teaching TipNatural Diuretics Alcohol andcaffeine are diuretics, which causethe excretion of excessive amountsof urine. Alcohol inhibits therelease of antidiuretic hormone.This causes more water to beexcreted in the urine. Caffeinecirculates in the bloodstream to the kidneys where it inhibits thereabsorption of solutes and water.Thus, caffeine also increases urineproduction.

Group Activity Drugs and Their Effects Assignstudents to groups of four. Haveeach group make a table listing different kinds of drugs, theirdesired effects, and their possibleside effects. For each drug listed,have students categorize the drugsas medicinal or social, legal or ille-gal, and prescription or over thecounter. Their tables might includestimulants, depressants, narcotics,hallucinogens, inhalants, hormonereplacements, analgesics, antibi-otics, antihistamines, and appetitesuppressants. Students should findthat many drugs have more sideeffects than desired effects.

Verbal Co-op LearningLS

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Transparencies

TR J76 Psychoactive Drugs of Abuse

TR J74 Action of Cocaine

TR J77 Effects of Blood AlcoholConcentration

reabsorbed by presynaptic neurons after they have been released

into the synaptic cleft. Cocaine is an example of a drug that interferes

with a presynaptic neuron’s ability to reabsorb, or reuptake, neuro-

transmitter molecules. Cocaine affects dopamine (DOH pah meen)

neurons in the limbic system, which plays an important role in the

sensation of pleasure. The mechanism of cocaine action is summa-

rized in Figure 15.

Step At a normal synapse, reuptake receptors move molecules of

dopamine in the synaptic cleft back into the presynaptic

neuron.

Step Cocaine blocks the reuptake of dopamine molecules by

interfering with these reuptake receptors.

Step As a result, excess dopamine remains in the synaptic cleft,

overstimulating the postsynaptic cell. Overstimulation pro-

duces an intense feeling of exhilaration and well-being.

Because the post synaptic cell has been overstimulated, the

number of dopamine receptors will decrease over time.

Step If cocaine is removed from the synaptic cleft, the number

of dopamine molecules returns to normal. This level is now

too low to adequately stimulate the postsynaptic cell

because it has fewer receptor proteins. Addiction occurs

because more cocaine must be taken to maintain adequate

stimulation of the postsynaptic cell.

963

Cocaine removed from synapse

Dopamine release returns to normal, but the

postsynaptic cell is understimulated.

4Overstimulated postsynaptic cell

The number of receptor proteins on the

postsynaptic cell decreases.

3

Postsynaptic

cell

Presynaptic

neuron

Real Life

Caffeine is a stimulant

found in many foods and

beverages.

Chocolate, coffee, tea, and

some soft drinks often

contain caffeine.

Calculating

Make a list of everything

you eat and drink (includ-

ing medicines) for

24 hours; then use a

reference table to

determine your

total caffeine

intake.

963


DemonstrationShow the class a videotape, in slow motion if possible, of some-one performing a complicated highdive or balance-beam routine.Have students relate this type ofperformance to the function of thecerebellum. Alcohol has a signifi-cant effect on the synapses in thecerebellum. Have students discussalcohol’s effects on balance andcoordination. Interpersonal

Activity BAC Have students research lawsfor blood alcohol concentrationusing the web site in the InternetConnect box on this page. Havestudents write a report summariz-ing their findings. Verbal

DemonstrationInvite a local law enforcementofficer to come to the class todiscuss DUI (Driving Under theInfluence) and DWI (DrivingWhile Intoxicated), incidence ofDUI/DWI, and the dangers ofdrinking and driving. Have theofficer tell students what bloodalcohol concentration is considered“legally drunk” in your state. Havestudents find this BAC level inTable 4.

GENERAL

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Alcohol Concentration Tell students thatbeer, wine, and liquor differ in their ethanolconcentrations. However, a 12 oz. can of beer,a 4 oz. glass of wine, and a 1 oz. shot ofliquor all contain about the same amount ofabsolute ethanol. Tell students that “proof”reflects the alcohol concentration in a bever-age. Alcohol concentration is determined bydividing the proof number in half—80 proofliquor has a 40 percent alcohol content.


