SECTION 29.2
29.2
Plan and Prepare
Objectives
Neurons
KEY CONCEPT The nervous system is composed of highly specialized cells.
MAIN IDEAS
VOCABULARY
• Neurons are highly
specialized cells.
• Neurons receive and
transmit signals.
• Describe neurons as specialized
cells.
• Explain how neurons transmit and
receive signals.
Section Resources
neuron, p. 876
dendrite, p. 876
axon, p. 876
resting potential, p. 877
sodium-potassium pump, p. 877
Connect When you eat a snack, you might flick crumbs off of your fingers
Unit Resource Book
Study Guide pp. 27–28
Power Notes p. 29
Reinforcement p. 30
Pre-AP Activity pp. 49–50
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.
Interactive Reader Chapter 29
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
have three main parts, as shown in FIGURE 29.3.
Biology Toolkit pp. C8, C17, C36
Technology
Power Presentation 29.2
Media Gallery DVD
Online 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 immediate reactions possible? nervous
system
1
The cell body is the part of the neuron that contains the nucleus
and organelles.
2
Dendrites are branchlike extensions of the cytoplasm and the cell membrane 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 axon is a long extension that carries
electrical messages away from the cell body and passes them to other cells.
FIGURE 29.3 Structure of a Neuron
A neuron is a specialized cell of the nervous system that produces and transmits signals.
1
cell body
2
3
axon
Vocabulary
dendrite 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.
Answers
A Infer Having more than one
dendrite enables a neuron to receive
signals from multiple neurons.
876
Unit 9: Human Biology
b10hste-0929.indd 876
axon
terminals
myelin sheath
dendrites
Teach
action potential, p. 878
synapse, p. 879
terminal, p. 879
neurotransmitter, p. 879
colored LM; magnification 200⫻
A Infer Why might it be beneficial for a neuron to have more than one dendrite?
876
Unit 9: Human Biology
Differentiated Instruction
HANDS-ON ACTIVITY
PRE-AP
U
To 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 threequarters 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.
For 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.
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Biology Toolkit, SQ3R, p. C8
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8/06
M 1:46:01 PM
There are three types of neurons: (1) sensory neurons, (2) interneurons,
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.
Interneurons receive signals from sensory neurons and relay them within the
brain and the spinal cord. They process information and pass signals to motor
neurons. Motor neurons pass messages from the nervous system to other
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
sheath, insulates neurons’ axons and helps them to send messages.
TAKING NOTES
Answers
Use a flow chart to organize
your notes on how a neuron
transmits a signal.
Neuron is
stimulated.
A Analyze A neuron has a long axon
that carries signals long distances.
Na+ channels
open; action
potential
generated.
Take It Further
In 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.
Naⴙ–Kⴙ 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.
A 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 neurons
in your ear. The neurons send signals to your brain, which prompt you to
either get out of bed or hit the snooze button. Neurons transmit information
in the form of electrical and chemical impulses. When a neuron is stimulated,
it produces an electrical impulse that travels only within that neuron. Before
the signal can move to the next cell, it changes into a chemical signal.
Before a Neuron Is Stimulated
When 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 a
charge difference across their membranes, which keeps them ready to transmit impulses when they become stimulated.
While a neuron is at rest, the inside of its cell membrane is more negatively
charged than the outside. The difference in charge across the membrane is
called the resting potential, because it contains the potential energy needed to
transmit an impulse. The resting potential occurs because there are unequal
concentrations of ions inside and outside the neuron.
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.
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 compared
with its surroundings because there are fewer positive ions inside the neuron.
Proteins in the cell membrane of the neuron maintain the resting potential. Some are protein channels that allow ions to diffuse across the membrane—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
leave the cell much faster than they enter. This unequal diffusion of ions is the
main reason for the resting potential. In addition, the membrane also has a
protein called the sodium-potassium pump, which uses energy to actively
transport Na+ ions out of the cell and bring K+ ions into the cell. This process
also helps maintain the resting potential.
Connecting
CONCEPTS
Active Transport Recall from
Chapter 3 that energy and
specialized membrane proteins
are required to move molecules
and ions against the concentration gradient.
outside
inside
energy
Chapter 29: Nervous and Endocrine Systems
ENGLISH LEARNERS
BELOW LEVEL
Use 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.
