Objectives• Describe neurons as specialized

cells.• Explain how neurons transmit and

receive signals.

Section ResourcesUnit Resource Book

Study Guide pp. 27–28Power Notes p. 29Reinforcement p. 30Pre-AP Activity pp. 49–50

Interactive Reader Chapter 29Spanish Study Guide pp. 291–292

Biology Toolkit pp. C8, C17, C36

TechnologyPower Presentation 29.2Media Gallery DVDOnline Quiz 29.2

Activate Prior Knowledge Explain that a phobia is a fear of a specific thing that usually elicits an immediate response. Have students think of what they fear the most and how it makes them feel. Ask

• How do you respond to your fear? Students may suggest screaming, jumping back, increased heart and breathing rates, or sweating.

• What body system makes immedi-ate reactions possible? nervous system

Vocabularydendrite Tell students that the prefix of dendrite has two meanings: dendro- from the Greek dendron, meaning “tree,” or dendr-, meaning “earlier.” Relate this to dendrite structure. A dendrite branches like a tree; it is also where an impulse enters a neuron—its “earliest” point.

AnswersA Infer Having more than one

dendrite enables a neuron to receive signals from multiple neurons.

Plan and PreparePlan and Prepare

TeachTeach

U

FIGURE 29.3 Structure of a NeuronA neuronneuron is a specialized cell of the nervous system that produces and transmits signals.

Infer Why might it be beneficial for a neuron to have more than one dendrite?

dendrites

cell bodyaxon

myelin sheath

1

2

3 axonterminals

colored LM; magnification 200�

Connect When you eat a snack, you might flick crumbs off of your fingers without giving it much thought. The specialized cells of your nervous system, however, are hard at work carrying the messages between your fingers and your brain.

MAIN IDEA

Neurons are highly specialized cells.A neuronneuron is a specialized cell that stores information and carries messages within the nervous system and between other body systems. Most neurons have three main parts, as shown in FIGURE 29.3.

1 The cell body is the part of the neuron that contains the nucleus and organelles.

2 Dendrites Dendrites are branchlike extensions of the cytoplasm and the cell mem-brane that receive messages from neighboring cells. Neurons can have more than one dendrite, and each dendrite can have many branches.

3 Each neuron has one axon. An axonaxon is a long extension that carries electrical messages away from the cell body and passes them to other cells.

29.2 NeuronsKEY CONCEPT The nervous system is composed of highly specialized cells.

MAIN IDEAS• Neurons are highly

specialized cells.

• Neurons receive and transmit signals.

VOCABULARYneuron,neuron, p. 876

dendrite,dendrite, p. 876

axon,axon, p. 876

resting potential,resting potential, p. 877

sodium-potassium pump,sodium-potassium pump, p. 877

action potential,action potential, p. 878

synapse,synapse, p. 879

terminal,terminal, p. 879

neurotransmitter,neurotransmitter, p. 879

876 Unit 9: Human Biology

4.c Describe and differentiate among the organizational levels of organisms (e.g., cells, tissues, organs, systems, types of tissue).

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Differentiated Instruction

SECTION 29.2

HANDS-ON ACTIVITYTo demonstrate how a nerve impulse travels along the axon of a neuron, give each group of students eight dominoes. Have students set the dominoes on end, about three-quarters of a domino’s length away from one another, in a straight line. Then have them flick the first domino in the line. Ask, How is the action of the dominoes like a nerve impulse? requires a stimulus, moves one way at a set speed Discuss the need for a network to move a message any distance and the fact that the original condition must be reinstated before a new signal can be sent.

PRE-APFor this section, suggest students use the Survey/Question/Read/Recite/Review strategy. First they preview the section by surveying the material and developing questions. As they read, they will answer the questions, clarify the answers, and then use these to review what they have learned.

Biology Toolkit, SQ3R, p. C8

876 Unit 9: Human Biology

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AnswersA Analyze A neuron has a long axon

that carries signals long distances.

Take It FurtherIn a cell membrane, there are many openings, or channels, formed by proteins that extend through to both sides of the membrane. There are at least three types of protein channels in a neuron’s membrane:

ion channels These channels are always open and allow tiny quantities of ions to enter and leave the cell by diffusion. This normally helps create a balance inside and outside the cell.Na�–K� pump These channels use active transport to maintain a negative charge, or resting potential, inside the neuron.gated channels When a dendrite is stimulated by a neurotransmitter, gated channels in the dendrite open to allow Na� to enter the cell at that site. If the number of ions that enter crosses a threshold, the site becomes depolarized, which causes other gated channels to open sequentially down the axon.

