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Rationale: This lesson introduces students to inhibitory synapses. To review 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 effect on the activity of the pathway. Finally, the class engineers the neural pathway underlying why applying pressure reduces the amount of pain we feel.
Lesson
Plan
Objectives:
■■ Students will be able to describe the flow of ions at an inhibitory synapse.
Lesson 3.3 Why does applying
pressure relieve pain?
■■ Students will be able to describe how inhibitory synapses hyperpolarize
the membrane potential of the postsynaptic cell.
1. Do now (5-7 min):
Outline
OVERVIEW
23.3
LE S S O N
Le s s o n
Unit1.2
The
■■ Students will be able to explain why applying pressure relieves pain.
Activities: The students begin this lesson by reviewing the steps of synaptic transmission with a partner. The lesson continues with the students modeling 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 demonstration 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.
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 worksheet
2. Additional Materials
• White cotton balls
• Colored cotton balls
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1.
DO
NOW
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 homework, so the students can use either the slide or their homework worksheet to
review the steps.
Ask the students- What is the first step of synaptic 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?
Powerpoint
Slide 2
■■ 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.
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.
Part 1: Student Model of Pain
Pathway
Use 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.
LE S S O N
3.3
Powerpoint
Slide 3
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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2.
Activity
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.
Powerpoint
Slide 4
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 representing the axon.
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 postsynaptic neuron.
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.
LE S S O N
3.3
■■ 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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2.
Activity
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 neurotransmitters?
■■ 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 message 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 student yells “Ouch” to show the message was received.
LE S S O N
3.3
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 neurons 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 neurons 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 neurotransmitters (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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2.
Activity
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.
■■ 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.
■■ 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.
4. Discuss the effect the “special” neuron had on the
activity of the pathway with the students.
■■ 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.
3.3
Ask the class – what effect will the “special”
neuron have on the activity of the pathway?
Ask the class – if I tap both of these sensory neurons, what effect will that have on the activity of
the pathway?
2. Now, repeat the demonstration, this time tapping both
sensory neurons to show the students the effect on the
activity of the pathway.
LE S S O N
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 students – What happened to the activity of the pathway once the “special” neuron
released colored cotton balls?
Introduce an inhibitory neuron to the model
■■ The activity of the pathway decreased because the spinal neuron
needed a lot more input to fire an action potential and send neurotransmitters to the brain.
1. Recast one of the sensory neurons as a “special” neuron.
Ask the students- Why would you ever want to
slow the activity of neurons?
■■ 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.)
■■ Students may or may not have an answer – transition to the remaining slides in the PowerPoint to continue the discussion of this model.
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3.
Discussion
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.
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.
Powerpoint
Slide 6
Powerpoint
Slide 5
Ask the students- How could you 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.
LE S S O N
3.3
_____________________________________
■■ For homework last night, students had to respond to the question of
whether or not it mattered which ion channels opened on the postsynaptic 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 answering 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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3.
Discussion
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 depolarize 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.
LE S S O N
3.3
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 reaching 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.
________________________________________
Getting to Threshold
Use this slide to introduce the students to the concept of excitatory postsynaptic 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.
Powerpoint
Slide 7
Ask the students if they recognize this graph?
Does it look like anything else we have seen before?
■■ 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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3.
Discussion
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.
Ask the students – When a cell’s membrane
potential is more positive, is it more or less
likely to reach threshold and open Na+ channels to start an action potential? Additionally,
you could ask the students - when a cell’s membrane potential becomes more positive, is it
getting closer or further away from threshold?
■■ On this graph, it is the red dotted line.
■■ More likely to fire an action potential because the membrane potential
is getting closer to threshold.
Animate the slide to show the students the threshold textbox.
■■ Animate the slide to display the box about EPSPs.
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?
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.
■■ 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 depolarize 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 negative ions flow into the
cell, what will that do to the membrane potential?
■■ Make it more negative.
Ask the students – Where on this graph is the
membrane potential becoming more negative?
■■ The first three downward humps.
Ask the students – If positive ions flow into the
cell, what will that do to the membrane potential?
LE S S O N
3.3
■■ Make it more positive.
Ask the students – Where on this graph is the
membrane potential becoming more positive?
■■ The last three upward humps.
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3.
Discussion
Ask the students – after negative ions have entered 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 becomes more negative, is it getting closer or further 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.
______________________________________
Excitatory and Inhibitory Synapses
Use this slide to introduce the students to the concept of excitatory and inhibitory 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 produced, is an action potential going to fire?
■■ Yes, because threshold is reached.
Ask the students – what part of the neuron determines whether or not to fire an action potential? 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?
LE S S O N
3.3
Powerpoint
Slide 8
■■ 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
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is not fired.
3.
4.
Activity
Tell the students that the information in this picture can
also be presented in a different format without the spinal
cord and skin present.
_______________________________________
Build the Circuit in the Spinal Cord
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.
Use 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.
Powerpoint
Slide 10
Before having the students complete the worksheet, introduce them to how
the circuit is represented graphically by reviewing the next two slides.
The Activity Worksheet is
included in the Materials
Folder for this lesson
_________________________________________
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.
Powerpoint
Slide 9
LE S S O N
3.3
Now, allow the students a few minutes to complete the worksheet with a
partner to build the circuit.
_________________________________________
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 students that the projection neuron travels to the brain.
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5.
Wrap Up
Add the Pressure Neuron
Powerpoint
Slide 11
Powerpoint
Slide 11
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.
LE S S O N
3.3
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?
Ask the students – What type of synapse must
the pain neuron make with the projection neuron for a painful stimulus to be sent to the brain?
■■ A lot less pain.
■■ Excitatory.
■■ Animate the slide to show the students that a smaller painful stimulus is sent to the brain.
■■ Animate the slide to show the students that the pain neuron makes an
excitatory synapse with the projection neuron.
Ask the students – Does this circuit look like
any circuit we’ve seen before?
Ask the students – What signal is sent to the
brain?
■■ Yes, it is the final model system we setup in the beginning of the
class.
■■ Pain! And a lot of it!
_____________________________________
■■ Animate the slide to show the students that a painful stimulus is sent to
the brain.
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LE S S O N
3.3
Homework
5.
Unit Projects
■■ For homework, have the
students work on their unit
projects.
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