Have 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.
bhspe-092902.indd Sec2:877
Biology Toolkit, Connect to Content through
Visuals, p. C17
Biology Toolkit, Cause and Effect Chain,
p. C36
Vocabulary
877
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Academic 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.
Connecting
CONCEPTS
Active Transport It is important for
students to understand that active
transport is not involved in the propagation of a nerve impulse through the
axon. It is active transport that maintains
resting potential via the sodiumpotassium pump.
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
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.
Teach continued
View an animation
of transmission at
ClassZone.com.
TEACH FROM VISUALS
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.
+
ACTION POTENTIAL
+
–
Na+
–
+ + +
+ + +
–
–
+
–
–
area of detail
+
–
• 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.
–
–
+
–
+
K
+ + + + + + + +
–
–
–
–
–
–
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–
+ + + + + + + +
–
impulse
– –
+ + +
+ + +
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–
+ + + + + + + + + + + + +
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878
Unit 9: Human Biology
–
– –
+ + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + +
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+ + + + + + + + + + + + + + + +
impulse
– – –
+ + +
+ + +
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A
+ + + + + +
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+ + + + + +
CHEMICAL SYNAPSE
+ + + +
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+ + + +
+ + + +
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+ + + +
• When the impulse
reaches the axon terminal, vesicles in the
terminal fuse to the
neuron’s membrane.
• The fusing releases
neurotransmitters
into the synapse.
• The neurotransmitters bind to the
receptors on the next
neuron, stimulating
the neuron to open
its Na+ channels.
Na+
synapse
+
–
–
–
+ +
878
CRITICAL
VIEWING
–
–
+ + +
+
– –
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+ +
+ +
BB
+ + + + + + +
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– –
+ + + + + + +
+ +
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A
neurotransmitter
receptor
–
+
+ +
–
+ +
–
vesicles
+ +
–
Na+
A
impulse
–
– –
+ +
Answers
A 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.
+ +
–
How is an action potential generated, and how
does it move down the axon?
–
–
–
–
–
–
– –
+ + + +
–
–
–
impulse
– –
+ + +
+ + +
–
–
–
+ + +
–
–
–
–
–
–
+ + +
ACTION POTENTIAL
• Na+ channels in the second
neuron open quickly. Na+
rushes into the cell.
• A new impulse is generated.
Unit 9: Human Biology
Differentiated
Instruction
HANDS-ON ACTIVITY
Fil N
bh
2Sec2:878
03240
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C d
092902 i dd
Students 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 twoi pairs
of positives
and negatives PM
L M difi d 6/22/06 2 256/28/06
j
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.
U
1:46:17 PM
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Transmission Within a Neuron
History of Science
As 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, 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.
In 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.
Transmission Between Neurons
A
BB
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
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. Neurotransmitters
(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 generates 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.
Answers
A Contrast Signal transmission within a
neuron is electrical. Signal transmission
between neurons is chemical.
Assess and Reteach
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.
A Contrast How does signal transmission within and between neurons differ?
29.2
ONLINE QUIZ
ASSESS MENT
REVIEWING
MAIN IDEAS
1. What are the roles of the three
types of neurons?
2. Draw a picture to illustrate
resting potential, and explain
how it helps transmit signals in
neurons.
ClassZone.com
CRITICAL THINKING
3. Infer How does a threshold
prevent a neuron from generating
too many action potentials?
4. Predict What might happen if a
drug blocked neurotransmitter
receptors?
29.2 ASSESSMENT
00.2
ASSESSMENT
1. Sensory
neurons detect stimuli and transmit
bhspe-051401.indd Sec1:429
b10hspe-092902.indd
signals to interneurons in the brain and
spinal cord. Interneurons relay the signals
879
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.
Connecting
CONCEPTS
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
4. Neurotransmitters would not be able to
2. Drawings should show more Na⫹ outside
2/13/06 8:39:56 AM
bind with the receptors and initiate
the neuron and more K⫹ inside the neuron.
impulses
in the neurons.
The resting potential sets up the ion
9/2/08 1:49:26
M
gradient necessary so that Na⫹ will move
5. Students’ responses should discuss the
into the cell and K⫹ will move out of
importance of sodium ions in generating
the cell.
action potentials and conclude that low
amounts of sodium would make neurons
3. A threshold ensures that action potentials
less able to transmit signals.
are not produced unless the neuron has
received enough stimulation.
Chapter 29: Nervous and Endocrine Systems 879
b10hste-0929.indd 879
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