SECTION 29.

2
Plan and Prepare 29.2 Neurons
KEY CONCEPT The nervous system is composed of highly specialized cells.

Objectives MAIN IDEAS

• Neurons are highly
VOCABULARY
neuron, p. 876 action potential, p. 878
• Describe neurons as specialized specialized cells. dendrite, p. 876 synapse, p. 879
cells. • Neurons receive and terminal, p. 879
axon, p. 876
• Explain how neurons transmit and transmit signals.
resting potential, p. 877 neurotransmitter, p. 879
receive signals. sodium-potassium pump, p. 877

Section Resources
Connect When you eat a snack, you might flick crumbs off of your fingers
Unit Resource Book without giving it much thought. The specialized cells of your nervous system,
Study Guide pp. 27–28 however, are hard at work carrying the messages between your fingers and
your brain.
Power Notes p. 29
Reinforcement p. 30
Pre-AP Activity pp. 49–50 MAIN IDEA

Interactive Reader Chapter 29

Neurons are highly specialized cells.
Spanish Study Guide pp. 291–292 A neuron is a specialized cell that stores information and carries messages
within the nervous system and between other body systems. Most neurons
Biology Toolkit pp. C8, C17, C36 have three main parts, as shown in FIGURE 29.3.
Technology 1 The cell body is the part of the neuron that contains the nucleus
Power Presentation 29.2 and organelles.
Media Gallery DVD 2 Dendrites are branchlike extensions of the cytoplasm and the cell mem-
brane that receive messages from neighboring cells. Neurons can have
Online Quiz 29.2 more than one dendrite, and each dendrite can have many branches.
3 Each neuron has one axon. An axon is a long extension that carries
Activate Prior Knowledge Explain that electrical messages away from the cell body and passes them to other cells.
a phobia is a fear of a specific thing that
usually elicits an immediate response.
Have students think of what they fear FIGURE 29.3 Structure of a Neuron
the most and how it makes them feel. A neuron is a specialized cell of the nervous system that produces and transmits signals.
Ask
• How do you respond to your fear?
Students may suggest screaming, 1 3 axon
cell body axon
jumping back, increased heart and terminals
breathing rates, or sweating.
• What body system makes immedi-
ate reactions possible? nervous
system myelin sheath
2
dendrites colored LM; magnification 200⫻

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

Vocabulary 876 Unit 9: Human Biology

Differentiated Instruction
dendrite Tell students that the prefix of
dendrite has two meanings: dendro- from HANDS-ON ACTIVITY PRE-AP
b10hspe-092902.indd 876 U 9/9/09 7:49:51 PM
the Greek dendron, meaning “tree,” or
dendr-, meaning “earlier.” Relate this to To demonstrate how a nerve impulse travels For this section, suggest students use the
dendrite structure. A dendrite branches along the axon of a neuron, give each group Survey/Question/Read/Recite/Review
like a tree; it is also where an impulse of students eight dominoes. Have students strategy. First they preview the section by
enters a neuron—its “earliest” point. set the dominoes on end, about three- surveying the material and developing
quarters of a domino’s length away from one questions. As they read, they will answer the
another, in a straight line. Then have them questions, clarify the answers, and then use
Answers flick the first domino in the line. Ask, How is these to review what they have learned.
A Infer Having more than one the action of the dominoes like a nerve
dendrite enables a neuron to receive impulse? requires a stimulus, moves one way Biology Toolkit, SQ3R, p. C8
signals from multiple neurons. 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
876 Unit 9: Human Biology reinstated before a new signal can be sent.

