Acting Potential

Acting Potential
This activity was made possible by Grant Number 8 R25 RR/AA09832-03 from the National Institutes of Health. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Institutes of Health; the grant recipient, the National Association of Biology Teachers; or the subcontractor, the Society for Neuroscience.
Thomas J. Conley
Parkway West High School
Ballwin, MO

Type of entry:
Lesson/class activity
Type of activity:
Hands-on, simulation
Target audience:
General Biology-adaptable to all levels
Background information:
Objectives
The students should be able to

predict the movement of Na+ and K+ across a permeable membrane.
state the mechanism that allows a neuron to remain at rest.
demonstrate an action potential moving down an axon.
design a diagram for a resting neuron and for an action potential moving down a axon.

Integration into curriculum

Before doing this lesson, the topics of cell membrane, cell transport-including both passive and active transport and the anatomy of the neuron should be covered.
This activity is designed for a general biology class. An advanced class could use activities 2 and 3 based on their previous knowledge and with less teacher direction. For a lower level class, activity 1 and part of activity 2 could be used when studying cell transport.

Class time needed

One 55 minute class to do the three activities
Additional time is needed to discuss application questions of activity three

Materials Needed

Four to five hours to prepare materials depending on the number of groups one has in the class but materials can be saved for future years
This activity can be done in small groups or as a class using the overhead. The materials listed will be for the overhead and if doing as small groups one will just have to make additional set ups.

Materials:

1-12X6.5 inch clear plastic storage box (size does not need to be exact and does not have to be clear if doing in small groups.
10-12 mm beads for Na+ or 3/4 X 11/2 in. dowel rod
10-16 mm X10 mm discs for K+ or 7/8 X 11/4 in. dowel rod
10-20 mm beads for negative molecules-what is important is 3 different sizes of beads
1-piece of foam board

All of this can be purchased at a craft store. The sizes are not an absolute but it is important for the negative intracellular molecules to be the largest and for the two ions not to be able to move through the wrong channels. Different sizes and shapes for the Na+ and K+ ions were used to accomplish this. One could also use different sizes of dowel rods as indicated.
For the membrane use a 4 cm wide strip of foam board long enough to fit the length of your box. The pores for the Na+ and K+ need to be of appropriate size for the beads used. The pump was made by cutting a 25 mm circle from the foam board and by cutting out two notches on opposite sides. One notch has the shape and 1/2 the diameter of Na+ and the other notch was cut to fit the K+ ion. Make the following cuts: Na+ pore, K+ pore, pump site, Na+ pore, K+ pore, pump site. Put a small dowel rod through the center of the disc. The cut out in the membrane for the pump must be large enough to fit the carrier protein and the two molecules it carries. Using the beads from this setup, the opening was 21/2 X 11/2 inches. For the gates use the piece of foam board cut out when you made the opening in the strip and tape it to the membrane.See the following diagram which is not drawn to a set scale but is meant to give one a visual of these descriptions:

Notes for teacher
Students will work through all three activities. Do not stress the use of terms until the students have worked through each activity.

Activity 1
Exploration

You will need to set up the cell membrane as described in background but do not let the students worry about the sodium-potassium pump portions for now. The gates for Na+ and K+ should be completely opened

When you add the sodium and potassium ions, make sure there are more sodiums on the outside of the membrane and more potassiums on the inside. A few sodiums maybe placed on the inside of the membrane or a few potassiums on the outside.

This activity is designed for the students to interact with each other. Let them mentally and orally interact and writing can be done at the end of each activity.

When the students have arranged the beads properly, they will work through the four questions on their handout.
Concept/Term Introduction

Make sure they understand that diffusion is occurring and the idea that cells can perform functions that are needed for their survival even when the task goes against the moleculesš own forces.

Activity 2
Exploration

For this activity the sodium-potassium pump will be used and place the negatively charged particles on the inside portion of the membrane. Put out more negatively charged particles than there are positively charged particles on the inside.

Have the gates closed for sodium but partially opened for potassium.

Do not tell them that the model is representing the axon of a neuron.

Let them work out any possibilities as they do questions 1-3.

If in their decision making no one has considered the role of the large negative molecules, then you will need to remind them of the attraction of opposites. Similarly, if they do not notice that some ions could leak through, you can add this variable.
Concept/Term Introduction

When they have looked at all of their possibilities and have given an answer to question 4 then you can mention that this represents the axon of a neuron and that it wants to maintain this imbalance so that the cell is positive on the outside and negative on the inside with a small intracellular negative voltage. This disequilibrium is maintained by the sodium-potassium pump.

Activity 3
Concept/Term Introduction

At this point you will need to work with them as they read through the description on their handout. If your textbook has a better description, use it.

Start with the first sodium gate opening and continue down the axon. It is important that they see this is a self-perpetuating system. As the first sodiums flow in, this triggers the full opening of the potassium channels and the opening of the next sodium channels.

Also after the impulse passes one set of sodium channels, the impulse cannot go backwards because there is a brief period in which previous channels cannot be opened regardless of the voltage applied.

Third, there is not just one channel opening at a time but many channels in the same area are being stimulated simultaneously by the change in voltage.

