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Learning goals

m Neurons: The BuildingBlocks of the Nervous

System

m How Neurons

Communicate

m The Structure of the

Nervous System

w The Endocrine System

Students will be able to:

1 Identify and describe the functions of the partsof a neuron.

2 Explain the process of neural transmission.3 Explain the roles of neurotransmitters in

neural transmission.

4 Identify and describe the divisions of thenervous system.

s Discuss the nature and function of endocrinesystem communication.

mpreviewing Key Terms:neuron

dendrite

soma

axon

axon terminal

action potential

refractory period

resting potential

all-or-none principle

synapse

neurotransmitter

excitatory effect

inhibitory effect

receptor cells

sensory nerves

interneurons

acetylcholine (ACh)

antagonist

agonist

dopamine

serotonin

motor nerves

central nervous system(CNS)

peripheral nervoussystem (PNS)

somatic nervous systemautonomic nervous

system

sympathetic division

parasympathetic division

endocrine systemhormone

pituitary gland

Module 6 The Nervous System and the Endocrine System 95

Your body is an incredible organization of biological systems, each withits own important functions. Your skeletal system supports your body.Your digestive system extracts nutrients from food. Your immune sys-tem wards off disease. Your respiratory system allows you to take inoxygen and rid your cells of carbon dioxide. But the biological systemsthat psychologists focus on are the nervous system and the endocrine(hormonal) system. These two biological systems enable communica-tion and information processing within our bodies.

Neurons: The Building Blocks of theNervous System

The Computer and the BrainBoth have amazing capabilities

and get their power frommillions of "switches"

(electronic bits in thecomputer, neurons in the brain)

that can be either on or off.

THIN KI NG C RITICAL LY What are the parts of a neuron, and what dothey do?

The nervous system is your body's electrochemical communication sys-tem. Through it, your brain tells your body parts to move, your face toexpress emotion, and your internal organs to go about their business.Your nervous system, in partnership with your sensory systems, alsogathers information so that your brain can respond appropriately tostubbed toes, fire alarms, and the smell of popcorn. Like every othersystem in your body, your nervous system is built of cells. Looking atthose cells is a good starting point for understanding the whole system.

Your brain, spinal cord, and nerves are formed from nerve cells orneurons, the basic building blocks of the nervous system. Neurons arethe highly specialized and unique cells of the nervous system. A neuronexists only to perform three tasks:

To receive information (in the form of electrochemical impulses)from the other neurons that feed into it

To carry this information down its length

To pass the information on to the next neurons in line

Every behavior, thought, and emotion you've ever experienceddepends on the neuron's remarkable ability to process information inthese three ways.

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The wonder of it all is that neurons are actuallyquite limited in function-their main capability issimply transmitting an impulse, or "firing." In someways, the guts of modern powerful computers oper-ate similarly. A computer's central processor controlsmany electronic switches that can be either "on" or"off." All of a computer's extraordinary capabilities-its communication functions, elaborate games,"number crunching," mind-dazzling graphics, andsound-are ultimately accomplished by settingswitches in the proper on-or-off pattern.

96 Biopsychological Domain Biological Bases of Behavior Chapter

Neurons work in a similar way: They can "fire" (that is, send animpulse down their length) or not "fire." That's it. The beautiful colorsyou see in a sunset, the intense emotions you experienced during yourfirst crush, the memory of your first day of kindergarten, the taste ofpepperoni pizza, the thrill you feel when riding a roller coaster, and thedevastating depression so many thousands suffer from-all emergefrom a certain sequence of neurons either firing or not firing.

Neurons, like trees and dogs, come in a tremendous variety of shapesand sizes, but all neurons have similar stmctures. Take a minute now tolook at Figure 6.1, which shows these structures in a motor neuron, a nervecell that carries messages to muscles and glands. In this discussion, weexamine neuron parts following the order in which information travels-from the dendrites to the soma, the axon, and the axon terminals.