AlcoholOf all the psychoactive drugs, alcohol (ethanol) is one of the most

widely used and abused. Alcohol, found in wine, beer, and liquor, is

a depressant that produces a sense of well-being when taken in

small amounts. A is a drug that generally decreases the

activity of the central nervous system. As more alcohol is con-

sumed, reaction time increases, and coordination, judgment, and

speech become impaired. This produces a state of intoxication

known as being “drunk.” Drunkenness results as the blood-alcohol

concentration (BAC) increases. BAC can be measured by a breath

test, illustrated in Figure 16, that detects the level of alcohol vapors

in the breath. Table 4 shows the effects of alcohol at various con-

centrations in the blood.

depressant

Table 4 Effects of Blood Alcohol Concentration

BAC* Condition

0.02–0.04 Slight impairment and sedation

0.05–0.06 Slight impairment of coordination; increased reaction time

0.07–0.09 Slurred speech; blurred vision; intoxication

0.10–0.15 Severe intoxication; impaired coordination, vision, and balance

0.15–0.30 Dizziness; confusion; inability to walk; extremely severe intoxication

0.30–0.50 Unconsciousness

0.50–0.60 Coma or death

*in mg of alcohol per mL of blood

www.scilinks.org

Topic: Blood AlcoholConcentration

Keyword: HX4026

Figure 16 Breath test.

Law enforcement officials use

a device that detects the level

of alcohol vapors in the breath

to estimate the BAC of drunk-

driving suspects.

Alcohol is absorbed into the blood through the stomach and small

intestine. Alcohol affects neurons throughout the nervous system,

changing the shape of receptor proteins. Such widespread changes

in receptor proteins have various effects on normal brain function-

ing.

Addiction to alcohol, or alcoholism, is the most prevalent drug-

abuse problem in the United States. People who drink excessive

amounts of alcohol over long periods of time develop serious health

problems. For example, many alcoholics do not eat properly when

drinking heavily. This can lead to malnutrition, abnormalities in the

circulatory system, and inflammation of the stomach lining. In

addition, the liver begins to use alcohol as an energy source. After

exposure to alcohol over time, the liver accumulates fat deposits. If

drinking of alcohol continues, a potentially fatal liver condition

called cirrhosis (sih ROH sis) may develop. In a cirrhotic liver, cells

are replaced with scar tissue, and liver functioning is impaired.

964


also adds carbon monoxide to the blood. Thisadded carbon monoxide causes fatty acids tobe deposited on the inside of the arteries andthus is a major contributor to cardiovasculardisease. The combined effect of the strain ofcardiovascular disease and the lack of oxygencontributes heavily to the damage seen insmokers’ cardiovascular systems.

Group Activity Social Drug Use Have studentsbring in newspaper and magazinearticles that deal with smoking cig-arettes and drinking alcohol. Havesome extra articles on hand to useif needed. Assign students togroups of four and ask students toconsider the image of social druguse that is portrayed in the media.Have each group discuss its articleand report back to the class with abrief summary. Display the articleson a bulletin board, and encouragethe class to read them as they studythis section.

Interpersonal Co-op LearningLS

GENERAL


Nicotine is extremely toxic. In fact, a sulfatederivative of nicotine has been used as acommercial insecticide for years. The lowerdoses of nicotine experienced by smokers arenot immediately noticeable as toxic, however.Initially, nicotine elevates blood pressure byconstricting blood vessels and increasing theheart rate. It stimulates the release of freefatty acids into the bloodstream. Smoking


Answer

About 90 percent of people whodevelop cancer of the mouth,pharynx, and larynx use tobaccoin some form. Smoking cigarettes,cigars, or pipes is associated withcancers in the mouth, pharynx,larynx, esophagus, and otherorgans. Using smokeless tobaccois associate with cancers of thecheek, gums, and lips.

Source: American Cancer Society

Real Life

NicotineAbout 50 million Americans smoke cigarettes despite convincing

evidence that smoking causes mouth cancer, heart disease, lung

cancer, and emphysema. So why do people continue to smoke?

Many smokers say they would like to stop smoking but find the

habit too difficult to overcome. They are addicted to nicotine, a

drug in cigarette smoke.

Effects of NicotineNicotine is the highly addictive stimulant found in the leaves of the

tobacco plant, Nicotiana tabacum, shown in Figure 17. Nicotine is

extremely toxic; a dose of only 60 mg is lethal in humans. Tobacco

leaves are dried or crushed and are then smoked in cigarettes, cig-

ars, and pipes. Tobacco is also chewed and snuffed.

