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OVERVIEWRationale: This lesson introduces students to inhibitory synapses. To re-view synaptic transmission, the class involves a student model pathway to act out synaptic transmission. After the class has reviewed synaptic transmission, an inhibitory neuron is introduced into the model and students explore the ef-fect on the activity of the pathway. Finally, the class engineers the neural path-way underlying why applying pressure reduces the amount of pain we feel.

Objectives: ■ Students will be able to describe the flow of ions at an inhibitory synapse.

■ Students will be able to describe how inhibitory synapses hyperpolarize the membrane potential of the postsynaptic cell.

■ Students will be able to explain why applying pressure relieves pain.

Activities: The students begin this lesson by reviewing the steps of synap-tic transmission with a partner. The lesson continues with the students mod-eling the pain pathway. This model is designed to reinforce the connection between the action potential and synaptic transmission. Once the class has reviewed the model, an inhibitory neuron is introduced into the model, and students explore its effect on the activity of the pathway. The model demon-stration is followed by a Socratic discussion of the pain pathway and why it is important to have inhibitory as well as excitatory neurons. Finally, the students work in small groups to build the neural circuit that allows pressure stimuli to modulate pain perception.

Homework: Students are assigned to work on their unit projects.

Lesson Plan

The

Lesson 3.3 Why does applying pressure relieve pain?

1. Do now (5-7 min): Students review the previous night’s homework with a partner.

2. Activity Part 1 (10-15 min):Student model of pain pathway.

3. Discussion (10-15 min): Through a Socratic discussion students are introduced to inhibitory and excitatory postsynaptic potentials.

4. Activity Part 2 (5-7 min):Build the Circuit: Students work in small groups to build the neural circuit that allows pressure stimuli to modulate painful stimuli.

5. Wrap up (3 min):Review the neural circuit that underlies how pressure modulates painful stimuli.

6. Homework:Work on unit project.

7. Materials:1. Printed Materials • Activity worksheet2. Additional Materials • White cotton balls • Colored cotton balls

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Have the students review the steps of synaptic transmission with a partner. The steps of synaptic transmission were a part of the previous night’s home-work, so the students can use either the slide or their homework worksheet to review the steps.

Synaptic Transmission

Use this slide to quickly review the steps of synaptic transmission with the class. The steps are animated so will appear one at a time.

NOWDO1.Ask the students- What is the first step of syn-aptic transmission?

■ The action potential invades the axonal terminal.

Ask the students – What happens next?

■ The voltage-gated Ca2+ channels open and Ca2+ flows into the cell.

Ask the students – What happens next?

■ The Ca2+ sensitive proteins fuse synaptic vesicles to the membrane and neurotransmitters are released into the synapse.

Ask the students – What happens next?

■ The neurotransmitters bind to receptors on the postsynaptic cell.

Ask the students – What happens next?

■ Ion channels open on the postsynaptic membrane, allowing ions to flow into the cell.

Ask the students – What happens next?

■ Excess neurotransmitters are degraded by enzymes or pumped back into the presynaptic cell.

Part 1: Student Model of Pain PathwayUse a student model of the pain pathway to demonstrate how neurons pass messages between each other and to review how the action potential and synaptic transmission are related processes.

Prepare the students for the activity by telling them that they will model the neurons in the pain pathway. Tell the students that they will be playing the part of a neuron – one hand will represent their dendrites and the other hand will represent their axon.

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To setup the initial model:

1. Ask for three student volunteers to act out the model. Have the students decide which of their hands will represent the dendrite, and which will represent the axon. Put five white cotton balls in the hand repre-senting the axon.

Ask the class what do the cotton balls represent?

■ Neurotransmitters

2. Cast each of the students as a neuron within the pain pathway.

■ Cast one student as the neuron that senses pain in the skin.

■ Cast another student as the projection neuron in the spinal cord.