VocabularyAcademic Vocabulary The words potent and potential have the same root, meaning “to be able”:potent, capable of exerting a strong effect

potential, capable of being but not yet in existence

Students may have seen the word used in physics: potential energy versus kinetic energy. There is a similar use here: resting potential versus action potential.

Active Transport It is important for students to understand that active transport is not involved in the propaga-tion of a nerve impulse through the axon. It is active transport that maintains resting potential via the sodium-potassium pump.

Connecting CONCEPTSConnecting CONCEPTSM

neuron

neuron

Dendrites

axon

neuron,dendrite,axon,resting potential,sodium-potassium pump,

action potential,synapse,terminal,neurotransmitter,

8/06 1:46:01 PM

TAKING NOTESUse a flow chart to organize your notes on how a neuron transmits a signal.

Neuron is stimulated.

Na+ channels open; action potential generated.

ConnectingActive Transport Recall from Chapter 3 that energy and specialized membrane proteins are required to move molecules and ions against the concentra-tion gradient.

CONCEPTS

outside inside

energy

There are three types of neurons: (1) sensory neurons, (2) interneurons,and (3) motor neurons. Sensory neurons detect stimuli and transmit signalsto the brain and the spinal cord, which are both made up of interneurons.Interneurons receive signals from sensory neurons and relay them within thebrain and the spinal cord. They process information and pass signals to motorneurons. Motor neurons pass messages from the nervous system to othertissues in the body, such as muscles.

The nervous system also relies on specialized support cells. For example,Schwann cells cover axons. A collection of Schwann cells, called the myelinsheath, insulates neurons’ axons and helps them to send messages.

Analyze How does a neuron’s shape allow it to send signals across long distances?

MAIN IDEA

Neurons receive and transmit signals.When your alarm clock buzzes in the morning, the sound stimulates neuronsin your ear. The neurons send signals to your brain, which prompt you toeither get out of bed or hit the snooze button. Neurons transmit informationin the form of electrical and chemical impulses. When a neuron is stimulated,it produces an electrical impulse that travels only within that neuron. Beforethe signal can move to the next cell, it changes into a chemical signal.

Before a Neuron Is StimulatedWhen a neuron is not transmitting a signal, it is said to be “at rest.” However,this does not mean that the neuron is inactive. Neurons work to maintain acharge difference across their membranes, which keeps them ready to trans-mit impulses when they become stimulated.

While a neuron is at rest, the inside of its cell membrane is more negativelycharged than the outside. The difference in charge across the membrane iscalled the resting potential,resting potential, because it contains the potential energy needed totransmit an impulse. The resting potential occurs because there are unequalconcentrations of ions inside and outside the neuron.

Two types of ions—sodium ions (Na+) and potassium ions (K+)—causethe resting potential. More Na+ ions are present outside the cell than inside it.On the other hand, there are fewer K+ ions outside the cell than inside it.Notice that both ions are positively charged. The neuron is negative comparedwith its surroundings because there are fewer positive ions inside the neuron.

Proteins in the cell membrane of the neuron maintain the resting poten-tial. Some are protein channels that allow ions to diffuse across the mem-brane—Na+ ions diffuse into the cell and K+ ions diffuse out. However, themembrane has many more channels for K+ than for Na+, so positive chargesleave the cell much faster than they enter. This unequal diffusion of ions is themain reason for the resting potential. In addition, the membrane also has aprotein called the sodium-potassium pump,sodium-potassium pump, which uses energy to activelytransport Na+ ions out of the cell and bring K+ ions into the cell. This processalso helps maintain the resting potential.

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ENGLISH LEARNERSUse FIGURE 29.4 as a prereading stimulus for a picture-imaging activity. As background, choose points from pages 877 and 879, “Neurons receive and transmit signals.” Ask questions using what, where, when; size, color, number, shape, sound; movement, direction, pattern, background, perspective. Students provide details about the diagram and then compare the description to pages 877 and 879.

Biology Toolkit, Connect to Content through Visuals, p. C17

BELOW LEVELHave students use a graphic organizer, such as a cause and effect chain, to interpret and remember the details of a nerve impulse. Have students work with the text on page 877 as well as FIGURE 29.4.

Biology Toolkit, Cause and Effect Chain, p. C36

Chapter 29: Nervous and Endocrine Systems 877

B

A

B

FIGURE 29.4 Have students study the illustration and walk them through each step. Ask

• When a neuron is at rest, what is the charge of its inner cell membrane? negative

• What causes an area of the inner membrane to become positively charged? How does this happen? a stimulus; Na� channels open, allowing Na� ions to rush into the cell.