Nicotine quickly enters the bloodstream and circulates through the

body. In the brain, nicotine mimics the action of the neurotransmit-

ter acetylcholine. Scientists have extensively studied the behavior of

the brain when exposed to nicotine. Nicotine binds to brain cells at

specific sites usually reserved for acetylcholine. These sites are the

central controls of the brain—mechanisms the brain uses to adjust

levels of many of its activities. Like twisting the dial on a central con-

trol, the binding of nicotine to these sites produces many changes.

After a while, the smoker’s body makes adjustments, and systems

almost return to normal—as long as the smoker keeps smoking. Take

away the nicotine, however, and all those adjustments throw every-

thing out of balance all at once. The only way to keep things “normal”

is to keep smoking. The smoker is addicted.

Effects of TobaccoSmokers get more than nicotine from cigarette smoke. Inhaled

smoke contains hundreds of toxic and mutagenic chemicals that

pass through the mouth, air passages, and lungs. These chemicals,

also called tars, are produced by burning tobacco. Because tars and

other chemicals in tobacco smoke are powerful mutagens, smoking

causes lung cancer. Almost all cases of lung cancer, a major cause

of death in the United States, are attributed to smoking.

In the United States, smoking-related illnesses cause more than

400,000 deaths each year. Smoking is associated with cancer of the

mouth and larynx, and smoking may increase the risk of cancer of

the pancreas and bladder. Smoking is also a major contributor to

often-fatal respiratory disorders, such as emphysema. The tars in

smoke irritate mucous membranes in the mouth, nose, and throat.

They accumulate in the lungs and paralyze cilia that move debris

from the lungs. Tars also blacken lung tissue and decrease breath-

ing capacity. People who are exposed to secondhand smoke are at

risk for the same diseases as people who smoke. Women who

smoke during pregnancy are more likely to have miscarriages or to

give birth to stillborn babies.

Real Life

Is smokeless tobacco

harmful?

The use of smokeless

tobacco, such as chewing

tobacco, causes cancers

of the lips, mouth, and

gums. When chewing

tobacco is placed

between the cheek and

gum, nicotine and other

chemicals are absorbed

into the bloodstream.

Finding Information

Find out about mouth

cancers caused by

tobacco.

Figure 17 Tobacco plant.

Tobacco leaves are dried and

crushed and are then smoked

in cigarettes, cigars, and

pipes. Tobacco is also chewed

and snuffed.

965


Anticipation Guide Ask volun-teers to discuss how their opinionsformed in the Reading Activity onthe opening page of this chapterchanged or stayed the same as aresult of their reading. Studentsshould point to specific passages inthe text that support their reasoning.

Verbal

ReteachingHave students work in pairs. Askeach student to write definitions forthe key terms listed on the firstpage in this section. Ask studentpairs to quiz each other on theterm, and then describe the effectsof psychoactive drugs on the centralnervous system.

Quiz1. Why are pain receptors impor-

tant? (They notify you that bodytissues have been injured.)

2.What is the function of neuro-transmitters? (They transmit nerveimpulses across the synapse.)

3. Compare stimulants withdepressants. (A stimulantgenerally increases the activity of the central nervous system; adepressant generally decreases theactivity.)

AlternativeAssessmentHave students design an informa-tive poster to educate people aboutthe dangers of smoking.

GENERAL

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READINGREADING


1. Repeated exposure to a drug leads to tolerance,a condition in which increased amounts of thedrug are needed to achieve the desired effect.

2. Cocaine overstimulates postsynaptic neuronsby blocking dopamine reuptake, ultimatelydecreasing the number of dopamine receptors.When cocaine is removed, insufficient dopa-mine is available to stimulate postsynaptic cellsat a normal level.

3. Stimulants, such as nicotine, increase centralnervous system activity. Depressants, such asalcohol, decrease central nervous system activity.

4. All psychoactive drugs alter the function of thecentral nervous system.

5. Addictive drugs alter the normal functioning ofneurons and synapses.

6. A. Correct. It interferes with the presynapticneuron’s ability to reabsorb the neurotransmitter.B. Incorrect. This prevents pain signals fromreaching the brain. C. Incorrect. Sensory percep-tion is everything your senses respond to in theenvironment. D. Incorrect. Synaptic transmissionis the transmission of signals between cells.


