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Outline
OVERVIEW
This lesson has been
differentiated for more
advanced students by
adding in more information about the action
potential.
22.2
LE S S O N
Le s s o n
Unit1.2
Rationale: This lesson introduces the action potential, the process by which
axons signal electrically. Since the concepts involved in explaining the action
potential can be quite abstract, this lesson uses analogies and a model to
demonstrate the concepts.
This is one of two lessons that introduces the action potential. This lesson
(Lesson 2.2 Differentiated) contains many more details than the other lesson
(Lesson 2.2), and is best for classes where students already have mastered
the concepts of diffusion, threshold and impermeability of the cell membrane.
If your students will need to review these concepts, use Lesson 2.2 as it reviews those concepts as it presents the action potential.
In order to allow adequate time for the students to use their models to simulate
the action potential, this lesson may take two class periods.
Objectives:
■■ Students will be able to describe how Na+ and K+ ions flow in and out of
axons to create the action potential.
■■ Students will be able to explain how the action potential moves down the
length of the axon.
■■ Students will be able to describe how Novocain works.
Activities: This lesson begins with a brief brainstorming session in which
students try to determine how Novocain, the local anesthetic used for most
dental procedures, works. The lesson continues with a Socratic discussion
in which the basic principles underlying the action potential are introduced.
Students then use a model axon to simulate the action potential and how it
moves down the axon. The class concludes with a discussion of how Novocain works.
Homework: Students complete their activity worksheets to review the action potential and concepts presented in class.
The
Lesson
Plan
Lesson 2.2 (differentiated):
How do our neurons signal
electrically?
1. Before class begins:
Prepare action potential model setups.
2. Do Now (5 min):
The students brainstorm ways that Novocain relieves pain.
3. Socratic discussion (20 min):
Socratic discussion of principles underlying action potential, including
impermeability of the cell membrane, voltage-gated channels and membrane potential.
4. Activity (10- 15 min):
Simulation of the action potential.
5. Wrap up (5 min):
How does Novocain work?
6. Homework:
Students complete their activity worksheets.
7. Materials:
1. Printed Materials
• Activity worksheet
2. Other Materials
• Action potential model setups: Black beans and toothpicks.
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Before class begins:
Prepare the action potential model setups for the activity.
“Package” the kits for each group’s supplies in Ziploc bags
to save time during the class and from year to year.
For each model, which will be used by 4-6 students, place the
following in a plastic bag:
This lesson has been
differentiated for more
advanced students by
adding in more information about th action
potential.
•
100 black eyed peas in small Ziploc bag
•
100 lima beans in small Ziploc bag
•
3 blue toothpicks
•
3 red toothpicks
•
3 black toothpicks
____________________________________
1.
DO
NOW
Have the students work with a partner to brainstorm ways that Novocain
might work to prevent pain.
Ask the students – how do you think Novocain
works?
■■ Students will likely have a variety of different ideas. Urge them to think
about what Novocain might do to our neurons that would stop them from
relaying a painful stimulus.
■■ Animate the slide to show the students that Novocain stops our neurons
from signaling electrically.
■■ Animate the slide to show the students the next question.
Ask the students – how do our neurons signal
electrically?
■■ Some students might be familiar with the concept of the action potential.
However, it is very unlikely that students will be able to fully answer this
question, because it is unlikely that they understand how the action potential works.
■■ Since how the action potential works is the focus of today’s lesson, it is
not important that students be able to fully answer this question now. Use
this question to lead into the Socratic discussion of the action potential.
2.
Discussion
Neurons Send Signals
Powerpoint
Slide 2
LE S S O N
2.2
Use this slide to review the two types of signals neurons send, as well as
how these signals relate when neurons are signaling in a chain.
Powerpoint
Slide 3
After giving the students 5 minutes to complete this task, review the students’
ideas.
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2.
Remind the students that neurons send two different types
of signals. Tell the students that neurons signal chemically
via synapses, and electrically via axons.