Dendrites are the branching extensions of a neuron that receive infor-mation and conduct impulses toward the cell body. Dendrites look likebranches, and in fact the word dendrite comes from the Greek word for"tree." From the dendrites, the information moves to the soma, the cell

body of a neuron. The soma contains the cell nucleus and other parts thatkeep the cell healthy and functioning properly. From there, irformationtravels along the axon, the extension of a neuron through which neuralimpulses are sent. The neuron's purpose is to move information from pointA to point B, and the axon creates distance between these points. Axons ofneurons in the brain may be very short, because information doesn't haveto travel far between the cells. But in some neurons in the leg, axons extendmore than 3 feet, making these giant redwoods of the nervous system thelongest cells in your body! Longer axons are covered by a myelin sheaththat protects the axon and speeds up the transmission of information.Finally, the information reaches the axon terminals, the endpoint of aneuron where neurotransmitters (discussed soon) are stored. As you willsee, axon terminals are the points of departure for information as it makesits way to the dendrites of the next neurons in the sequence.

neuron

A nerve cell; the basic buildingblock of the nervous system.

dendrite

The branching extensions of aneuron that receive information

and conduct impulses toward thecell body (soma).

soma

The cell body of a neuron, whichcontains the nucleus and other

parts that keep the cell healthy.

axon

The extension of a neuron

through which neural impulsesare sent.

axon terminal

The endpoint of a neuron whereneurotransmitters are stored.

Figure 6.1 A Typical MotorNeuron Information travels

from lefk to right in this neuron.Messages are received at thedendrites, travel through thesoma and down the axon, andarrive at the axon terminals.

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Axon terminals

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Neural impulse(electrical signal travelingdown the axon)

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THINKING CR?TICALLY SUMMARY Dendritesreceiveinformationand conduct impulses toward the soma (cell body). Information travels alongthe axon until it reaches the axon terminals, which transmit information to thenext neuron.

How Neurons Communicate

THINK?NG CRITICALLY Howdoesaneuronfire?

Now we're ready to look more closely at what happens when a neuronfires. This involves changes both within a neuron and between neurons.

The Neural ImpulseWhen a neuron fires, the neural impulse is called an action potential. Itis a brief electrical charge that travels down the axon of a neuron. It worksits way from the dendrites to the axon terminals, much as a bite of swal-lowed food makes its way from your mouth to your stomach. This actionpotential represents the "on" condition of the neuron. Each action poten-tial is followed by a brief recharging phase known as the refractoryperiod, when a neuron, after firing, cannot generate another actionpotential. Think of a camera flash that has to recharge before it can beused again and you'n get the idea. After the refractory period, the neuronis capable of another action potential when it is stimulated. When the ceuis recharged, at rest, and capable of generating another action potential, aresting potential exists. Table 6.1 illustrates these steps.

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Action potential

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The neural impulse created when aneuron "fires.' The impulse travels fromthe dendrites down the axon to the

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Refractory period

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The state of a neuron when it is

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An interesting fact about how a neuron fires is called the all-or-noneprinciple, which states that a neuron always fires with the same inten-sity. All action potentials are the same strength. It doesn't matter if thereis strong stimulation or weak stimulation at the cell's dendrites. As longas there is enough energy to trigger the neuron, it will fire with the sameintensity.

One of the best analogies to a neuron and how it fires is, perhapsunfortunately, a toilet. Stop for a moment and think of how a toilet issimilar to a neuron. Here are some similarities (perhaps you will beable to think of more):

Like a neuron, a toilet has an action potential. When you flush, an"impulse" is sent down the sewer pipe.

Like a neuron, a toilet has a refractory period. There is a shortdelay after flushing when the toilet cannot be flushed againbecause the tank is being refilled.

Like a neuron, a toilet has a resting potential. The toilet is"charged" when there is water in the tank and it is capable of beingflushed again.

Like a neuron, a toilet operates on the all-or-none principle-italways flushes with the same intensity, no matter how much forceyou apply to the handle (as long as you provide enough force totrigger the mechanism).

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action potentialA neural impulse; a brief electri-cal charge that travels down theaxon of a neuron.