Drugs of AbuseNarcotics are extremely addictive psychoactive drugs that relieve

pain and induce sleep. Some of the most potent narcotics are derived

from the poppy plant, Papaver somniferum, shown in Figure 18. The

sap that oozes from the cut seed pod forms a thick, gummy sub-

stance called opium. Drugs derived from opium, called opiates or

narcotics, include codeine (KOH deen), morphine, and heroin, a more

potent form of morphine. Codeine is widely prescribed by doctors for

pain relief. Morphine is one of the most effective pain-relieving drugs

used today. Heroin addiction and abuse are among the most serious

illegal-drug problems in society.

Recall that pain receptors throughout the body detect painful stim-

uli. As uncomfortable as it may feel, pain plays a very important role

in the body. Pain notifies you that body tissues have been injured or

damaged. Imagine how your body would look and function today if

you did not have the ability to sense pain. Pain begins as a signal at

damaged nerve endings. Nerve impulses generated by pain receptors

travel to the spinal cord toward the brain. After reaching the spinal

cord, a pain signal is suppressed by a class of neurotransmitters called

enkephalins (ihn KEHF uh lihnz). When enkephalins bind to neurons

in the spinal cord, they prevent pain signals from reaching the brain.

Narcotics mimic the action of enkephalins by binding to the same

receptor proteins in the spinal cord. These receptor proteins are

called opiate receptors because scientists observed opiates binding to

them before enkephalins were ever discovered. Narcotics also affect

the limbic system, producing a feeling of well-being.

MarijuanaIn addition to alcohol and tobacco, marijuana, though illegal, is a

widely consumed drug. Marijuana comes from various species of

the hemp plant, Cannabis, shown in Figure 19. Hashish also comes

from the hemp plant. The active ingredient in marijuana and

hashish is commonly known as THC. When marijuana is smoked, it

may cause disorientation, impaired judgment, short-term memory

loss, and general loss of motivation. Scientists continue to research

the effects of THC on the nervous system.

Describe how tolerance to a drug develops.

Summarize how cocaine produces addiction.

Distinguish between stimulants anddepressants. Give an example of each.

Critical Thinking Recognizing

Relationships What do all psychoactive drugs have in common?

Critical Thinking Applying Information

Why is drug addiction considered a physiological condition?

Cocaine interferes withthe normal functions of the limbic system byblocking

A reuptake of dopamine. C sensory perception.

B release of D synaptic enkephalins. transmission.


Section 4 Review

Figure 18 Opium poppy.

Opium is a narcotic derived

from the poppy plant, Papaver

somniferum.

Figure 19 Hemp. Marijuana

is produced from the hemp

plant, Cannabis.

966


AlternativeAssessmentDivide the class into four groups ofstudents. Assign one section of thischapter to each group. Tell studentgroups that they are to write a cre-ative poem, short play, or very shortstory (three paragraphs) thatincludes all of the key terms foundin their assigned section. Tell stu-dents that the definitions of eachterm should appear in their writing.When students have completed theircreative writing, ask for volunteersfrom each group to present theirproduct to the rest of the class.

GENERAL

Answer to Concept Map

The following is one of several possible answersto Performance Zone item 15.


• Science Skills Worksheet

• Critical Thinking Worksheet

• Test Prep Pretest

• Chapter Test GENERAL

GENERAL

GENERAL


has divisions called

is composed of

send signals to

is composed ofis connected to the CNS by

Nervous system

nerves

synapse

neurotransmitters

nerves

spinal cord brain

such as

motor neurons

receive signals from

which travel to the

made of

sensory neurons

using chemicals called

pass signals across

peripheral nervous systemcentral nervous system

neurons

Key Concepts

Study CHAPTER HIGHLIGHTS

ZONE

Neurons and Nerve Impulses

● Neurons are specialized cells that rapidly transmit

information as electrical signals throughout the body.

● At the resting potential, the inside of a neuron is negatively

charged with respect to the outside of the neuron.

● An action potential moves rapidly down an axon.

● Synaptic transmission involves the release of

neurotransmitters at synapses.

Structures of the Nervous System

● The central nervous system consists of the brain and

spinal cord.

● The brain contains three major parts: the cerebrum, the

cerebellum, and the brain stem.

● The spinal cord links the brain to the peripheral nervous

system, which branches throughout the body.

Sensory Systems

● Sensory receptors detect various sensory stimuli.

● Photoreceptors in the eyes convert light into electrical

signals that are interpreted by the brain.

● The ear converts sound into electrical signals that

are interpreted by the brain.

● The semicircular canals monitor the position of

the head.

● Taste and smell are related chemical senses.

Drugs and the Nervous System

● Psychoactive drugs affect the central nervous system.

