Download ND Lesson 2.2-Differentiated

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ND Lesson 2.2-Differentiated: Activity Worksheet
Introduction: In this activity, you will set up a model to simulate how neurons signal
electrically. First a quick review of some things you’ll need to know:
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Ions
 Black-eyed peas represent sodium ions. There are more sodium ions outside the
axon than inside, so place more peas “outside” the axon.
 Lima beans represent potassium ions. There are more potassium ions inside the
axon than outside, so place more lima beans “inside” the axon.
 In a real cell, there would be millions of ions, but there isn’t enough room for that
many peas and beans on your model.
 One force acting on ions is for them to move from areas of higher concentration to
areas of lower concentration.
 A positive charge attracts a negative charge, and vice versa. However, positive
charges repel each other, and so do negative charges.
 Electrical charge (membrane potential) is the result of excess ions on one side of the
cell membrane.
Ion channels and pumps
 Neurons have specialized proteins in their membranes called channels. Neurons
have channels for many different ions, but each channel only recognizes one ion.
For example, if a sodium channel opens, sodium ions, but no other ions, can pass
through. The colored toothpicks in your model represent sodium ion channels,
potassium ion channels and the sodium/potassium pump.
 The ion channels open once a threshold has been reached. The first Na+ channel in
your diagram reaches threshold after another neuron (not diagramed) sends a signal
to your model neuron. The second and subsequent Na+ channels open in response to
the opening of the Na+ channel immediately before it. K+ channels also reach
threshold and open when the Na+ channel immediately before it opens.
 The sodium/potassium pump moves three Na+ ions out for every two K+ ions it
moves into the cell.
Directions: Set up your model axon according to the diagram below. You will use your model to
simulate the steps described and answer the questions that follow.
Questions:
1. Start this simulation with all of your ion channels closed. Look at the number of blackeyed peas representing sodium ions inside and outside the cell. If a sodium channel were
suddenly opened so that sodium ions could move across the cell membrane, which
direction would they tend to move based on their concentration and charge: into or out of
the cell? Explain.
2. Your model neuron has just received a signal from another neuron. This signal causes the
first sodium channel to reach threshold and open. A sodium channel opens for about one
millisecond. Have one of your group members time the opening of the sodium channel
for 5 seconds, representing one millisecond. Before he/she begins timing, decide which
direction the sodium ions will move based on your answer to question 1. Once ready,
have the timekeeper begin timing 5 seconds. Immediately open the first sodium channel
by moving the toothpick as shown in the diagram. Then have another group member drag
the peas through the sodium channel one at a time in the direction you think they will go
until the timekeeper reaches 5 seconds. Then close the channel.
i. Look at the number of sodium ions on each side of the cell membrane now.
Compared to the number before the sodium channel opened there are: (circle one)
a. More sodium ions inside the cell now than there were before.
b. Fewer sodium ions inside the cell now than there were before.
ii. Do you think the internal medium of the cell is: (circle one)
a. More negative than it was before.
b. More positive than it was before.
3. Look at the number of lima beans representing potassium ions inside and outside the cell.
If the potassium channel was suddenly opened so that potassium ions could move across
the cell membrane, which direction would they tend to move based on their concentration
and charge: into or out of the cell? Explain.
4. A potassium channel opens for one millisecond. Follow the directions in step 2 above,
except now open the first potassium channel. Remember to decide which direction the
potassium will move before you begin timing.
i. Look at the number of potassium ions on each side of the cell membrane now.
Compared to the number before the potassium channel opened there are: (circle
one)
a. More potassium ions inside the cell now than there were before.
b. Fewer potassium ions inside the cell now than there were before.
ii. Do you think the internal medium of the cell is: (circle one)
a. More negative than it was before.
b. More positive than it was before.
5. Now that this segment of the axon has more sodium ions inside and more potassium ions
outside, the first sodium potassium pump moves 3 sodium outside and 2 potassium ions
inside.
i. Do you think the internal medium of the cell is: (circle one)
a. More negative than it was before.
b. More positive than it was before.
6. In response to the opening of the first sodium channel, the second sodium channel
reaches threshold and opens. Repeat step 2, only this time for the second sodium channel.
7. In response to the opening of the second sodium channel, the second potassium channel
reaches threshold and opens. Repeat step 2, only this time for the second potassium
channel.
8. Now that this segment of the axon has more sodium ions inside and more potassium ions
outside, the second sodium potassium pump moves 3 sodium outside and 2 potassium
ions inside.
9. In response to the opening of the second sodium channel, the third sodium channel
reaches threshold and opens. Repeat step 2, only this time for the third sodium channel.
10. In response to the opening of the third sodium channel, the third potassium channel
reaches threshold and opens. Repeat step 2, only this time for the third potassium
channel.
11. Now that this segment of the axon has more sodium ions inside and more potassium ions
outside, the third sodium potassium pump moves 3 sodium outside and 2 potassium ions
inside.
12. In an axon, something happens called sodium channel inactivation. This means that after
the sodium channels open and close, they cannot open again for a few milliseconds. What
effect would this have on the action potential?
13. In order to continue to function properly, the axon must somehow get back to its resting
state with fewer sodium ions inside the axon. How does the cell do this?
Homework
Directions: Write a summary of what is happening at each stage of the action potential
diagrammed below.