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Communication Between Neurons

So far, we have been discussing how information passes down thelength of a single neuron. But how do messages travel from one neuronto the next? Amazingly, this happens without any two neurons actuallycoming in contact with each other! At every place where an axon termi-nal of one neuron and the dendrite of an adjacent neuron meet (andthere may be thousands of such places on any single neuron), there is atiny, fluid-filled gap called a synapse that action potentials cannotjump. In this gap, chemical messengers known as neurotransmitterstravel across the synapse to carry the information from one neuron tothe next. It is the neurotransmitter that influences whether the nextneuron will generate an action potential ("fire") or not. When an actionpotential works its way to the end of a neuron, it causes the release ofneurotransmitters from the axon terminals. The neurotransmitter mol-ecules, which have a distinctive chemical shape, rapidly cross thesynapse and fit into receptor sites on the dendrite of the next neuron(see Figure 6.2).

The neurotransmitters can come to rest only in receptor sitesdesigned to fit their shape, just as a key can open locks only with a cer-tain configuration. Once in the receptor site, neurotransmitters can

relractory periodThe "recharging phase" duringwhich a neuron, after firing, can-not generate another action po-tential.

resting potentialThe state of a neuron when it isat rest and capable of generatingan action potential.

all-or-none principleThe principle stating that if a neu-ron fires it always fires at thesame intensity; all action poten-tials have the same strength.

synapseThe tiny, fluid-filled gap betweenthe axon terminal of one neuronand the dendrite of another.

neurotransmitter

A chemical messenger that trav-els across the synapse from oneneuron to the next and influences

whether a neuron will generatean action potential.

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Figure 6.2Communication Between

Neurons The action

potential triggers therelease of a neurotrans-

mitter from the axon

terminals of the sendingneuron. The neurotrans-

mitter crosses the

synapse and locks intoreceptor sites located on

the dendrites of the

receiving neuron.

1. Electrical impulses (action potentials) travelfrom one neuron to another across a tiny gapknown as a synapse.

Sending neuron

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2. When an action potentialreaches an axon terminal,it stimulates the release ofneurotransmitters. Theseneurotransmitters cross

the synaptic gap and bindto receptor sites on thereceiving neuron. Theneurotransmitters either

excite or inhibit a new

action potential in thereceiving neuron.

excitatory effectA neurotransmitter effect that

makes it more likely that the re-ceiving neuron will generate anaction potential or "fire."

inhibitory effectA neurotransmitter effect that

makes it Iess likely that a receiv-ing neuron wilt generate an ac-tion potential or "fire."

serve two broad functions. Under some circumstances, neurotransmit-

ters have an excitatory effect, which makes it more likely that thereceiving neuron will generate an action potential or "fire." Other times,neurotransmitters have an inhibitory effect, which makes it less likelythat the receiving neuron will generate an action potential. The excita-tory role is like a green light. It shouts, "Just do it!" The inhibitory roleis like a red light. Its message is "Just say no!"

There are dozens of neurotransmitters, although so far researchershave not learned the specific functions of all of them. Different neuro-transmitters serve different functions, depending not only on the typeof receptor site each locks into but also on the place where they arereleased in the brain (see "Psychology Is a Science: Neurotransmittersand Drugs," page 102).

The Neural Chain

The neural chain describes the path information follows as it isprocessed by the nervous system. To understand it, consider theexample of playing your favorite radio station on your stereo system.


What is necessary for this task? First, the radio station must broad-cast the music over radio waves. Second, your stereo's antenna has topick up the radio waves and send them as an electronic messagealong a wire to the stereo's radio receiver. The receiver must processthis information by filtering and amplifying it. Then the electronicinformation is sent to the stereo speakers, again along a wire. Finally,the speakers vibrate to create the sound of your favorite new song.The stereo goes through this process of receiving, processing, andoutputting information continuously.

Your nervous system also specializes in receiving and processinginformation, and it contains functional components similar to thosethat make up your stereo system. First, you need to gather informationfraom your environment. Your "antennae" are receptor cells, special-ized cells in the sensory systems of the body. These amazing receptorcells can turn other kinds of energy into action potentials (impulses)your brain can understand. Your eyes, for example, have receptor cellsthat take light energy and turn it into nerve impulses. Your ears havereceptor cells that process sound energy, and elsewhere in your bodythere are other receptor cells that process smells, tastes, and touch intonerve impulses. Without these receptor cells, your brain would be help-less. By itself, your brain cannot detect light, sound, or smell. Just asyou need your stereo to turn radio waves into something meaningful(music), your brain needs your senses and their receptor cells to gatherand transform information into a form your brain can understand.