■ Cast the final student as the neuron in the brain that interprets painful stimuli. Tell this student once he/she receives a message they will yell “Ouch” to show the message was received. This student does not need cotton balls since they are the end of our model pathway.

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3. Have the students assemble themselves in a representative pathway.

■ The pathway should be as follows: neuron in skin, then neuron in spinal cord, then neuron in brain.

4. Explain to the students that you will provide the sensory input (as a tap) to the first neuron (student) within the model pathway.

■ The students are then to demonstrate what happens within the pathway by passing their neurotransmitters (cotton balls) onto the next neuron. Tell the students they are to pass only one cotton ball for each stimulation they receive.

To ensure the students understand the how neurotransmitters move between neurons, ask the class – where do neurotransmitters go once they are released?

■ They cross the synaptic cleft and bind to receptors on the postsyn-aptic neuron.

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Ask the class – What happens after they bind? ■ They get degraded by enzymes in the synaptic cleft, or are pumped back

into the presynaptic cell.

Ask the class – How can the student neurons model this movement and recycling of neu-rotransmitters?

■ The students will likely come up with several of ideas, one of which could be that after neurotransmitters bind to dendrites, the neurotransmitters should be dropped on the floor. After the neurotransmitters are on the floor, the presynaptic neuron can pick them up again.

5. Tap the first student and allow the students to model neuronal communication.

■ After receiving sensory input (tap), the first student needs to send a mes-sage to the second student by releasing neurotransmitter (one cotton ball) from the axon into the synapse.

■ The dendrites of the second student receive the neurotransmitter and release neurotransmitter from their axon terminal.

■ When the third student in the chain receives neurotransmitters, the stu-dent yells “Ouch” to show the message was received.

6. Since the model will get more complicated in a minute, discuss it with the class to ensure that all students understand what it represents, as well its limitations.

Ask the students- What causes neurons to start signaling?

■ A tap from the teacher or stimulation form the previous neuron.

Ask the students- Within the first neuron, what must happen along its axon in order for it to be able to release neurotransmitter?

■ It fires an action potential.

Ask the students- How could our model neu-rons represent firing an action potential?

■ Students will likely come up with several solutions, one of which could be that the students wiggle their arms that represent the axons before passing the neurotransmitters (cotton balls) to the next neuron.

Ask the students – how does a neuron decide when to fire an action potential?

■ It must reach threshold, which is the voltage that the Na+ channels open.

Ask the students – what causes a neuron to reach threshold?

■ After neurotransmitters bind to the postsynaptic cell’s receptors, ion channels open. If positive ions enter, the postsynaptic cell may reach threshold and fire an action potential.

Ask the students – how could our model neu-rons represent the need to reach a threshold before firing an action potential?

■ Students will likely come up with several solutions, one of which could be that the neurons (students) need to receive at least two neu-rotransmitters (cotton balls) in order to fire an action potential.

7. Now, repeat the demonstration, this time incorporate acting out the action potential and the need for each neuron to reach threshold.

■ Be sure the students pay attention to the fact that each neuron must be stimulated, reach threshold, and fire an action potential before it can release neurotransmitters which then stimulate the next neuron in the pathway.

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Introduce a second sensory neuron to the model

1. Ask for another student volunteer. ■ For now, cast this student as another sensory neuron. Place this

student next to the other student representing a sensory neuron in the skin. Have this student make a synaptic connection with the neuron in the spinal cord. Give this student white cotton balls.

Ask the class – if I tap both of these sensory neu-rons, what effect will that have on the activity of the pathway?

■ It will speed up the signal because the two sensory neurons will signal at the same time and send neurotransmitters to the neuron in the spinal cord. With double the input, the spinal neuron will reach threshold sooner

and signal to the brain faster.

2. Now, repeat the demonstration, this time tapping both sensory neurons to show the students the effect on the activity of the pathway.

■ Be sure the students pay attention to the fact that the presence of two sensory neurons speeds up the activity because the neuron in the spinal cord reaches threshold sooner.