b10hste-0929.indd 876 9/10/09 1:55:58 PM

 There are three types of neurons: (1) sensory neurons, (2) interneurons, TAKING NOTES
and (3) motor neurons. Sensory neurons detect stimuli and transmit signals
to the brain and the spinal cord, which are both made up of interneurons.
Use a flow chart to organize
your notes on how a neuron
Answers
Interneurons receive signals from sensory neurons and relay them within the
transmits a signal. A Analyze A neuron has a long axon
brain and the spinal cord. They process information and pass signals to motor Neuron is Na+ channels that carries signals long distances.
stimulated. open; action
neurons. Motor neurons pass messages from the nervous system to other potential
generated.
tissues 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 myelin
Take It Further
sheath, insulates neurons’ axons and helps them to send messages. In a cell membrane, there are many
openings, or channels, formed by
A Analyze How does a neuron’s shape allow it to send signals across long distances?
proteins that extend through to both
sides of the membrane. There are at
MAIN IDEA least three types of protein channels in a
Neurons receive and transmit signals. neuron’s membrane:
When your alarm clock buzzes in the morning, the sound stimulates neurons ion channels These channels are always
in your ear. The neurons send signals to your brain, which prompt you to open and allow tiny quantities of ions to
either get out of bed or hit the snooze button. Neurons transmit information enter and leave the cell by diffusion.
in the form of electrical and chemical impulses. When a neuron is stimulated, This normally helps create a balance
it produces an electrical impulse that travels only within that neuron. Before inside and outside the cell.
the signal can move to the next cell, it changes into a chemical signal. Naⴙ–Kⴙ pump These channels use
active transport to maintain a negative
Before a Neuron Is Stimulated charge, or resting potential, inside the
When a neuron is not transmitting a signal, it is said to be “at rest.” However, neuron.
this does not mean that the neuron is inactive. Neurons work to maintain a
charge difference across their membranes, which keeps them ready to trans- gated channels When a dendrite is
mit impulses when they become stimulated. stimulated by a neurotransmitter, gated
channels in the dendrite open to allow
While a neuron is at rest, the inside of its cell membrane is more negatively
Na to enter the cell at that site. If the
charged than the outside. The difference in charge across the membrane is
number of ions that enter crosses a
called the resting potential, because it contains the potential energy needed to
transmit an impulse. The resting potential occurs because there are unequal
threshold, the site becomes depolarized,
concentrations of ions inside and outside the neuron.
which causes other gated channels to
open sequentially down the axon.
Two types of ions—sodium ions (Na+) and potassium ions (K+)—cause
the resting potential. More Na+ ions are present outside the cell than inside it. Connecting CONCEPTS

On the other hand, there are fewer K+ ions outside the cell than inside it. Active Transport Recall from
Chapter 3 that energy and
Notice that both ions are positively charged. The neuron is negative compared
with its surroundings because there are fewer positive ions inside the neuron.
specialized membrane proteins Vocabulary
are required to move molecules
and ions against the concentra- Academic Vocabulary The words
Proteins in the cell membrane of the neuron maintain the resting poten- tion gradient. potent and potential have the same
tial. Some are protein channels that allow ions to diffuse across the mem- root, meaning “to be able”:
outside inside
brane—Na+ ions diffuse into the cell and K+ ions diffuse out. However, the
membrane has many more channels for K+ than for Na+, so positive charges energy
potent, capable of exerting a strong
leave the cell much faster than they enter. This unequal diffusion of ions is the effect
main reason for the resting potential. In addition, the membrane also has a potential, capable of being but not yet
protein called the sodium-potassium pump, which uses energy to actively in existence
transport Na+ ions out of the cell and bring K+ ions into the cell. This process
also helps maintain the resting potential.
Students may have seen the word used in
physics: potential energy versus kinetic
energy. There is a similar use here: resting
Chapter 29: Nervous and Endocrine Systems 877 potential versus action potential.

8/06
M 1:46:01 PM ENGLISH LEARNERS
bhspe-092902.indd Sec2:877 BELOW LEVEL 6/28/06 1:46:10

Use FIGURE 29.4 as a prereading stimulus for a Have students use a graphic organizer, such as Connecting CONCEPTS
picture-imaging activity. As background, a cause and effect chain, to interpret and Active Transport It is important for
choose points from pages 877 and 879, remember the details of a nerve impulse. students to understand that active
“Neurons receive and transmit signals.” Ask Have students work with the text on page transport is not involved in the propaga-
questions using what, where, when; size, 877 as well as FIGURE 29.4. tion of a nerve impulse through the
color, number, shape, sound; movement, axon. It is active transport that maintains
direction, pattern, background, perspective. Biology Toolkit, Cause and Effect Chain, resting potential via the sodium-
Students provide details about the diagram p. C36 potassium pump.
and then compare the description to pages
877 and 879.
Biology Toolkit, Connect to Content through
Visuals, p. C17 Chapter 29: Nervous and Endocrine Systems 877
 FIGURE 29.4 Transmission Through and Between Neurons
BIOLOGY
Once a neuron is stimulated, a portion of the inner membrane becomes positively View an animation
Teach continued charged. This electrical impulse, or action potential, moves down the axon. Before
it can move to the next neuron, it must become a chemical signal.
of transmission at
ClassZone.com.