The sense organs are not actually located in the brain, so your ner-vous system must literally move the information your receptor cells pullin. This movement occurs as billions of neurotransmitter molecules

pass messages among millions of neurons-from your fingertips, youreyeballs, your ears, your nose, and your mouth to the proper area of thebrain for processing. Just as a stereo uses metal wires to move informa-tion, your body uses living wires known as nerves, which are bundles ofindividual neurons. Sensory nerves carry information fraom the sensereceptors to the brain and spinal cord. Without sensory nerves, yourbrain would be no more effective than your stereo receiver would be ifsomebody cut the wire bringing information from the antenna.

The brain, like a stereo receiver, is the real powerhouse of the sys-tem. The brain must process a constant barrage of sensory data flowingin from the sensory nerves. Your brain receives information about whatyou see, hear, taste, smell, and feel throughout your body (althoughmost of it is ignored as probably insignificant). It is the brain's respon-sibility to deal with all of the information and make appropriate deci-sions, just as your stereo receiver properly filters and amplifies anincoming radio signal. The billions of neurons in your brain and spinalcord that process information are called interneurons.

receptor cellsSpecialized cells in every sensorysystem of the body that can turnother kinds of energy into actionpotentials (neural impulses) thatthe brain can process.

sensory nerves

Nerves that carry informationfrom the sense receptors to thespinal cord and brain.

interneurons

Nerve cells in the brain and

spinal cord responsible forprocessing information.


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Neurntransmitters and Orugs

x"' PSY(HOl06Y 15 A S(IEN(E

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The synapse is where it's at when it comes to theeffects of many drugs. Let's take a look at theroles of three key neurotransmitters (see Table6.2), and see what happens when outside chemi-cals are added to the mix.

One neurotransmitter, acetylcholine (ACh),triggers muscle contraction and affects bothIearning and memory (Alzheimer's disease isassociated with low Ievels of ACh). ACh is pres-ent in every synapse of motor nerves. Certaindrugs can disrupt the normal effects of ACh,however. Some native tribes in South America

use such a drug, a poison called curare, to coatthe tips of the darts they use in their blowguns.When these darts strike an animal, the result isparalysis. Why? Because the curare moleculesfill the receptor sites on dendrites that normallyreceive ACh, but the curare molecules do not

stimulate an action potential in the receivingneuron the way ACh would. This means thatACh is blocked from doing its job and move-ment ceases. Substances such as curare that

block the effects of a neurotransmitter are

called antagonists.

Black widow spider venom also interacts withACh, but not in the same way curare does. Thevenom fills the ACh receptor sites, but its chemicalstructure is so similar to ACh that it mimics ACh's

effect on the receiving neuron. So now two sub-stances, ACh and spider venom, are doing thesame thing. The result is excessive and uncontrol-lable movement in the form of convulsions. The

spider venom is called an agonist, a drug thatboosts the effect of a neurotransmitter. Figure 6.3illustrates how antagonists and agonists interactwith neurotransmitters.

Another neurotransmitter with interesting effectsis dopamine, which influences Iearning, attention,and emotion. Schizophrenia, a serious illness thatdisrupts a person's sense of reality, is associatedwith high levels of dopamine. Drugs commonly pre-scribed for schizophrenia alleviate some of thesymptoms by blocking the action of dopamine at thesynapse. These drugs are dopamine antagonists.

Another disorder, depression, is associatedwith low Ievels of the neurotransmitter serotonin,which affects hunger, sleep, arousal, and mood.Some medications, the most famous of which is

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Examples of Neurotransmitter Functions aNeurotransmitter

Acetylcholine (ACh)

Serotonin

Dopamine

Affected Functions

a Muscle action

a Learning

it Memory

it Learning

a Attention

it Emotion

a Hunger

a Sleep

it Arousal

a Mood

Associated Problems

ACh-producing neurons havedeteriorated in people withAlzheimer's disease.