Introduce an inhibitory neuron to the model

1. Recast one of the sensory neurons as a “special” neuron. ■ Give this student colored cotton balls. (Other than the change to colored

cotton balls, this student should still follow all of the rules previously established for the model – it needs to be stimulated to fire an action potential, and once it fires and action potential it releases one cotton ball onto the next neuron’s dendrites.)

2. Tell the students that if a COLORED cotton ball is released onto their dendrites, they need double the number of white cotton balls to be able to reach threshold and signal an action potential.

Ask the class – what effect will the “special” neuron have on the activity of the pathway?

■ Students will likely predict that it will stop the activity of the pathway. In fact the presence of the “special” neuron will slow down the activity of the pathway, because the spinal neuron is receiving both negative and positive input.

3. Now, repeat the demonstration, this time incorporating the “special” neuron to show the students its effect on the activity of the pathway.

4. Discuss the effect the “special” neuron had on the activity of the pathway with the students.

Ask the students – What happened to the ac-tivity of the pathway once the “special” neuron released colored cotton balls?

■ The activity of the pathway decreased because the spinal neuron needed a lot more input to fire an action potential and send neu-rotransmitters to the brain.

Ask the students- Why would you ever want to slow the activity of neurons?

■ Students may or may not have an answer – transition to the remain-ing slides in the PowerPoint to continue the discussion of this model.

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Why would you ever want to decrease the activity of neurons?

Use this slide to stimulate a discussion about when it might be beneficial for our bodies to decrease the activity of neurons.

Ask the students – Why would you ever want to slow the activity of neurons?

■ Given the picture on the slide, students should remember the video about the football player and his desire to work through a very painful experience.

■ If we need, or want to work through a painful situation, we would want to slow down the activity of neurons signaling pain.

■ Let students brainstorm briefly about other times when it would be helpful to modulate the levels of signaling with inhibitory synapses.

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How could you decrease the activity of neurons?Use this slide to review the second part of the previous night’s homework assignment, and demonstrate the molecular mechanism underlying inhibitory synapses.

Ask the students- How could you decrease the activity of neurons?

■ For homework last night, students had to respond to the question of whether or not it mattered which ion channels opened on the post-synaptic cell following the binding of neurotransmitter. So, hopefully that will give students a hint to the answer.

■ To decrease the activity of neurons, you want to prevent them from reaching threshold and firing an action potential. To prevent a neuron from firing an action potential, you would want to allow negative ions into the postsynaptic neuron to hyperpolarize it, which means that the membrane potential is far from threshold.

If the students are having a hard time answer-ing this question, lead them to the answer by first asking – what is the voltage called that neurons have to reach before firing an action potential?

■ Neurons must reach threshold before firing an action potential.

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Discussion

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Then, ask the students – how does a neuron reach threshold?

■ After neurotransmitters bind, they open ion channels, and positive ions flow into the postsynaptic cell.

Animate the slide to show the students the inward flow of Na+ and Ca2+ ions make the postsynaptic cell more positive. Excitatory neurotransmitters depo-larize the postsynaptic cell in this fashion. After an excitatory neurotransmitter binds to the postsynaptic cell, ion channels open that allow positive ions to flow in, which depolarizes the membrane and if threshold is reached, allows an action potential to fire.

Ask the students - If the inward flow of positive ions allows a neuron to reach threshold, what ions would you want to flow into the neuron to stop it from reaching threshold?

■ Allow negative ions to enter the postsynaptic cell.

■ Additionally, hyperpolarization can result from stopping the inward flow of positive ions, but to simplify this complicated concept, today we are focusing on the inward flow of negative ions.

Animate the slide to show the students the inward flow of Cl- ions makes the postsynaptic cell more negative. Inhibitory neurotransmitters hyperpolarize the postsynaptic cell in this fashion. After inhibitory neurotransmitters bind to the postsynaptic cell, ion channels open that allow negative ions to flow into the cell, which decreases the cell’s membrane potential, preventing an action potential from firing.

