Teacher Material

Ion Flow in Neurons

Lynda Jones & Mark Little

Overview

In order for a neuron to fire, there are two types of electrical potentials produced. The first is a non-propagated local potential called an electrotonic potential and the second is a propagated impulse called an action potential. These are found universally in the nervous system and are the only responses of neurons (Ganong, p. 39). Smaller neurons in relation to their length, such as neurons in the brain, have only electrotonic potentials while longer neurons require both (Reichert, p.33). Action potential conduction velocity in neurons has been studied and shown to range from 0.5 to 120 m/sec (Ganong, p. 47). In this activity, the student will be investigating the speed of the localized electrotonic potential. A piece of absorbant cotton floss soaked in saline solution simulating the neuron will be stimulated with an electrical pulse and the impulse rate of ion conduction will be measured on an oscilloscope.

Biological Concepts

• Nerve Impulses

• Electrotonic Potential

• Local Response

• Current Flow

• Action Potential

Class Time

This activity will require one class period for a prelab discussion, one class period to run the

experiment, and one class period for a postlab discussion.

Background Information rane ahead of the action potential." (Ganong, p. 43). (See Figure l.) The membrane repolarizes by opening the K⁺¹ ion channels allowing K⁺¹ ions to flow out of the cell, reestablishing the resting state of the cell.

In this experiment, the student will determine the velocity of ion flow in an electrotonic potential and compare it to the velocity of action potentials. The student should expect values in the range of 75,000 to 187,000 m/sec for ion flow over 30 mm as compared to known action potential values ranging from 0.5-2 m/sec for reflex, pain, and temperature dorsal root receptors to 70-120 m/sec for somatic motor neurons.

Materials

• TEMNA 72-915 20 MHz Oscilloscope (3 leads--one from the input in Ch. 1 to the pre amp output, one from the external input to the prepulse and ground on the stimulator, and one from the ground to the ground wire on the Faraday box.) (See Figure 2.)

• Faraday Box (See Teacher Preparation for how to make an inexpensive box with a ground wire.)

• Electrode Grid (See Figure 3.) Printed circuit board etching kits are available from Radio Shack for $8-$10.

• Stimulator (2 leads--one connects the prepulse and ground ports on the stimulator to the external input on the oscilloscope and 1 lead connects the + and - ports on the stimulator to the #1 pair of + and - electrodes on the grid.)

• Pre amplifier (2 leads--one connects the + and - ports on the pre amp to the #6 pair of + and - electrodes on the grid and one connects the output on the pre amp to the Ch. l input on the oscilloscope.)

• Power unit (+, -, and ground lead connects to the same on the pre amp)

• Absorbant Cotton Embroidery Floss

• Scissors

• Drosophila Ringer Solution (See Teacher Preparation.)

• Eye dropper

• Double distilled water

• Scotch tape

• Petri dish

• Aluminum foil

• Ground wire

• Velcro

• Plastic knobs (for the bottom of the Faraday Box)

• Kimwipes or other low lint wipes

• Plastic cover (top of a reaction plate or something similar)

Teacher Preparation

• Making a Faraday Box:

1. Obtain a cardboard box about 12 inches x 24 inches x 6 inches and break it down so it can lie flat.

2. Connect a ground wire to one end of the box and cover it smoothly with tape.

3. Obtain a role of aluminum foil and adhesive glue.

4. Cut foil pieces to approximate size of the box dimensions.

5. Spray or apply the glue to the cardboard.

6. Attach the aluminum foil and cut off excess. Try to attach it as smoothly as possible.

7. Fold back and velcro in place. The use of velcro will make the boxes easier to use than tape and easier to store. (See Figure 2.)

• Making Drosophila Ringer Solution

1. Measure out 7.50 grams of NaCl, 0.35 grams of KCl, and 0.21 grams of CaCl₂.

2. Add the salts to a graduated cylinder and add up to l liter of distilled water.

• Setting Up The Equipment

1. Obtain an oscilloscope. (Note: The physics teacher or technology teacher may be able to help with the equipment.) Plug oscilloscope into an outlet.

2. Attach one lead to the input on Ch. 1 input on the oscilloscope and connect it to the output port on the pre amplifier.

3. Connect the ground of the oscilloscope with a lead to the ground wire on the Faraday box.

4. Connect the external input port on the oscilloscope to the stimulator on the prepulse and ground ports. Plug stimulator into an outlet.

5. Connect the + and - ports on the stimulator and connect to the #1 pair of + and - electrodes on the grid. (Note: Each pair of electrodes is 10 mm apart and there are 2-3 mm between the + and - electrodes in each pair.)

6. From the pre amp, connect the +, -, and ground leads to the corresponding ports on the power unit. Plug power unit into an outlet.

7. From the pre amp, connect the + and - leads to the #6 pair of + and - electrodes on the grid.

8. Set the oscilloscope:

• Volts/division 10 millivolts

• Time/division 20 microseconds

• Coupling setting On

• Source On

• Storage mode On

• Sweep mode On

9. Set Stimulator:

• Prepulse On

• Frequency 20 on dial and 10 X on knob

• Delay 2 on dial and .01 X on knob

• Duration 4 on dial and .1X on knob

• Volts 1 on dial and .1 on knob

10. Pre amp settings:

• Gain on 100x

• To turn it on--turn to AC.