TEACH FROM VISUALS

FIGURE 29.4 Have students study the ACTION POTENTIAL
illustration and walk them through each area of detail
step. Ask Na+
• When a neuron is at rest, what is the
+ – – – + • Na+ channels open quickly. Na+ rushes
into the cell, and it becomes positive.
charge of its inner cell membrane? – + + + – • The next Na+ channels down the axon
negative spring open, and more Na+ rushes into
the cell. The impulse moves forward.
• What causes an area of the inner – + + + – • K+ channels open slowly. K+ flows out of
membrane to become positively
charged? How does this happen? + +
– – – + the cell, and it becomes negative again.

a stimulus; Na channels open, K

allowing Na ions to rush into impulse +
+ +
the cell. + + + + + + + + + – – – + + + + + + + + + + + + + – – + + + + + +
– – – – – – – – – + + + – – – – – – – – – – – – – – – – – – – –
• How does an area of positive charge, A
or impulse, move down the axon of– – – – – – – – – + + + – – – – – – – – – – – – –
– –
– – – – – –
+ + + + + + + + + – – – + + + + + + + + + + + + + + + – + + + + + +
a neuron? The Na channels along +
the axon open in sequence, allowing impulse
+ + +
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 CHEMICAL SYNAPSE
move out of the cell. impulse
• What happens when the impulse Na+ – – – BB
reaches the axon terminal? Vesicles+– + + + + • When the impulse synapse
– + + + + + + + + + +
reaches the axon ter- – – +
inside the terminal release neuro- –
– – – –
minal, vesicles in the + + +
– – – – – – – –

transmitters into the synapse. +

–
+ + + +
– – – terminal fuse to the
neuron’s membrane. – + + – + +
– –
+ +
– –
– –
+ + + + + + +
– – – –

• How do the neurotransmitters • The fusing releases

– A
generate an impulse in an adjacent neurotransmitters neurotransmitter
into the synapse.
neuron? The neurotransmitters bind + impulse
to receptors on the adjacent neuron,
• The neurotransmit-
receptor + +
+ + + + + ters bind to the + + – – – – – + + +
–
causing Na channels in that neuron– – – – – receptors on the next vesicles – – – + + + – – –
–
to open. –
+
–
+ + + +
– – – neuron, stimulating
the neuron to open + + + +
–
+ + + +
–
– –
–
+ + +
– – –
– –
+ + +
–

its Na+ channels. + – –

– ACTION POTENTIAL
Na+
• Na+ channels in the second
Answers neuron open quickly. Na+
rushes into the cell.
A Critical Viewing An action potential A
CRITICAL How is an action potential generated, and how
• A new impulse is generated.
is generated when a neuron is stimulated VIEWING does it move down the axon?
and a portion of the inner membrane
becomes positively charged. The action 878 Unit 9: Human Biology
potential moves down the axon as Differentiated Instruction
portions of the inner membrane down
the axon become positively charged in C d HANDS-ON ACTIVITY
lhspe-092902.indd 2Sec2:878
Fil N
03240 bh 092902 i dd U
the first twoi pairs
j
of positives and negatives PM
L M difi d 6/22/06 2 256/28/06 1:46:17 PM
bhspe-051401.indd
bhspe-092902.i