• How does an area of positive charge, or impulse, move down the axon of a neuron? The Na� channels along the axon open in sequence, allowing an area of positive charge to move down the axon.

• How is the negative charge of the axon’s inner membrane restored? K� channels open, causing K� to move out of the cell.

• What happens when the impulse reaches the axon terminal? Vesicles inside the terminal release neuro-transmitters into the synapse.

• How do the neurotransmitters generate an impulse in an adjacent neuron? The neurotransmitters bind to receptors on the adjacent neuron, causing Na� channels in that neuron to open.

AnswersA Critical Viewing An action potential

is generated when a neuron is stimulated and a portion of the inner membrane becomes positively charged. The action potential moves down the axon as portions of the inner membrane down the axon become positively charged in sequence.

Teach continuedTeach continued

TEACH FROM VISUALSTEACH FROM VISUALS

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• Na+ channels in the second neuron open quickly. Na+ rushes into the cell.

• A new impulse is generated.

ACTION POTENTIAL

area of detail

ACTION POTENTIAL

Na+

K+

CHEMICAL SYNAPSE

Na+

Na+

• When the impulse reaches the axon ter-minal, vesicles in the terminal fuse to the neuron’s membrane.

• The fusing releases neurotransmitters into the synapse.

• The neurotransmit-ters bind to the receptors on the next neuron, stimulating the neuron to open its Na+ channels.

synapse

receptor

neurotransmitter

impulse

• Na+ channels open quickly. Na+ rushes into the cell, and it becomes positive.

• The next Na+ channels down the axon spring open, and more Na+ rushes into the cell. The impulse moves forward.

• K+ channels open slowly. K+ flows out of the cell, and it becomes negative again.

impulse

impulse

impulse

vesicles

View an animationof transmission at ClassZone.com.

BIOLOGY

CRITICAL VIEWING

FIGURE 29.4 Transmission Through and Between Neurons

Once a neuron is stimulated, a portion of the inner membrane becomes positively charged. This electrical impulse, or action potential, action potential, moves down the axon. Before it can move to the next neuron, it must become a chemical signal.

How is an action potential generated, and how does it move down the axon?

+ + ++ ++ ++ ++ ++ ++ ++ –– –– –– ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++– – –– –– –– –– –– –– –– ++ ++ ++ –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– ––

– – –– –– –– –– –– –– –– ++ ++ ++ –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– ––+ + ++ ++ ++ ++ ++ ++ ++ –– –– –– ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++

++––

++––

++––

––++

––++ ––

++

+ + ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ –– –– –– ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++– – –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– ++ ++ ++ –– –– –– –– –– –– –– –– –– –– ––

– – –– –– –– –– –– –– –– –– –– –– –– –– –– –– –– ++ ++ ++ –– –– –– –– –– –– –– –– –– –– ––+ + ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ –– –– –– ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++

++––

++––

++––

––++

––++ ––

++

+ + ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++– – –– –– –– –– –– –– –– –– –– –– ––

– – –– –– –– –– –– –– –– –– –– –– ––+ + ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++

––++

––++

––++

++––

++–– ++

––

+ + ++ ++ ++ ++ ++ –– –– –– ++ ++ ++– – –– –– –– –– –– ++ ++ ++ –– –– ––

– – –– –– –– –– –– ++ ++ ++ –– –– ––+ + ++ ++ ++ ++ ++ –– –– –– ++ ++ ++

++––

++––

++––

––++

––++ ––

––++

––++

––++

––++

––++

++––

++––

++–– ++

––

++––

++ –– –– –– ++–– ++ ++ ++ ––

–– ++ ++ ++ ––++ –– –– –– ++

878 Unit 9: Human Biology

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A

action potential,

synapse. terminal,

Neurotransmitters

neurons

resting potential, action potentials

neurotransmitter

bhspe-092902.i

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Differentiated Instruction

HANDS-ON ACTIVITYStudents can make their own animation of an action potential moving down an axon. Give each group of students six index cards. Tell them to draw two lines along the length of one of the cards, with a series of ten evenly spaced negatives on the inside of each line and a series of matching positives on the outside of each line, as shown here.

On a second card, have students draw a reversal of

the first two pairs of positives and negatives along each line. On a third card, they should draw a reversal of the second two pairs of positives and negatives along each line, restoring the first two pairs. They should continue reversing two pairs of negatives and positives on each subsequent card until the

last two pairs are reversed on the last card. Tell students to stack the cards in the order in which they were made and to flip through them to watch the action potential move.