● Drug addiction involves physiological changes in neurons.

● Alcohol is an addictive depressant that widely affects the

central nervous system.

● Nicotine is an addictive stimulant found in tobacco products.

4

3

2

1

Key Terms

Section 1

neuron (944)

dendrite (944)

axon (944)

nerve (944)

membrane potential (945)

resting potential (946)

action potential (946)

synapse (948)

neurotransmitter (948)

Section 4

psychoactive drug (961)

addiction (962)

tolerance (962)

withdrawal (962)

stimulant (962)

depressant (964)

Section 2

central nervous system (950)

peripheral nervous system (950)

sensory neuron (950)

motor neuron (950)

brain (950)

cerebrum (951)

cerebellum (951)

brain stem (951)

thalamus (952)

hypothalamus (952)

spinal cord (952)

reflex (952)

interneuron (953)

Section 3

sensory receptor (956)

retina (958)

rod (958)

cone (958)

optic nerve (958)

cochlea (959)

semicircular canal (959)

967


brain. These conflicting signals can causedizziness and vertigo.

12. Cone cells enable color vision. If damage or achemical disorder occurs in these cells, colorblindness could be the result.

Alternative Assessment

13. Sample answer: Alzheimer’s disease: cause:unknown—origin of plaque buildups in thebrain may be neurochemical, environmental,infectious, or genetic; symptoms: impairedmemory, loss of intellectual function, atrophyof brain tissue, degradation of language ability;part of the nervous system: brain; availabletreatment: none available—symptoms are

treated with medication, physical therapy, andoccupational therapy.


CHAPTER 41

ANSWERS

Understanding Key Ideas

1. c

2. c

3. a

4. b

5. c

6. The fish cerebrum is muchsmaller than a human cerebrum(relative to other parts of thebrain). The sense of smell proba-bly is very important to the fish.

7. Voltage-gate sodium channels,which close shortly after anaction potential, cannot openagain for a brief moment afterthe action potential. This pre-vents the nerve impulse frommoving in the opposite direction.

8. When binding to receptor proteinson the postsynaptic cell, someneurotransmitters cause ion chan-nels to open, some stimulate theformation of second messengers inthe cell, and others stimulateenzyme activity in the cell.

9. Cell-death inhibitors are used formyelin-forming cells. Peripheralnerves are grafted to stimulateaxon growth.

10. One possible answer to the con-cept map is found at the bottomof the previous page.

Critical Thinking

11. The semicircular canals detectacceleration and orientation ofthe body. Eyes detect movement,sending that message to the brain.If the hair cells in the semicircularcanals do not, however, detect thesame degree of movement, theysend a conflicting signal to the

Understanding Key Ideas

1. A myelin sheath on the axon of a neurona. covers the axon completely.b. decreases the rate of impulseconduction.

c. increases the rate of impulseconduction.

d. has no effect on impulse conduction.

2. When a neuron is at the resting potential,a. the inside is positively charged.b. the outside is negatively charged.c. the inside is negatively charged.d. None of the above

3. During an action potential,a. sodium ions flow into a neuron.b. sodium ions flow out of a neuron.c. potassium ions flow into a neuron.d. there is no movement of ions.

4. In a spinal reflex, the signal travelsa. immediately to the brain.b. to the spinal cord and out to a muscle.c. only through sensory neurons.d. only through motor neurons.

5. Drug addiction is considered a physiologicalcondition because addictive drugsa. can be purchased illegally.b. must be injected.c. alter the functioning of neurons.d. are used in social settings.

6. The diagram below shows the brain of afish. How is the cerebrum of the fish braindifferent from that of a human brain? Whatdo the large olfactory bulbs of the fish brainindicate about the relative importance ofthe sense of smell to the fish?

7. Action potentials travel in only one direc-tion along a neuron—toward the axonterminals and away from the cell body.What structures of the neuron ensure that this pattern is always followed?

8. List three ways that the binding of a neurotransmitter to a receptor protein on apostsynaptic cell could cause changes in thecell. (Hint: See Chapter 4, Section 2.)

9. Although neurons in thespinal cord do not grow and regeneratewhen they have been injured, scientists havedeveloped ways to help prevent paralysis.Describe two of these ways.

10. Concept Mapping Make a conceptmap that describes the structures andfunctions of the nervous system. Try toinclude the following terms: spinal cord,brain, neuron, nerve, synapse, andneurotransmitter.