Ask the students – So, if we want to prevent a neuron from signaling, after a neurotransmitter binds, what type of ions do we want to allow to flow into the postsynaptic cell?

■ Which ions flow into or out of the neuron changes the cell’s membrane potential and thus whether or not the neuron will reach threshold and fire an action potential.

■ To prevent a neuron from signalling you want to prevent it from reach-ing threshold, so oyu would want negative ions to enter.

■ The concept of reaching threshold requires the students to think back to the action potential lesson. Since it’s an important concept, and it underlies how excitatory and inhibitory synapses work, use the next slides to further discuss it.

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Getting to ThresholdUse this slide to introduce the students to the concept of excitatory post-synaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). This slide is animated, so you can introduce the concepts of threshold, EPSPs, and IPSPs to the students individually, and then label them on the graph.

Ask the students if they recognize this graph? Does it look like anything else we have seen be-fore?

■ This graph looks like the action potential graph they saw in the action potential lesson.

■ This graph is measuring the membrane potential of a neuron over time.

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Discussion

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Ask the students – What is threshold? And where is it on this chart?

■ Students should remember that threshold is the voltage at which Na+ channels open to start the action potential.

■ On this graph, it is the red dotted line.

Animate the slide to show the students the threshold textbox.

If the students are having a hard time answering this question, lead them to the answer by asking – what is the voltage called that neurons have to reach before firing an action potential?

■ Neurons must reach threshold before firing an action potential.

Then, ask the students – how does a neuron reach threshold?

■ After neurotransmitters bind, they open ion channels, and positive ions flow into the postsynaptic cell.

Animate the slide to show the students the inward flow of Na+ and Ca2+ ions make the postsynaptic cell more positive. Excitatory neurotransmitters depo-larize the postsynaptic cell in this fashion. After an excitatory neurotransmitter binds to the postsynaptic cell, ion channels open that allow positive ions to flow in, which depolarizes the membrane and if threshold is reached, allows an action potential to fire.

Ask the students – If positive ions flow into the cell, what will that do to the membrane potential?

■ Make it more positive.

Ask the students – Where on this graph is the membrane potential becoming more positive?

■ The last three upward humps.

Ask the students – When a cell’s membrane potential is more positive, is it more or less likely to reach threshold and open Na+ chan-nels to start an action potential? Additionally, you could ask the students - when a cell’s mem-brane potential becomes more positive, is it getting closer or further away from threshold?

■ More likely to fire an action potential because the membrane potential is getting closer to threshold.

■ Animate the slide to display the box about EPSPs.

Tell the students that when positive ions enter the cell, an Excitatory Postsynaptic Potential (EPSP) occurs, and this makes it more likely that threshold is reached to open voltage-gated Na+ channels and fire an action potential. These potentials are called excitatory because they enhance the likelihood of firing an action potential.

Ask the students – If negative ions flow into the cell, what will that do to the membrane poten-tial?

■ Make it more negative.

Ask the students – Where on this graph is the membrane potential becoming more negative?

■ The first three downward humps.

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Discussion

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Ask the students – after negative ions have en-tered the cell, and the cell is more negative is it more or less likely to reach threshold to start an action potential? Additionally, you could ask the students - when a cell’s membrane potential be-comes more negative, is it getting closer or fur-ther away from threshold?

■ Less likely to fire an action potential because the membrane potential is further away from threshold.

Animate the slide to display the box about IPSPs.

Tell the students that when negative ions enter the cell, an Inhibitory Postsynaptic Potential (IPSPs) occurs. These potentials are called inhibitory because they inhibit the likelihood of firing an action potential.

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Excitatory and Inhibitory Synapses

Use this slide to introduce the students to the concept of excitatory and inhibi-tory synapses. Again, this slide is animated, so you can introduce one concept at a time.