This lesson has been
differentiated for more
advanced students by
adding in more information about th action
potential.
■■ Animate the slide to show the students that after the first neuron sends
an electrical signal down its axon, it signals chemically to the second
neuron. Then the second neuron sends an electrical signal down its
axon, followed by a chemical signal to the third neuron. The third neuron
then sends an electrical signal down its axon.
Tell the students that today we will be focusing on how
neurons send electrical signals down their axons. We will
discuss how neurons signal chemically in the next unit.
________________________________
What is the Action Potential?
Use this slide to introduce students to the terms “action potential” and
“membrane potential”, as well as review the concept of potential energy. All
of these terms are very important in neuroscience, and it is important that the
students learn them.
We will address the term “membrane potential” in a minute, but introduce
the concept now that it is a form of potential energy, meaning that energy
is stored in the membrane just as energy is stored within a ball sitting at
the top of a hill which has the potential to roll down the hill.
_________________________________
First what is electricity?
Use this slide to review the concept of electricity and impermeability of
the cell membrane.
Powerpoint
Slide 5
Ask the students – what is electricity?
Powerpoint
Slide 4
■■ Electricity is the flow of charged particles – also known as ions.
■■ Animate the slide to show the students the definition of electricity.
■■ Animate the slide to show the diagram of the axon and next question. Remind the students that if we zoom in on the axon, it (like all
cells) is surrounded by a cell membrane.
LE S S O N
2.2
Tell the students that the process by which axons signal
electrically is called the action potential.
Tell the students that neurons do this by using the potential
energy stored in their membrane potentials.
Ask the students – can ions just flow into the
axon?
■■ No. Ions can’t just flow through the cell membrane.
■■ Animate the slide to show that Na+ bounces back upon hitting the
cell membrane.
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2.
The Axon’s Ion Channels and Pumps
Use this slide to introduce the axon’s ion channels and pumps.
This lesson has been
differentiated for more
advanced students by
adding in more information about th action
potential.
Powerpoint
Slide 6
Tell the students that the axon has ion channels and pumps that
act as doors through which ions enter and exit the axon.
Tell the students that the axon has ion channels and pumps that
act as doors through which ions enter and exit the axon.
______________________________________
The Axon’s Ion Channels and Pumps (2)
Tell the students that the voltage-gated sodium channel allows
sodium ions to enter and exit the axon. Also make sure that the
students are familiar with the chemical abbreviation (Na+) for
sodium as it will be used throughout the remainder of the lesson
and module.
The term “voltage-gated” is most likely new to the students and
we will address it in the next slide. So, if students ask now, just
have them hold onto that question.
Animate the slide to show the potassium channel. Tell the
students that the voltage-gated potassium channel allows
potassium ions to enter and exit the axon. Again, make sure that
the students are familiar with the chemical abbreviation (K+) for
potassium.
Animate the slide to show the Na+/K+ pump. Tell the students
that the Na+/K+ pump, literally pumps Na+ out of the axon and K+
into the axon.
_________________________________
Wait, what does “Voltage-gated” mean?
Use this slide to explain the term “voltage-gated”.
Use this slide to introduce the ion channels and pump involved in the action
potential. The slide is animated, so you can introduce one at a time.
Powerpoint
Slide 8
Powerpoint
Slide 7
LE S S O N
2.2
Tell the students that there are two ion channels and one pump in
the axon that help to conduct the action potential.
Ask the students – what do you think “voltagegated” means? Encourage the students to break
down the term into its parts: voltage and gate.
■■ “Voltage-gated” means that the channel is gated (or opens) at a
specific voltage. This means that the voltage of the membrane is a key
that opens the channel.
■■ Animate the slide to display the answer.
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2.
The Axon’s Ion Channels and Pumps
The Axon’s Membrane Potential (2)
Use this slide to introduce the voltages that open the voltage-gated Na+ and
K+ channels.
Use this slide to show the students the difference in charge between the
inside and outside of the axon.