sequence. Students can make their own animation of an along each line. On a third card, they should
action potential moving down an axon. Give draw a reversal of the second two pairs of
each group of students six index cards. Tell positives and negatives along each line,
them to draw two lines along the length of restoring the first two pairs. They should
one of the cards, with a series of ten evenly continue reversing two pairs of negatives and
spaced negatives on the positives on each subsequent card until the
inside of each line and a          
last two pairs are reversed on
series of matching positives the last card. Tell students to
on the outside of each line,           stack the cards in the order
as shown here. in which they were made and
          to flip through them to
On a second card, have watch the action potential
students draw a reversal of           move.
878 Unit 9: Human Biology
 Transmission Within a Neuron
As you tap your finger on a desk, pressure receptors in your fingers stretch. History of Science
The stretching causes a change in charge distribution that triggers a moving
In 1952, a series of papers was published
electrical impulse called an action potential, shown in FIGURE 29.4.
summarizing the work of Alan Hodgkin
An action potential requires ion channels in the membrane that have gates and Andrew Huxley on neural action
that open and close. When a neuron is stimulated, gated channels for Na+ potential. Their work contributed to our
open quickly, and Na+ ions rush into the cell. This stimulates adjacent Na+ current understanding of how an action
channels down the axon to spring open. Na+ ions rush into the cell, and then
potential is generated. Working at
those ion channels snap shut. In this way, the area of positively charged
Cambridge University, Hodgkin and
membrane moves down the axon.
Huxley chose an unusual subject for
At the same time Na+ channels are springing open and snapping shut, K+ their studies—the squid. A squid has an
ion channels are opening and closing more slowly. K+ ions diffuse out of the axon that is about 1 millimeter (0.04 in.)
axon and cause part of the membrane to return to resting potential. Because in diameter, enabling them to work a
K+ channels are slow to respond to the change in axon’s charge, they appear to wire down its axis. In 1963, Hodgkin and
open and close behind the moving impulse.
Huxley won a Nobel Prize for their work.
Transmission Between Neurons
Before an action potential moves into the next neuron, it crosses a tiny gap
between the neurons called a synapse. The axon terminal, the part of the axon
A through which the impulse leaves that neuron, contains chemical-filled
vesicles. When an impulse reaches the terminal, vesicles bind to the terminal’s
Answers
membrane and release their chemicals into the synapse. Neurotransmitters A Contrast Signal transmission within a
(NUR-oh-TRANS-miht-urz) are the chemical signals of the nervous system. neuron is electrical. Signal transmission
They bind to receptor proteins on the adjacent neuron and cause Na+ channels between neurons is chemical.
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. Assess and Reteach
BB Once neurotransmitters have triggered a new action potential, they must
be removed from the synapse so that ion channels on the second neuron will Assess Use the Online Quiz or Section
close again. These neurotransmitters will be broken down by enzymes in the Quiz (Assessment Book, p. 572).
synapse, or they are transported back into the terminal that released them. Reteach Have students view nerve
A Contrast How does signal transmission within and between neurons differ? impulse transmission at ClassZone.com.
Then have them draw and label their
own diagrams to illustrate impulse
ONLINE QUIZ transmission.
29.2 ASSESS MENT ClassZone.com

REVIEWING MAIN IDEAS CRITICAL THINKING Connecting CONCEPTS

1. What are the roles of the three 3. Infer How does a threshold 5. Cell Chemistry Hyponatremia
types of neurons? prevent a neuron from generating occurs when people have very
2. Draw a picture to illustrate too many action potentials? low amounts of sodium in their
resting potential, and explain 4. Predict What might happen if a body. How might the nervous
how it helps transmit signals in drug blocked neurotransmitter system be affected if a person
neurons. receptors? had this condition?

Chapter 29: Nervous and Endocrine Systems 879

29.2 ASSESSMENT
00.2 ASSESSMENT
1. Sensory neurons detect stimuli and transmit 2. Drawings should show more Na⫹ outside 4. Neurotransmitters would not be able to
bhspe-051401.indd Sec1:429 2/13/06 8:39:56 AM
signals to interneurons in the brain and the neuron and more K⫹ inside the neuron. bind with the receptors and initiate
b10hspe-092902.indd spinal cord. Interneurons relay the signals
879 The resting potential sets up the ion impulses
9/2/08 1:49:26 M in the neurons.
within the brain and spinal cord, process gradient necessary so that Na⫹ will move 5. Students’ responses should discuss the
information, and pass signals to motor into the cell and K⫹ will move out of importance of sodium ions in generating
neurons. Motor neurons pass signals from the cell. action potentials and conclude that low
the brain and spinal cord to other parts of 3. A threshold ensures that action potentials amounts of sodium would make neurons
the body, such as muscles. are not produced unless the neuron has less able to transmit signals.
received enough stimulation.

Chapter 29: Nervous and Endocrine Systems 879

b10hste-0929.indd 879 9/10/08 1:58:51 PM