Tell the students that at an excitatory synapse (shown here in red) neurotransmitters open Na+ channels and produce EPSPs.

Tell the students that excitatory synapses are often labeled with a plus sign (+) because the membrane potential is getting more positive and the cell is likely to fire an action potential. (This notation will be used throughout the remainder of this module, so it is important that students understand what it means.)

Ask the students – after EPSPs have been pro-duced, is an action potential going to fire?

■ Yes, because threshold is reached.

Ask the students – what part of the neuron de-termines whether or not to fire an action po-tential? Where is the action potential started?

■ Axon hillock.

Animate the slide to display the inhibitory synapse.

Tell the students that at an inhibitory synapse (shown here in blue) neurotransmitters open Cl- channels and produce IPSPs.

Tell the students that inhibitory synapses are often labeled with a negative sign (-) because the membrane potential is getting more negative and the cell is less likely to fire an action potential. (This notation will be used throughout the remainder of this module, so it is important that students understand what it means.)

Ask the students – when both EPSPs and IPSPs are produced in the same cell, what will happen at the axon hillock?

■ The balance between excitatory and inhibitory will determine whether an action potential will fire.

■ The axon hillock adds the IPSPs and EPSPs together to determine if threshold is reached.

In this figure, the IPSPs are greater than the EPSPs so an action potential is not fired.

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Discussion

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Part 2: Pain and Pressure

Decreasing Neuronal Activity: Why do we instinctively apply pressure to something that is painful?

Divide the students into groups and give them each a copy of the Activity Worksheet for this lesson. This worksheet guides the students through the process of building the neural circuit that allows pressure to regulate how much pain is felt.

Before having the students complete the worksheet, introduce them to how the circuit is represented graphically by reviewing the next two slides.

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Build the Circuit in the Spinal Cord Use this slide to review with the students the different neurons involved in the neural circuit

Show the students that both the pain neuron and the pressure neuron receive stimuli from the skin and synapse with the projection neuron. Show the stu-dents that the projection neuron travels to the brain.

Tell the students that the information in this picture can also be presented in a different format without the spinal cord and skin present.

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Build the Circuit in the Spinal CordUse the animations in slide 11 to review the neural circuitry that underlies the phenomena of why applying pressure reduces the amount of pain we feel.

Tell the students that the picture from the previous can also be presented with simple line drawings, like the one shown here of the pain neuron and the projection neuron.

Now, allow the students a few minutes to complete the worksheet with a partner to build the circuit.

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The Activity Worksheet is included in the Materials

Folder for this lesson

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Activity4.

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Ask the students – What is the first thing you do after you stub your toe? Or hurt a part of your body? And what does that do to the amount of pain you feel?

■ You rub it.

■ Applying pressure decreases the amount of pain you feel.

Ask the students – What type of synapse must the pain neuron make with the projection neu-ron for a painful stimulus to be sent to the brain?

■ Excitatory.

■ Animate the slide to show the students that the pain neuron makes an excitatory synapse with the projection neuron.

Ask the students – What signal is sent to the brain?

■ Pain! And a lot of it!

■ Animate the slide to show the students that a painful stimulus is sent to the brain.

Add the Pressure Neuron

Ask the students – Like the pain neuron, the pressure neuron also makes a synapse with the projection neuron. What type of synapse could the pressure neuron make with the projection neuron to help modulate the feelings of pain?

■ Inhibitory.

■ Animate the slide to show the students that the pressure neuron makes an inhibitory synapse with the projection neuron.

Ask the students – So what signal is sent to the brain now?

■ A lot less pain.

■ Animate the slide to show the students that a smaller painful stimu-lus is sent to the brain.

Ask the students – Does this circuit look like any circuit we’ve seen before?

■ Yes, it is the final model system we setup in the beginning of the class.

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Wrap Up5.

Powerpoint Slide 11

Powerpoint Slide 11

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Unit Projects

■ For homework, have the students work on their unit projects.