Excess dopamine activity isassociated with schizophrenia.

Low levels of serotonin are asso-

ciated with depression.

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enough to the neuro-transmitter to occupy itsreceptor site and blockits action, but not similarenough to stimulate thereceptor. Curare paralyzesits victims by blockingACh receptors involved inmuscle movement.

This agonist excites. It issimilar enough in structureto the neurotransmittermolecule that it mimics its

effects on the receivingneuron. Black widow spidervenom, for instance, mimicsthe action of ACh by stim-ulating receptors in brainareas involved in movement,causing convulsions.

Figure 6.3 Antagonists and Agonists When a drugblocks the effect of a neurotransmitter, it's called anantagonist. When a drug boosts the effect of a neuro-transmitter, it's called an agonist.

acetylcholine [ah-seat-el-KO-Ieen](ACh)A neurotransmitter that triggers musclecontraction and affects learning andmemory.

Prozac, work to reduce depression by enhancingthe availability of serotonin at the synapse.Prozac, therefore, is a serotonin agonist.

Prescribed medications are not the only sub-stances that exert their effects at the synapse. Allmind-altering chemicals, ranging from caffeine tococaine, operate by influencing neurotransmis-sion. A single drug, such as alcohol, might influ-ence several neurotransmitters in different waysdepending on the synapse it enters. Research onneurotransmitters is always in progress and bringsfascinating and important results.

antagonistA drug that blocks the effect of aneurotransmitter.

agonistA drug that boosts the effect of aneurotransmitter

dopamine [DO-pa-mean?A neurotransmitter that affects learning,attention. and emotion.

serotonin [sare-oh-TON-in?A neurotransmitter that affects hunger,sleep. arousal, and mood.

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motor nerves

Nerves that carry informationaway from the b?ain and spinalcord to the muscles and glands.

Your brain determines when action is necessary to deal withincoming information. If your brain detects a ball moving towardyour head, you need to either catch the ball or duck to avoid being hit.If your brain detects a question asked by your teacher, you need todecide on an appropriate answer and say it. If your brain detects thatyou're overheating, you need to begin sweating. The point is that whilethe brain can determine a course of action on its own (such as speak-

ing or sweating), it cannot actually do these things. To trigger actionsthe brain must get word to the body's muscles and glands, just as yourstereo system must convey the processed sound signal from the stereoreceiver to its speakers. Your stereo uses more wires for this purpose.Similarly, your nervous system uses motor nerves to carry informa-tion away from your brain and spinal cord to your muscles and glandsso that they can take action. Without motor nerves, your brain couldnot accomplish anything. (Your stereo wouldn't be much good with-out speakers, would it?)

Figure 6.4 shows a neural chain so basic that the initial action isdetermined by the spinal cord without the involvement of the brain. Inthis case, the response to the heat from the flame is a simple reflex. Toreact quickly to a dangerous situation, an interneuron in the spinal cordsends the command to withdraw the finger even before other interneu-rons relay the information to your brain.

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Figure 6.4 A NeuralChain When you burn your

finger, a neural chain isactivated. Receptor cells,

sensory nerves, interneurons,motor nerves, and muscles allwork together to minimize the

damage from the flame.

2. Sensory information(incoming information)Sensory nerves carrythe information to the

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1 . Skin receptorsSkin receptors detectthe heat of the flame

and generate nerveimpulses.

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TH?NKING CRITICALLY SUMMARY A small electrical charge calledan action potential makes its way down the axon of a neuron. The chargecauses neurotransmitters to be released from the axon terminals into the

synapse. Neurotransmitters can have an excitatory or inhibitory effect on thefiring of the next neuron.

central nervous system (CNS)The brain and the spinal cord.

The Structure of the Nervous System

THI NKIN G CRITICAL LY What are the divisions of the nervous

system, and what do they do?

So far, we've examined the nervous system by zooming in on its small-er pieces-sensory and motor nerves made up of bundles of neuronsthat send their neurotransmitters to one another. Now it's time to take

a step back for a broader view of the whole nervous system in whichthese smaller pieces function.