Finally, the impulse is both a flow of electrical charge and a flow of molecules across the membrane. Application

As the sodium moves in and changes the voltage of this area of the axon, the changed voltage also stimulates the next group of sodium channels. Thus the impulse is self-perpetuating.

Impulse cannot flow backwards once initiated at the beginning of the axon because of the refractory period that results as the Na+ channels close and the K+ channels are activated.

Since the stimulus is in the center of the axon and no refractory condition exists, the stimulus should cause the opening of Na+ channels on both sides of the stimulus. The result of this would be the flowing of one impulse towards the axonšs terminal end and one impulse moving towards the cell body and dendrite.

Since the amplitude of the potential cannot increase regardless of the stimulus (remember the all or none principle), the increase in sensation is due to more frequent impulses being sent down the axon.

Fingertip Information

One method to demonstrate the stimulus threshold and the all or none response is to use a slinky. Set it at the top of the steps and begin to tap it easily. Increase the strength of your tap until the stimulus is great enough to cause the slinky to move down the stairs.

Do not be afraid to let your students work through their hypotheses without your directions. Give them the chance to find out the answer on their own. 9. Modifications

For an honors biology class or an advanced class, the students could be made responsible for setting the beads up themselves in Activity 2 without teacher input based on having read their textbook. Likewise, Activity 3 could be worked through without the students having the benefit of the written description.

For these advanced students if you have time, you could have them design the materials for these three activities.

For the lower level biology students Activity 1 and Activity 2 (Questions 1-3) could be used as the topics of cell movement are covered.

Abstract of Activity:
How does the nerve impulse/the action potential flow along the axon of the neuron? Acting potential is a student participation activity in which the students will manipulate the protein molecules of the neural membrane and the Na+ and K+ ions involved in an impulse as it is generated and begins to move down an axon. It is expected that after participation in this activity the students will have a better visualization of an action potential and they will be able to diagram and explain the movement of the impulse down the axon.
This activity is based on the Learning Cycle model. During Activity 1 and 2, the students explore the movement of molecules across a typical cell membrane and then a neuron. At the end of Activity 2 and the beginning of Activity 3, terms important for the understanding of an action potential are introduced. Understanding of the concept can be checked by students answering the application level questions. Students can also be asked to research problems related to nerve transmission, such as epilepsy.

Student lesson
Activity 1
In this activity you will be using different beads to represent sodium or potassium ions and a model of a cell membrane. You will have to follow your teacheršs directions in setting up this activity and subsequent activities.

Once the beads are in position examine the set up of Activity 1 and predict what should happen to the movement of sodium and potassium. Write down your prediction.
Work through all predictions

Based on the demonstrations which hypothesis seems correct? Explain the process that occurred in this activity.

If this cell is found in humans, do you believe that this process would always occur or could a cell exist in this disequilibrium. Explain your answer and check with other students to get their feedback.

What answer did the class agree upon.

Activity 2
Follow your teacher's directions for setting the beads up for this activity. In this activity the membrane will represent a highly specialized cell of your body.

What do you notice that is the same in the set up of activities 1 and 2.

What do you notice that is different?

Predict what you think will happen to the sodium, potassium and negatively charged intracellular molecules. Explain your answer.

In making your prediction did you remember all that you had learned about the cell membrane and the movement of materials through a cell membrane.

Do all of you agree on the outcome?

Try all possibilities.

How could this cell maintain this disequilibrium and what effect would the imbalance of positive and negative charges have on the cell?
Think about electricity.

Activity 3
Now that you know that Activity 2 was looking at an axon of a neuron, you want to demonstrate a nerve impulse or what is called an action potential.

When an impulse reaches the axon, the voltage within the cell is reduced and this starts the action potential. This change in voltage causes an immediate opening of the sodium gates and half a millisecond later causes the gate to close. Within a few milliseconds the sodium gates could be reopened, if given the right stimulus. As the sodium moves into the cell, the voltage increases which causes two events. One result is the full opening of the potassium channels which allows potassium to stream out. The movement of potassium to the outside drops the voltage which causes the potassium gates to return to their resting state. Only a relatively few number of sodium ions have to move in to change the voltage and a few potassium ions need to move out to return the voltage to the resting potential. The sodium-potassium pump restores the concentration of these ions to their original level. The second result is that this voltage change also stimulates the next group of sodium channels so that the impulse once started can continue to move down the axon at the same amplitude and without further stimuli.

Try to work this out with your teacheršs help.

Once the impulse started with the first sodium gate, what kept the impulse moving down the axon?

Can the impulse once generated flow back to the cell body instead of towards the synaptic end of the axon? Explain.

What could happen to the flow of the impulse if the threshold stimulus occurred in the middle of the axon instead of at the beginning by the cell body?

Using your textbook examine a graph of an action potential. Explain what happens when as a stimulus increases one feels more of the sensation?

Using the information from these activities, design a diagram(s) to show a resting neuron and an action potential moving down the neuron. Label the appropriate molecules in the membrane and in the extra-cellular and intracellular fluids.

This activity describes what happens in a non-myelinated axon. What happens to the process when a myelin sheath covers the axon.

As an application students can research the cause and treatments of epilepsy, a disease of the CNS in which many neurons are being stimulated and results in the seizure.

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