This lesson has been
differentiated for more
advanced students by
adding in more information about th action
potential.
Powerpoint
Slide 8
■■ Animate the slide and tell the students that the voltage-gated Na+ channels open when the membrane potential is at -50 mV.
■■ Animate the slide and tell the students that the voltage-gated K+ channels open when the membrane potential is at +40 mV.
■■ Tell the students to keep this in mind for later.
_________________________________________
The Axon’s Membrane Potential
Powerpoint
Slide 10
■■ Animate the slide and tell the students that there are more negatively charged proteins (black stars with white negative signs) inside
the cell than outside the cell.
■■ Animate the slide and tell the students there are more Na+ ions
outside the cell than inside the cell.
■■ Animate the slide and tell the students there are more K+ ions inside
the cell than outside the cell.
____________________________________________
Use this slide to further explore the concept of membrane potential.
The Axon’s Membrane Potential (3)
Powerpoint
Slide 9
Powerpoint
Slide 11
LE S S O N
2.2
Use this slide to show the students the simplified version of charge difference across the membrane – inside is negative relative to outside.
■■ Tell the students that the term “membrane potential” refers to the potential energy stored at the membrane.
■■ Tell the students that there is potential energy because there is a charge
difference between the inside and the outside of the axon.
____________________________________________
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2.
Tell the students that when the membrane is at rest there are
more negative charges inside the cell relative to outside the cell,
therefore we say that the membrane potential is negative. When
measured relative to the outside it is actually -70 mV.
This lesson has been
differentiated for more
advanced students by
adding in more information about th action
potential.
Note: The remainder of the slides in this presentation will leave
out the negatively charged proteins for simplicity, but remember
that most of the negative charge inside the cell comes from the
proteins.
______________________________________
The Action Potential
Use the next set of slides to describe the steps of the action potential. They
are animated so you can present each step individually.
Powerpoint
Slide 14
The Axon’s Membrane Potential (4)
Use this slide to introduce the idea of graphing the membrane potential. At
rest, the membrane potential is -70 mV, therefore it is graphed with a straight
line.
Powerpoint
Slide 13
■■ Tell the students that after the neuron, diagramed here, receives stimulation from another neuron, positive charges flow down the axon. (How
this actually happens is the focus of the next unit.)
Ask the students – does anyone remember the
voltage at which the voltage-gated Na+ channels open?
■■ The voltage-gated Na+ channels open at -50 mV.
■■ Animate the slide to show the students that Na+ channels open at -50
mV, and that that voltage is called “threshold”.
■■ Tell the students that like most things in science, you can graph the
membrane potential. At rest, the membrane potential is -70 mV, so it is
graphed with a single straight line at -70 mV.
______________________________________________
LE S S O N
2.2
■■ Make sure the students are familiar with the concept of threshold – a
level that must be reached in order for an effect to happen.
■■ Animate the slide to show the opening of the voltage-gated Na+ channels.
Ask the students – now that a voltage-gated
Na+ channel is open, what direction will the
Na+ ions want to flow? In or out of the cell?
Why?
■■ The Na+ ions will flow into the cell, because they flow down their concentration gradient and towards the negative interior of the cell.
■■ Animate the slide to show that Na+ ions flow into the cell.
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2.
Ask the students – now that Na+ ions have
rushed into the cell, do you think the inside of
the cell is more negative or more positive than it
was before?
■■ The inside is more positive than it was before because you have added
positive Na+ ions to the interior.
This lesson has been
differentiated for more
advanced students by
adding in more information about th action
potential.
Powerpoint
Slide 16
■■ Use the next slide to show the change in membrane potential.
______________________________________________
The Action Potential (2)
Use this slide to show the students how the inward flow of Na+ ions increases the cell’s membrane potential.
Animate the slide and tell students that after depolarization, the
Na+ channels close.
Animate the slide to display the next question and ask the students – does anyone
remember the voltage at which the voltagegated K+ channels open?