One good way to understand the nervous system is to study itsmajor divisions, which you can see in Figure 6.5. The nervous systemhas two major components, the central nervous system and the periph-eral nervous system.

The central nervous system (CNS) includes the brain and thespinal cord, both of which are so important they are encased in bonefor protection. The brain is where most information processing takesplace, and the spinal cord is the main pathway information follows asit enters and leaves the brain. In shape, the spinal cord tapers fromabout the thickness of a broomstick where it joins the brain to thediameter of a pencil lead at the base of the back. The interneurons thatmake up the CNS are responsible for processing information.

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Figure 6.5 Divisions of theNervous System The nervoussystem is made up of severalimportant divisions.

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module we have a word

built from the Greek soma,which means "body"?

The peripheral nervous system (PNS) contains an sensory nervesand motor nerves that connect the brain and the spinal cord to the rest ofthe body. The word peripheral means "outer region" (perhaps you've heardthe phrase "peripheral vision," which refers to your ability to see things onthe outer regions of your visual field). The PNS divides into two subsys-tems-the somatic nervous system and the autonomic nervous system:

The somatic nervous system is the division of the peripheralnervous system that controls the body's skeletal muscles. It con-tains the motor nerves you use to activate muscles voluntarily. Youdevelop the idea to walk across a room using your central nervoussystem, but you rely on your somatic nervous system to carry theCNS's commands to the muscles of your legs and to get feedbackabout what your legs are actually doing.

The autonomic nervous system is the division of the peripheralnervous system that controls the glands and muscles of the inter-nal organs. It monitors the automatic functions of your body. Yourautonomic nervous system controls your breathing, blood pres-sure, and digestive processes.

The autonomic nervous system has two subdivisions-a sympathet-ic division and a parasympathetic division (see Figure 6.6). These two

SYMPATHETIC CENTRAL PARASYMPATHETIC

NERVOUS SYSTEM NERVOUS SYSTEM NERVOUS SYSTEM

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the Autonomic Nervous SystemThe sympathetic divisionarouses us and expends

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divisions work together in a masterful example of checks and bal-ances-it's not just our government that relies on this principle! Thesympathetic division is the part of the autonomic nervous system thatarouses the body to deal with perceived threats. It controls a number ofresponses collectively referred to as the fight-or-flight response. If youhear footsteps closing in behind you late at night on a deserted side-walk, if a teacher announces a pop quiz at the beginning of class, or ifyou're about to make a nervous phone call to ask somebody out on adate, then your sympathetic nervous system will kick in.

The parasympathetic division is the part of the autonomic nervoussystem that calms the body. The sympathetic division may send yourblood pressure higher when your parent catches you coming in afteryour curfew; your parasympathetic division brings your blood pressureback down to normal when your parent responds calmly to your expla-nation of car trouble.

THINKING CRIT?CALLY SUMMARY The nervous system is dividedfirst into the central nervous system and the peripheral nervous system. Theperipheral nervous system is further divided into the somatic (skeletal) ner-vous system and the autonomic (glands and internal organs) nervous sys-tem. The two divisions of the autonomic nervous system are the sympathetic(arousing) and parasympathetic (calming) divisions.

The Endocrine System

THINK?NG CRITICALLY How does the endocrine system communi-cate, and what does it do within the body?

Besides the nervous system, your body has another system for commu-nicating information. This system, slower to awaken and slower to shutdown than the nervous system, is the endocrine system, a set of glandsthat produce hormones. Hormones are chemical messengers that cir-culate throughout the body in the blood. Hormones and neurotransmit-ters are similar in function: Both carry messages, and both communi-cate by locking into receptor sites.

Figure 6.7 illustrates the major endocrine glands. The most impor-tant is the pituitary gland, the endocrine system's "master gland." Thepituitary gland, in conjunction with the brain, controls the otherendocrine glands. The brain may call on the pituitary to release hor-mones that stimulate or inhibit the release of other hormones from

other endocrine glands. The pea-sized pituitary is located at the base ofthe brain, and it connects to a part of the brain called the hypothalamus.In this connection, the tissue is part glandular and part neural, whichillustrates the close relationship between the nervous and the endocrinesystems.

