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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 demonstrations, analogies and a model to demonstrate the concepts.

This is one of two lessons that introduces the action potential. This lesson is best for classes where students are unfamiliar with the concepts of diffusion, threshold, and impermeability of the cell membrane. The second lesson (Les-son 2.2 Differentiated) contains many more details about the action potential, and is therefore best for classes where students have already mastered the concepts of diffusion, threshold and impermeability of the cell membrane.

You do not need to teach both lessons - just choose the lesson that suits your class best.

Objectives:

■ Students will be able to describe the concept of threshold – once inputs reach a certain point; an action potential is fired.

■ Students will be able to describe how Na+ ions flow into the axon to cre-ate the action potential.

■ 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. The students then use a model to simulate the action of the Na+ channels and movement of Na+ ions into the axon during an action potential. The lesson concludes with a discussion of how Novocain works. It inhibits the Na+ chan-nels which initiate the action potential.

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

Lesson Plan

The

Lesson 2.2: How do our neu-rons 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 diffusion, permeability of the cell membrane, threshold, and action of Na+ channels.

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 tooth-picks.

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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 beans in small Ziploc bag

• 3 blue toothpicks

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

■ 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 answer this question now. Use this question to lead into the Socratic discussion of the action potential.

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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 neu-rons from signaling electrically.

Animate the slide to show the students the next question. Ask the students – how do our neurons signal electrically?

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.

________________________________The Axon’s Electrical Signal: The Action PotentialUse this slide to introduce students to the term “Action Potential”. This is a very important term in neuroscience, and it is important that the students learn it.

Tell the students that the process by which axons signal electrically is called the action potential.

Before we model the Action Potential, let’s review: DiffusionUse these next several slides to review important concepts that the students must understand to appreciate the action potential and how Novocain works. Use this slide to review the concept of diffusion - the movement of molecules from areas of high concentration to areas of low concentration.

You may choose to actually set up this slide as a demonstration in your classroom. All you need is a clear cup filled with water and some food col-oring. (If you heat the water, the process of diffusion will be faster.) Then, at this point in the lecture, just add two drops of food coloring to the water

and let it sit for the remainder of the class.

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2.Ask the students – if you drop some blue food coloring in a glass of water, what will happen if you let it just sit for a few minutes?

■ The blue coloring will diffuse throughout all the water in the glass.

■ Animate the slide to show the students the picture of an entirely blue glass of water.

Ask the students – why does this happen?

■ The blue food coloring is at high concentration when it gets dropped into the water. It gradually diffuses or moves to areas of lower concen-tration until it is equally dispersed throughout all the water, which turns the water blue.

_____________________________________Diffusion of Sodium Ions (Na+)

Use this slide to show the students that diffusion occurs even if you can’t see it happening – for example with the sodium ions (Na+) shown here.

Make sure the students are aware of the chemical abbreviation for sodium (Na+). This abbreviation will be used throughout the remainder of this lesson and module.

Ask the students – if you dropped some so-dium ions in a glass of water, what will happen if you let it just sit for a few minutes?

■ Just like the blue food coloring, the sodium ions will diffuse through-out the entire glass.

■ Animate the slide to show the students the picture of a sodium ions distributed evenly throughout the entire glass.

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Before we model the Action Potential, let’s review: The Cell Membrane

Use this slide to review that the neuron is surrounded by a cell membrane which is composed of a phospholipid bilayer.

Tell the students that if we zoom in on the axon, it (like all cells) is surrounded by a cell membrane.

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2.The Cell MembraneUse this slide to review the impermeability of the cell membrane, as well as introduce the idea of ion channels that allow ions to enter and exit the cell. This slide is animated, so you can present the three circumstances individu-ally.

Ask the students – can Na+ just go through the cell membrane?

■ No. Na+ can’t just flow through the cell membrane.

■ Animate the slide to show that Na+ bounces back upon hitting the cell membrane.

Animate the slide to show the students the closed Na+ channel. Tell the students that our cell membranes have channels that act like doors through which ions can enter and exit the cell.

Ask the students – can Na+ go through a closed channel in the cell membrane? Could you go through a closed door?

■ No. Na+ can’t go through a closed channel, just like we can’t walk through closed doors.

■ Animate the slide to show that Na+ bounces back upon hitting a

closed Na+ channel.

Animate the slide to show the students the open Na+ channel. Ask the students – can Na+ go through an open Na+ channel in the cell membrane?

■ Yes. Na+ can go through an open Na+ channel.

■ Animate the slide to show that Na+ enters the cell by passing

through the open Na+ channel.

To connect the concept of ion channels back to diffusion, and lay the ground work for a principle to come, ask the students – why would Na+ want to go through an open Na+ channel? (Hint: What concentration would Na+ be outside to want to go inside the axon?).

■ Na+ will flow through a Na+ channel to get away from an area of high concentration to an area of low concen-tration.

Animate the slide to show the students the next question. Ask the students – what causes the Na+ channels to open?

■ Students will likely not know the answer to this question, but allow the students to brainstorm.

■ Animate the slide to show the students the answer. That Na+ chan-nels open at threshold.

Since threshold is another important concept, use the next slide to review threshold.

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2.Before we model the Action Potential, let’s review: ThresholdUse this slide, which presents the toilet flush analogy, to review the concept of threshold.