■■ The voltage-gated K+ channels open at +40 mV.
Powerpoint
Slide 15
Animate the slide to show that voltage-gated K+ channels open
at +40 mV.
Animate the slide to show the opening of the voltage-gated K+
channel.
LE S S O N
2.2
Ask the students – now that a voltage-gated K+
channel is open, what direction will the K+ ions
want to flow? In or out of the cell? Why?
Tell the students that after Na+ ions rush into the cell, the inside
of the cell becomes more positive. Shown in the graph by the line
going from -70 mV to +40 mV.
■■ The K+ ions will flow out of the cell, because they flow down their
concentration gradient and are pushed away from the positive interior
of the cell.
Animate the slide and tell the students that this increase in
positive charge is referred to as “depolarization”.
_________________________________________
■■ Animate the slide to show that K+ ions flow out of the cell.
The Action Potential (3)
Use this slide to describe the next step of the action potential – the movement of K+ ions.
Ask the students – now that K+ ions have
rushed out of the cell, do you think the inside
of the cell is more negative or more positive
than it was before?
■■ The inside is more negative than it was before because you lost positive K+ ions.
■■ Use the next slide to show the change in membrane potential.63
2.
The Action Potential (4)
Use this slide to show the students how the outward flow of K+ ions decreases the cell’s membrane potential.
Animate the slide to display the next question and ask the students – how does the
axon return to its resting membrane potential with Na+ outside and K+ inside the cell?
■■ The Na+/K+ Pump moves Na+ out and K+ into the axon.
■■ Animate the slide to display the answer.
This lesson has been
differentiated for more
advanced students by
adding in more information about th action
potential.
Powerpoint
Slide 17
■■ Then animate the slide to show the movement of Na+ out and the
movement of K+ back into the cell.
__________________________________________
The Action Potential (6)
Tell the students that after K+ ions rush out of the cell, the inside
of the cell becomes more negative. Shown in the graph by the line
decreasing from +40 mV to -90 mV.
Animate the slide and tell the students that this decrease in charge
is referred to as “hyperpolarization”.
________________________________________________
Use this slide to show the students how the movement of Na+ out and K+
into the cell returns the membrane to its resting membrane potential.
Powerpoint
Slide 19
The Action Potential (5)
Use this slide to describe how the axon returns to resting membrane potential.
■■ Tell the students that after Na+ ions are pumped out and K+ ions are
pumped in, the cell returns to resting membrane potential of -70 mV.
Powerpoint
Slide 18
__________________________________________
LE S S O N
2.2
Animate the slide and tell students that after hyperpolarization, the
K+ channels close.
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3.
Activity
Model the Action Potential along the
length of the axon
This lesson has been
differentiated for more
advanced students by
adding in more information about th action
potential.
Have the students work in small groups to simulate how an action potential
moves down an axon.
Prepare the students for the activity by telling them that they will be simulating the action potential using dried beans and peas to represent the Na+
and K+ ions and toothpicks to represent Na+ and K+ channels. They will
apply what they know about threshold and diffusion to model how an action
potential moves down an axon.
Give each set of students an Activity Worksheet and a Model Setup. Give
the students 10-15 minutes to set up and use their models.
The Actvity worksheet is
included in the Materials
Folder for this lesson.
Powerpoint
Slide 20
________________________________________________
The Action Potential along the Axon
Use this slide to review what the students should have observed when
simulating the action potential with their models.
LE S S O N
2.2
Powerpoint
Slide 21
Ask the students - what happens when a signal
is received from another neuron?
■■ The first Na+ channel opens and Na+ rushes into the cell.
■■ Animate the slide to show that the first Na+ channel opens and Na+
rushes into the cell, causing the cell membrane to depolarize and
send a signal to the next set of channels.
■■ For simplicity, this slide only models the movement of 4 Na+ and 4 K+
ions. Remind the students that there are many more Na+ and K+ ions
involved in the action potential.
