Outline This lesson has been about the action 2Lesson Unit1.2 OVERVIEW 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 This lesson (Lesson Differentiated) contains many more details than the other lesson (Lesson ), 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 as it reviews those concepts as it presents the action 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 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 (differentiated): How do our neurons signal electrically? 1. Before class begins: Prepare action potential model setups. 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 4. Activity (10-15 min): Simulation of the action 5. Wrap up (5 min): How does Novocain work? 6. Homework: Students complete their activity worksheets. 7. Materials: 1. Printed Materials Activity worksheet Other Materials Action potential model setups: Black beans and toothpicks. 57
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 1. DO NOW Have the students work with a partner to brainstorm ways that Novocain might work to prevent pain. Slide 2 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 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 Discussion Neurons Send Signals 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. After giving the students 5 minutes to complete this task, review the students ideas. Slide 3 58
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. 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. Slide 5 Slide 4 Tell the students that the process by which axons signal electrically is called the action Tell the students that neurons do this by using the potential energy stored in their membrane potentials. 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 question. 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. 59
The Axon s Ion Channels and Pumps Use this slide to introduce the axon s ion channels and pumps. 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) Use this slide to introduce the ion channels and pump involved in the action The slide is animated, so you can introduce one at a time. 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. Slide 7 Slide 8 Tell the students that there are two ion channels and one pump in the axon that help to conduct the action 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. 60
The Axon s Ion Channels and Pumps Use this slide to introduce the voltages that open the voltage-gated Na+ and K+ channels. 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. This lesson has been Slide 8 Slide 10 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 Use this slide to further explore the concept of membrane Slide 9 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. The Axon s Membrane Potential (3) 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. Slide 11 61
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. The Axon s Membrane Potential (4) Use this slide to introduce the idea of graphing the membrane At rest, the membrane potential is -70 mv, therefore it is graphed with a straight line. Slide 13 Tell the students that like most things in science, you can graph the membrane At rest, the membrane potential is -70 mv, so it is graphed with a single straight line at -70 mv. The Action Potential Use the next set of slides to describe the steps of the action They are animated so you can present each step individually. 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. Slide 14 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+ 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. 62
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 The Action Potential (2) Use this slide to show the students how the inward flow of Na+ ions increases the cell s membrane 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. The Action Potential (3) Slide 15 Use this slide to describe the next step of the action potential the movement of K + ions. Slide 16 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. 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 positive K+ ions. Use the next slide to show the change in membrane 63
The Action Potential (4) Use this slide to show the students how the outward flow of K+ ions decreases the cell s membrane 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. The Action Potential (5) Slide 17 Use this slide to describe how the axon returns to resting membrane Slide 18 Animate the slide and tell students that after hyperpolarization, the K + channels close. 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. Then animate the slide to show the movement of Na+ out and the movement of K+ back into the cell. 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 Slide 19 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. 64
3. This lesson has been The Actvity worksheet is included in the Materials Folder for this lesson. Activity Model the Action Potential along the length of the axon 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. 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. 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 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
3. Activity 4. Wrap Up This lesson has been about the action 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+ channels 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. How does Novocain work? Use this slide to discuss how Novocain works. Slide 22 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. 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 Animate the slide to show that the axon can t send an action 66
4. Wrap Up Ask the students if you can t send an action potential, can you feel pain? This lesson has been about the action 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 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 67