Ask the students – how do you flush a toilet?

■ You flush a toilet by pressing the lever.

Animate the slide to show the students the next question. Ask the students – what if you just gently jiggle the handle? Will the toilet flush?

■ No, gently jiggling will not flush the toilet. You need to actually press the lever to flush the toilet.

Animate the slide to show the students the next question. Ask the students – can you stop a toi-let from flushing after it’s started to flush?

■ No. Unless you do something crazy, once a toilet starts flushing, it completes flushing.

Tell the students that threshold is like flushing a toilet. You could jiggle the handle a little bit, but there is a definite point that causes the toilet to flush completely. That point is called threshold. Also, once the toilet has started to flush, it will complete flushing. Toilets either totally flush or don’t flush – an All-or-None principle, like the action potential. Once an action potential starts firing along an axon, it signals all the way down the axon.

_________________________Getting to Threshold Use this slide to describe how neurons reach threshold and open the Na+ channels. This concept will be elaborated on in the next unit after we discuss synaptic signaling, but for now make sure the students know that a neuron reaches threshold because it receives chemical signals from another neuron.

Tell the students that when a neuron receives a chemical signal from another neuron, it may reach threshold. If it reaches threshold, Na+ channels open.

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2.Note: This version of this lesson is not getting into the details of voltage-gating, but if students want to know what is it that actually opens the Na+ channels, you could say that it’s a positive charge (other ions) that flow into the neuron after a chemical signal is received. We will get into more detail on this concept in the next unit. ____________________________________Getting to Threshold (2)Use this slide to illustrate how a signal from another neuron sets off a cascade of toilet flushes. This slide is animated so you can show each step individually.

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Tell the students that for now we’ll represent the Na+ channels as toilets. When a signal comes down the axon from another neuron, it causes the first toilet to reach threshold.

Ask the students – What happens when the first toilet reaches threshold?

■ It flushes.

■ Animate the slide to show the first toilet flushing and that it sends a signal down to the next toilet.

Tell the students that that first toilet’s flushing also sends a signal down to the second toilet, which causes it to reach threshold and also flush. Animate the slide to show the second toilet flushing and that it sends a signal to the third toilet.

Ask the students – what happens next?

■ The third toilet reaches threshold and flushes, sending the signal onto the next

■ Animate the slide to show the students that the third toilet flushes and sends a signal down the axon.

Remind the students that the toilet flushing is an analogy for the opening of the Na+ channels in the axon’s membrane - when a neuron receives a signal from another neuron, if it reaches threshold, the Na+ channels open.

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ActivityModeling the Action PotentialHave 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 simu-lating the action potential using dried beans to represent Na+ ions and toothpicks to represent Na+ 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 minutes to set up and use their models.

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Model of the Action PotentialUse this slide to review what the students should have observed when simulating the action potential with their models.

Ask the students - what happens when a signal is received from another neuron?

■ The first Na+ channel opens.

■ Animate the slide to show that the first Na+ channel opens.

Ask the students – where is Na+ at a higher concentration – inside or outside of the cell?

■ Outside the cell.

■ Even though this slide shows 6 Na+ outside the axon, the concentra-tion of Na+ outside is MUCH greater than it is inside. For simplicity this slide only models the movement of 6 Na+. As we progress through the animations in this slide, it might visually appear that Na+ is at a higher concentration inside then out, but remind students that there are TONS more Na+ outside the cell than inside.

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The Activity Worksheet is included in the Materials Folder for this lesson

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ActivityAsk the students – based on the principle of dif-fusion, where will the Na+ go once the Na+ chan-nels open?

■ The Na+ will enter the axon, because it is at a lower concentration inside the axon than outside of it.

■ Animate the slide to show that Na+ enters the axon, and provides a signal to the next Na+ channel.

Ask the students – after the first Na+ channel opens and allows Na+ to enter, what happens to the second Na+ channel?

■ It opens, and allows more Na+ to enter the axon.

■ Animate the slide to show that the second Na+ channel opens and more Na+ enters the axon which provides a signal to the third Na+ channel.

Ask the students – after the second Na+ channel opens and allows Na+ to enter, what happens to the third Na+ channel?

■ It opens, and allows still more Na+ to enter the axon.

■ Animate the slide to show that the third Na+ channel opens and more Na+ enters the axon which provides a signal further down the axon.

Animate the slide to show the Action Potential arrow. Tell the students that this process of opening successive Na+ channels along the axon is the Action Potential and how neurons signal electrically.

■ Note: For simplicity, this lesson does not discuss how the axon pumps the Na+ ions out of the axon after firing an action potential. If the students are curious, you can tell them that there is a pump in the membrane that literally pumps the Na+ back outside. The pump is why there is so much more Na+ outside than inside.

■ 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 - it plugs the toilets.

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Wrap Up

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The Activity Worksheet is included in the Materials Folder for this lesson

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How does Novocain work?Use this slide to show how Novocain inhibits the Na+ channels, which prevents our axons from signaling and thus relaying a painful stimulus to the brain.

Ask the students – What happens if the Na+

channels can’t open?

■ If the Na+ channels can’t open you can’t send an action potential.

■ Animate the slide to show that the axon can’t send an action potential.

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: Action Potential.

■ For homework have the students complete their Activity Worksheets.

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