� � � � � � � � � �

� � � � � � � � � �

� � � � � � � � � �

� � � � � � � � � �878 Unit 9: Human Biology

00.2 ASSESSMENT

B

A

B

History of ScienceIn 1952, a series of papers was published summarizing the work of Alan Hodgkin and Andrew Huxley on neural action potential. Their work contributed to our current understanding of how an action potential is generated. Working at Cambridge University, Hodgkin and Huxley chose an unusual subject for their studies—the squid. A squid has an axon that is about 1 millimeter (0.04 in.) in diameter, enabling them to work a wire down its axis. In 1963, Hodgkin and Huxley won a Nobel Prize for their work.

AnswersA Contrast Signal transmission within a

neuron is electrical. Signal transmission between neurons is chemical.

Assess Use the Online Quiz or Section Quiz (Assessment Book, p. 572).Reteach Have students view nerve impulse transmission at ClassZone.com. Then have them draw and label their own diagrams to illustrate impulse transmission.

Assess and ReteachAssess and Reteach

bhspe-051401.indd Sec1:429 2/13/06 8:39:56 AM

29.2 ASSESSMENT

Connecting CONCEPTS

ONLINE QUIZClassZone.com

Transmission Within a NeuronAs you tap your finger on a desk, pressure receptors in your fingers stretch. The stretching causes a change in charge distribution that triggers a moving electrical impulse called an action potential,action potential, shown in FIGURE 29.4.

An action potential requires ion channels in the membrane that have gates that open and close. When a neuron is stimulated, gated channels for Na+

open quickly, and Na+ ions rush into the cell. This stimulates adjacent Na+

channels down the axon to spring open. Na+ ions rush into the cell, and then those ion channels snap shut. In this way, the area of positively charged membrane moves down the axon.

At the same time Na+ channels are springing open and snapping shut, K+

ion channels are opening and closing more slowly. K+ ions diffuse out of the axon and cause part of the membrane to return to resting potential. Because K+ channels are slow to respond to the change in axon’s charge, they appear to open and close behind the moving impulse.

Transmission Between NeuronsBefore an action potential moves into the next neuron, it crosses a tiny gap between the neurons called a synapse. synapse. The axon terminal,terminal, the part of the axon through which the impulse leaves that neuron, contains chemical-filled vesicles. When an impulse reaches the terminal, vesicles bind to the terminal’s membrane and release their chemicals into the synapse. NeurotransmittersNeurotransmitters(NUR-oh-TRANS-miht-urz) are the chemical signals of the nervous system. They bind to receptor proteins on the adjacent neuron and cause Na+ channels in that neuron to open, generating an action potential.

Typically, many synapses connect neurons. Before the adjacent neuron gen-erates an action potential, it usually needs to be stimulated at more than one synapse. The amount a neuron needs to be stimulated before it produces an action potential is called a threshold.

Once neurotransmitters have triggered a new action potential, they must be removed from the synapse so that ion channels on the second neuron will close again. These neurotransmitters will be broken down by enzymes in the synapse, or they are transported back into the terminal that released them.

Contrast How does signal transmission within and between neurons differ?

REVIEWING MAIN IDEAS

1. What are the roles of the three types of neuronsneurons?

2. Draw a picture to illustrate resting potential, resting potential, and explain how it helps transmit signals in neurons.

CRITICAL THINKING

3. Infer How does a threshold prevent a neuron from generating too many action potentialsaction potentials?

4. Predict What might happen if a drug blocked neurotransmitterneurotransmitter receptors?

5. Cell Chemistry Hyponatremia occurs when people have very low amounts of sodium in their body. How might the nervous system be affected if a person had this condition?

Chapter 29: Nervous and Endocrine Systems 879

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29.2 ASSESSMENT 1. Sensory neurons detect stimuli and transmit

signals to interneurons in the brain and spinal cord. Interneurons relay the signals within the brain and spinal cord, process information, and pass signals to motor neurons. Motor neurons pass signals from the brain and spinal cord to other parts of the body, such as muscles.

2. Drawings should show more Na� outside the neuron and more K� inside the neuron. The resting potential sets up the ion gradient necessary so that Na� will move into the cell and K� will move out of the cell.

3. A threshold ensures that action potentials are not produced unless the neuron has received enough stimulation.

4. Neurotransmitters would not be able to bind with the receptors and initiate impulses in the neurons.

5. Students’ responses should discuss the importance of sodium ions in generating action potentials and conclude that low amounts of sodium would make neurons less able to transmit signals.

Chapter 29: Nervous and Endocrine Systems 879

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