The brain monitors the levels of hormones circulating in the bloodand may be influenced by their levels. Hunger, for example, is aresponse to a complex interaction of the nervous system and the

peripheral nervous system(PNS)The sensory and motor nervesthat connect the brain and the

spinal cord to the rest of thebody.

somatic nervous systemThe division of the peripheralnervous system that controls thebody's skeletal muscles.

autonomic [aw-tuh-NAHM-ik]nervous systemThe division of the peripheralnervous system that controls theglands and muscles of the inter-nal organs. Its subdivisions arethe sympathetic (arousing) divi-sion and the parasympathetic(calming) division.

sympathetic divisionThe part of the autonomic ner-vous system that arouses thebody to deal with perceivedthreats.

parasympathetic divisionThe part of the autonomic ner-vous system that calms the body.

endocrine [EN-doh-krin?systemOne of the body's two communi-cation systems: a set of glandsthat produce hormones, chemicalmessengers that circulate in theblood.

hormone

A chemical messenger producedby the endocrine glands and cir-culated in the blood.

pituitary glandThe endocrine system's "mastergland? that, in conjunction withthe brain, controls the other en-

docrine glands.


Figure 6.7 MajorGlands of the Endocrine

System Endocrine glandssecrete hormones into

the bloodstream. The

hormones can influence

how we feel and behave.

Hypothalamus(brain regioncontrolling thepituitary gland)

r??!!W!!?7 ,/ Pituitary gland

] 't? (secretes manv't?V (secretes many different1. iJ hormones, some of which

affect other glands)

Thyroid glandI-=4'le-r-*o ?e=*-slirsliews .(affects metabolism,

ramong other things) m

r

1

J

L?

11

/Adrenal glands

f (inner part, calledthe medulla, helpstrigger the ?fight orflight? response)

Testis

(secretes male sexhormones)

J Pancreas

(regulates thelevel of sugarin the blood)

Ovary(secretes femalesex hormones)

rq

endocrine system. The hypothalamus and pituitary gland work togeth-er to monitor and control the levels of glucose (blood sugar that yourcells use for fuel) and insulin (a hormone the pancreas gland secretes,which allows the cells to use the available glucose). This, and a host ofother factors, determines how hungry you are at any given moment.The important pituitary also releases hormones related to physicalgrowth and pregnancy.

Other endocrine glands include the thyroid, the adrenals, and thesex glands (or gonads). The thyroid gland, located in the neck, helps reg-ulate energy level. The adrenal gla;rzds, which perch atop the kidneys,release epinephrme and norepinephrine (also called adrenaline and nor-adrenal'me). These substances enhance strength and endurance in emer-gency situations. The sex glands-ovaries in females and testes inmales-release hormones that influence emotion and physical develop-ment. The primary male hormone is testosterone and the primaryfemale hormone is estrogen, but both males and females have each hor-mone present in their systems.

I'm seated at my desk right now, working on a computer that willprocess e-mail, connect to the Internet, and fax with a click of the


mouse. It does this through a cable modem, which is also the source ofthe TV programming I can access with the remote control that sits nextto the telephone I used to talk to my son, 90 miles away, a few minutesago. Also on the desk is a stack of bills, delivered by the U.S. PostalService. Later this afternoon I will pay them-electronically-by usingthe computer to send instructions to my credit union. I depend on thesemethods of communication to function effectively in the world, just asmy body depends on the nervous system and the endocrine system forits communication needs. These systems are our personal informationhighways.

THINKING CRITICALLY SUMMARY The endocrine system is a setof glands that communicate chemically using hormones. The glands of theendocrine system are involved in many functions within the body using hor-mones, including hunger, energy levels, strength, endurance, and physicaldevelopment.

l'i Thinking About the Nervous System and the rii'l M e*wn4 IMAJILIO(;FlFle a751eFNl

LEaRNlNG GOAL li Identify and describe thefunctions of the parts of a neuron.

