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OVERVIEWRationale: 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 re-views 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 Novo-cain works.

Homework: Students complete their activity worksheets to review the ac-tion potential and concepts presented in class.

Lesson Plan

The

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 mem-brane 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.

This lesson has beendifferentiated for moreadvanced students byadding in more infor-mation about the action potential.

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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:

• 100 black eyed peas in small Ziploc bag

• 100 lima beans in small Ziploc bag

• 3 blue toothpicks

• 3 red toothpicks

• 3 black toothpicks

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Have the students work with a partner to brainstorm ways that Novocain might work to prevent pain.

After giving the students 5 minutes to complete this task, review the students’ ideas.

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This lesson has beendifferentiated for moreadvanced students byadding in more infor-mation about th action potential.

Discussion

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 po-tential 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.

Neurons Send SignalsUse 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.

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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.

■ 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.

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.

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.

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First what is electricity?Use this slide to review the concept of electricity and impermeability of the cell membrane.

Ask the students – what is electricity?

■ 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 ques-tion. Remind the students that if we zoom in on the axon, it (like all cells) is surrounded by a cell membrane.

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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The Axon’s Ion Channels and Pumps Use this slide to introduce the axon’s ion channels and pumps.

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.

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The Axon’s Ion Channels and Pumps (2)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.

Tell the students that there are two ion channels and one pump in the axon that help to conduct the action potential.

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”.

Ask the students – what do you think “voltage-gated” 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

Use this slide to introduce the voltages that open the voltage-gated Na+ and K+ channels.

■ Animate the slide and tell the students that the voltage-gated Na+ chan-nels open when the membrane potential is at -50 mV.

■ Animate the slide and tell the students that the voltage-gated K+ chan-nels open when the membrane potential is at +40 mV.

■ Tell the students to keep this in mind for later.

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The Axon’s Membrane Potential

Use this slide to further explore the concept of membrane potential.

■ Tell the students that the term “membrane potential” refers to the poten-tial 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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The Axon’s Membrane Potential (2)

Use this slide to show the students the difference in charge between the inside and outside of the axon.

■ Animate the slide and tell the students that there are more nega-tively 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.

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The Axon’s Membrane Potential (3)

Use this slide to show the students the simplified version of charge differ-ence across the membrane – inside is negative relative to outside.

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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.

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.

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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.

■ 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.

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The Action PotentialUse the next set of slides to describe the steps of the action potential. They are animated so you can present each step individually.

■ Tell the students that after the neuron, diagramed here, receives stimu-lation 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+ chan-nels 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”.

■ 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+ chan-nels.

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 con-centration 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.

■ Use the next slide to show the change in membrane potential.

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The Action Potential (2)Use this slide to show the students how the inward flow of Na+ ions increas-es the cell’s membrane potential.

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.

Animate the slide and tell the students that this increase in positive charge is referred to as “depolarization”.

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The Action Potential (3)Use this slide to describe the next step of the action potential – the move-ment of K+ ions.

Animate the slide and tell students that after depolarization, the Na+ channels close.

Animate the slide to display the next ques-tion and ask the students – does anyone remember the voltage at which the voltage-gated K+ channels open?

■ The voltage-gated K+ channels open at +40 mV.

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.

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?

■ 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 to show that K+ ions flow out of the cell.

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 posi-tive K+ ions.

■ Use the next slide to show the change in membrane potential.

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2.The Action Potential (4)Use this slide to show the students how the outward flow of K+ ions decreas-es the cell’s membrane potential.

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”.

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The Action Potential (5)Use this slide to describe how the axon returns to resting membrane poten-tial.

Animate the slide and tell students that after hyperpolarization, the K+ channels close.

Animate the slide to display the next ques-tion and ask the students – how does the axon return to its resting membrane poten-tial 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.

■ Then animate the slide to show the movement of Na+ out and the movement of K+ back into the cell.

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The Action Potential (6)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.

■ 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.

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The Actvity worksheet is included in the Materials

Folder for this lesson.

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Model the Action Potential along the length of the axonHave 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 simulat-ing 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.

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The Action Potential along the AxonUse this slide to review what the students should have observed when simulating the action potential with their models.

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This lesson has beendifferentiated for moreadvanced students byadding in more infor-mation about th action potential.

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+ chan-nel opens and Na+ enters the cell, what volt-age does the membrane reach? What happens then?

■ The membrane potential has reached +40 mV and the first K+ chan-nel 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+ chan-nel opens and Na+ enters the cell, what volt-age 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.

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Activity

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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.

Ask the students – after the second 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 second K+ chan-nels open, and K+ rushes out of the cell.

■ Animate the slide to show that the second K+ channel opens and K+ rushes out of the cell, causing the membrane to hyperpolarize.

Ask the students – after the second K+ channel opens and K+ leaves the cell, how does the axon return to its resting membrane potential?

■ 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.

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.

How does Novocain work?Use this slide to discuss how Novocain works.

Ask the students – now that you know how our axons signal electrically, how do you think Novocain works to stop this signaling and pre-vent pain?

■ Students may have a variety of answers. Encourage them to apply what they have learned about the action potential to propose pos-sible 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 poten-tial.

Wrap Up

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Ask the students – if you can’t send an action potential, can you feel pain?

■ 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.

This lesson has beendifferentiated for moreadvanced students byadding in more infor-mation about the action potential.

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Wrap Up

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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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