Critical Thinking

11. Inferring Relationships People who sufferfrom vertigo feel dizzy and disoriented incertain situations. What is the relationshipbetween vertigo and the semicircularcanals?

12. Recognizing Relationships Suggest a possi-ble defect of the retina or of retinal cells thatwould cause colorblindness, a condition inwhich a person cannot distinguish betweencertain colors.

Alternative Assessment

13. Summarizing Information Research thecauses and symptoms of various disordersof the nervous system caused by the degen-eration of neurons. Some disorders includeAlzheimer’s disease, Parkinson’s disease,and multiple sclerosis. Find out how thesedisorders affect the nervous system. Listthe types of drugs or other methods used totreat these disorders.

PerformanceZONE

CHAPTER REVIEW

Cerebrum

Olfactory bulbs

968

Section Questions

1 1, 2, 3, 7, 8

2 4, 6, 9, 13

3 11, 12

4 5

Assignment Guide


Question 2 Answer H is the cor-rect choice. The sodium–potassiumpump helps the neuron achieveresting potential by pumpingsodium ions out of the cell andpotassium ions into the cell.Answer F is incorrect because thenormal resting potential when thepump is working is negative inside.Answer G is incorrect because thefunctioning of voltage-gated ionchannels is not directly related tothe functioning of the sodium–potassium pump. Answer I isincorrect because the normal rest-ing potential of the cell has theseconcentrations, which are main-tained by the sodium–potassiumpump.

Question 4 The muscles that arerigid throughout the body arebeing stimulated by nerveimpulses.

Question 5 Answer H is the cor-rect choice. Drugs may target neu-rotransmitters in order to regulatesensations and moods. Answer F isincorrect because the cerebellumregulates balance, posture, andmovements. Answer G is incorrectbecause motor neurons commandmuscles and organs. Answer I isincorrect because the spinal cordlinks the brain to the peripheralnervous system. None of these islikely to have an effect on a per-son’s emotional state.

Question 6 Answer B is the cor-rect choice. The myelin sheathincreases the speed of actionpotentials. Answer A is incorrectbecause the axon terminal doesnot change the speed. Answer C isincorrect because the axon con-ducts impulses but does notchange their speed. Answer D isincorrect because the dendritereceives impulses but does notchange their speed.

Answers

1. B

2. H

3. A

4. Many muscles are being stimulated at once.

5. H

6. B


Standardized Test Prep

Understanding ConceptsDirections (1–3): For each question, write ona separate sheet of paper the letter of thecorrect answer.

1 Which of the following is a sensory receptor that is stimulated by light?A. cochlea C. interneuronB. cone D. optic nerve

2 If the sodium–potassium pump of a neuron failed, what effect would this likely have on the neuron’s function?F. The concentrations of positive and negative ions would cause the neuronto be negative inside.

G. Voltage-gated potassium channels andvoltage-gated sodium channels wouldno longer function.

H. The neuron could not conduct anotheraction potential until the resting potential was fully restored.

I. The concentration of sodium ionswould be higher outside the cell andthat of potassium ions would be higherinside.

3 Which part of the ear containsmechanoreceptors?A. cochleaB. eardrumC. hammerD. stirrup

Directions (4): For the following question,write a short response.

4 During an epileptic seizure, many neuronsin the brain produce large bursts of actionpotentials, causing the body to becomerigid and to jerk or convulse. From whatyou know about the brain’s control ofmuscles and posture, how might youexplain these symptoms?

Reading SkillsDirections (5): Read the passage below.Then answer the question.

Depression affects several millionAmericans. Symptoms of depression includewithdrawal, anger, poor communication, sadness, and indifference to surroundings.Depression may be triggered by the loss of afriend or relative, a major disappointmentat work, prescription drugs, prolongedillness, alcohol or drug withdrawal, or hormones. Treatments include counseling,several types of drugs, and exercise.

5 Which structure is likely a target for drugsthat treat depression?F. cerebellumG. motor neuronH. neurotransmitterI. spinal cord

Interpreting GraphicsDirections (6): Base your answer to question6 on the diagram below.

Neuron

6 Which structure increases the speed atwhich the axon conducts action potentials?A. A

B. B

C. C

D. D

Test

When using a diagram to answer a question, look in

the image for evidence that supports your potential

answer.

A

B

C

D

969

Standardized Test Prep


chapt 41 hbio nervous system · 2019-12-07 · a sodium-potassium pump is a carrier protein that...

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