Ask the students – after the first Na+ channel opens and Na+ enters the cell, what voltage does the membrane reach? What happens
then?
■■ The membrane potential has reached +40 mV and the first K+ channel opens, and K+ rushes out of the cell.
■■ Animate the slide to show that the first K+ channel opens and K+
rushes out of the cell, causing the membrane to hyperpolarize.
Ask the students – after the first Na+ channel opens and Na+ enters the cell, what voltage does the membrane reach? What happens
then?
■■ The Na+/K+ pump moves Na+ out of the cell and K+ into the cell.
■■ Animate the slide to show that the Na+/K+ pump moves Na+ out and
K+ in and repolarizes the membrane.
Remind the students that the signaling of this
first set of ion channels sent a signal down the
axon to the next set of ion channels. Ask the
students – what happens to the second set of
ion channels when they receive a signal from
the previous set of ion channels?
■■ The second Na+ channel opens because it reaches threshold, and
Na+ rushes into the cell.
65
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3.
Activity
■■ Animate the slide to show that the second Na+ channel opens and Na+
rushes into the cell, causing the cell membrane to depolarize and signal
to the next set of channels.
This lesson has been
differentiated for more
advanced students by
adding in more information about the action
potential.
How does Novocain work?
Use this slide to discuss how Novocain works.
■■ The membrane potential has reached +40 mV and the second K+ channels open, and K+ rushes out of the cell.
Powerpoint
Slide 22
■■ Animate the slide to show that the second K+ channel opens and K+
rushes out of the cell, causing the membrane to hyperpolarize.
■■ The Na+/K+ pump moves Na+ out of the cell and K+ into the cell.
■■ Animate the slide to show that the Na+/K+ pump moves Na+ out and K+
in and repolarizes the membrane.
■■ Continue to animate the slide to demonstrate the following points:
■■ Each segment along the axon actually fires its own action potential by
opening Na+ channels, then opening K+ channels, then activating the
Na+/K+ pump.
■■ The signaling of the previous segment actually triggers the action
potential for the next segment by passing along positive charge to allow
the voltage-gated Na+ channels to open.
2.2
Wrap Up
Ask the students – after the second Na+ channel
opens and Na+ enters the cell, what voltage does
the membrane reach? What happens then?
Ask the students – after the second K+ channel
opens and K+ leaves the cell, how does the axon
return to its resting membrane potential?
LE S S O N
4.
Tell the students that this process of opening successive Na+
channels along the axon is the Action Potential and how neurons
signal electrically.
Note: While this lesson does not discuss how this electrical signal
along the axon is converted into a chemical signal at the synapse,
that process will be discussed in the next unit on the synapse.
Ask the students – now that you know how
our axons signal electrically, how do you think
Novocain works to stop this signaling and prevent pain?
■■ Students may have a variety of answers. Encourage them to apply
what they have learned about the action potential to propose possible mechanisms.
■■ Novocain works by inhibiting the Na+ channels from opening.
■■ Animate the slide to show that Novocain inhibits the voltage-gated
Na+ channels.
Animate the slide to show the next question
and ask the students – what happens if the
Na+ channels can’t open?
■■
If the Na+ channels can’t open, the axon can’t send an action
potential.
■■ Animate the slide to show that the axon can’t send an action potential.
66
4.
Wrap Up
This lesson has been
differentiated for more
advanced students by
adding in more information about the action
potential.
LE S S O N
2.2
■■ No. If neurons are unable to signal that they sense pain, your brain
never receives a painful stimulus, meaning you can’t feel pain. In this
case it’s not “No brain, no pain”, but “No pain to the brain.”
■■ Animate the slide to show that if the axon can’t signal electrically, you
won’t be able to feel pain.
Homework
Ask the students – if you can’t send an action
potential, can you feel pain?
Worksheet: The stages of
the Action Potential.
■■ For homework have the
students complete their
Activity Worksheets,
and write a summary
of what is happening at
each stage of the action
potential.
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