W Neurons are made of dendrites, which receiveinformation and pass it along to the cell body(soma). The axon carries this information to theaxon terminals, which release neurotransmittersinto the synapse, carrying information to the nextneuron.

LEARNING GOAL 2: Explain the process of neuraltransmission.

s Within the cell, a small electrical charge (an actionpotential) travels down the axon.

m The charge causes the axon terminals to releaseneurotransmitters into the synapse.

LEARNING aoai. 3: Explain the roles ofneurotransmitters in neural transmission.

m The neurotransmitters released from axon

terminals into the synapse can either excite thenext neuron to fire or inhibit it from firing.

s There are many important neurotransmitters,including the following:

a Acetylcholine (ACh), which triggers musclecontraction

- Dopamine, which affects Iearning, attention, andemotion

a Serotonin, which affects hunger, sleep, arousal,and mood

wa Some drugs (antagonists) block the effect ofneurotransmitters, and others (agonists) mimic orboost the effect of neurotransmitters.

LEARNING GOAL 4i Identify and describe thedivisions of the nervous system.

wa The nervous system is divided into two mainsystems: the central nervous system (CNS) andthe peripheral nervous system (PNS).

wm The PNS is divided into the somatic and

autonomic nervous systems.

u The autonomic nervous system is divided into thesympathetic division, which speeds the body up,and the parasympathetic division, which slows itdown.

LEARNING GOAL s: Discuss the nature and

function of endocrine system communication.

wa The endocrine system is a set of glands thatcommunicate chemically using hormones.

wa These glands influence many functions within thebody using hormones, including hunger, energylevels, strength, endurance, and physicaldevelopment.

lI


lCheck Your Vocabulary lFor each definition, choose the best matching termfrom the list that follows.

13. The endpoint of a neuron whereneurotransmitters are stored.

Definitions

1 . A chemical messenger that travels acrossthe synapse from one neuron to the nextand influences whether a neuron will gener-

ate an action potential.

2. A nerve cell; the basic building block of thenervous system.

3. A neural impulse; a brief electrical chargethat travels down the axon of a neuron.

4. A neurotransmitter effect that makes it Iess

Iikely that a receiving neuron will generatean action potential.

s. A neurotransmitter effect that makes it more

likely that the receiving neuron will generatean action potential.

6. A neurotransmitter that affects hunger,

sleep, arousal, and mood.

7. A neurotransmitter that affects learning,attention, and emotion.

8. A neurotransmitter that triggers musclecontraction and affects Iearning and

memory.

9. One of the body's two communicationsystems; a set of glands that produce hor-mones.

10. The "recharging phase" during which aneuron, after firing, cannot generate anotheraction potential.

11 . The branching extensions of a neuron thatreceive information and conduct impulsestoward the cell body.

12. The cell body of a neuron, which containsthe nucleus and other parts that keep the

cell healthy.

14. The extension of a neuron through whichneural impulses are sent.

15. The part of the autonomic nervous systemthat arouses the body to deal with perceivedthreats.

16. The part of the autonomic nervous systemthat calms the body.

17. The principle stating that if a neuron fires italways fires at the same intensity; all actionpotentials have the same strength.

18. The state of a neuron when it is at rest

and capable of generating an actionpotential.

19. The tiny, fluid-filled gap between the axonterminal of one neuron and the dendrite of

another.

Terms

a. acetylcholine (ACh)b. action potentialc. all-or-none principled. axon

e. axon terminal

f. dendrites

g. dopamineh. endocrine systemi. excitatory effectj. inhibitory effectk. neuron

1. neurotransmitter

m. parasympathetic divisionn. refractory periodo. resting potentialp. serotoninq. soma

r. sympathetic divisions. synapse

!ktmly Your Knowledge

1 . Which of the following statements mostcompletely summarizes the function of theneuron?

a. Neurons rebuild our body's chemical,electrical, and hormonal systems afferstress.

b. Neurons receive, carry, and pass oninformation to other neurons.

c. Neurons create our thoughts and controlour conscious behaviors in the brain.

d. Neurons produce hormones that carrymessages to other glands in the endocrinesystem.

l


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