• The Experiment Itself

1. Double check the settings and make sure all the connections are plugged in.

2. Saturate a piece of floss in the Drosophila Ringer Solution for about 2-3 minutes.

3. Lay the saturated floss across the grid and make sure it crosses all of the grid wires.

4. Soak up any excess water with an eye dropper or use Kimwipes or other low lint products.

5. Cover with a plastic cover that fits the grid to prevent evaporation.

6. Place the pair of electrodes from the stimulator on electrode pair #1 and the ones from the pre amp on pair # 6.

7. Make sure the following are set on the oscilloscope: sweep mode on single (with the orange light on); CH 1 button is pushed.

8. Press pause button.

9. In the read out section, select one cursor and move it to the tip of the wave and set it.

10. Turn off pre amp.

11. Move the electrodes from pair #6 to pair #3.

12. Turn on pre amp.

13. Make sure the following are set on the oscilloscope: sweep mode on single (with the orange light on); CH 1 button is pushed.

14. Press pause button.

15. In the read out section, select one cursor and move it to the tip of the wave and set it.

16. Record the distance between each peak in milliseconds.

17. Repeat steps 6-16.

Extensions/Variations

• Vary diameter of floss to see if ion velocity varies with larger or smaller diameter thread.

• Try other types of absorbant fibers to see variation in ion velocity.

• Vary saline solution concentration to test changes in ion velocity.

Resources

Drewes, Charlie, Professor of Zoology and Genetics, Iowa State University, Ames, Iowa, cdrewes@iastate.edu.

Ganong, William F., Review of Medical Physiology, 14th Edition, Appleton & Lange, Norwalk, Connecticut, 1989.

Kandel, Eric R., Schwartz, James H, and Jessell, Thomas M., Essentials of Neural Science and Behavior, Appleton & Lange, Norwalk, Connecticut, 1995.

Reichert, Heinrich, PhD., Introduction to Neurobiology, Oxford University Press, New York, 1992.

About the Authors

Lynda Jones is a science teacher at Catlin Gabel School in Portland, Oregon. Lynda can be contacted at Catlin Gabel School, 8825 S.W. Barnes Road, Portland, Oregon 97225 or by e-mail at jones@catseq.catlin.edu.

Mark Little is a science teacher at Broomfield High School in Broomfield, Colorado. Mark can be contacted at Broomfield High School, #1 Eagle Way, Broomfield, Colorado 80020 or by e-mail at little@bvsd.k12.co.us.

Student Activity

NAME:_______________________DATE: ____________PERIOD:____________

ION FLOW IN NEURONS

STUDENT ACTIVITY SHEET

• Setting Up The Equipment

1. Obtain an oscilloscope and plug oscilloscope into an outlet.

2. Attach one lead to the input on Ch. 1 input on the oscilloscope and connect it to the output port on the pre amplifier.

3. Connect the ground of the oscilloscope to the ground wire on the Faraday box.

4. Connect the external input port on the oscilloscope to the stimulator on the prepulse and ground ports. Plug stimulator into an outlet.

5. Connect the + and - ports on the stimulator and connect to the #1 pair of + and - electrodes on the grid.

6. From the pre amp, connect the +, -, and ground leads to the corresponding ports on the power unit. Plug power unit into an outlet.

7. From the pre amp, connect the + and - leads to the #6 pair of + and - electrodes on the grid.

8. Set the oscilloscope:

• Volts/division 10 millivolts

• Time/division 20 microseconds

• Coupling setting On

• Source On

• Storage mode On

• Sweep mode On

9. Set Stimulator:

• Prepulse On

• Frequency 20 on dial and 10 X on knob

• Delay 2 on dial and .01 X on knob

• Duration 4 on dial and .1X on knob

• Volts 1 on dial and .1 on knob

10. Pre amp settings:

• Gain on 100x

• To turn it on--turn to AC.

• The Experiment Itself

1. Double check the settings and make sure all the connections are plugged in.

2. Saturate a piece of floss in the Drosophila Ringer Solution for about 2-3 minutes.

3. Lay the saturated floss across the grid and make sure it crosses all the of the grid wires.

4. Soak up any excess water with an eye dropper or use Kimwipes or other low lint products.

5. Cover with a plastic cover that fits the grid to prevent evaporation.

6. Place the pair of electrodes from the stimulator on electrode pair #1 and the ones from the pre amp on pair #6.

7. Make sure the following are set on the oscilloscope: sweep mode on single (with the orange light on); CH 1 button is pushed.

8. Press pause button.

9. In the read out section, select one cursor and move it to the tip of the wave and set it.

10. Turn off pre amp.

11. Move the electrodes from pair #6 to pair #3.

12. Turn on pre amp.

13. Make sure the following are set on the oscilloscope: sweep mode on single (with the orange light on); CH 1 button is pushed.

14. Press pause button.

15. In the read out section, select one cursor and move it to the tip of the wave and set it.

16. Record the distance between each peak in microseconds.

17. Repeat steps 6-16.

DATA TABLE

ANALYSIS

1. There are 30 mm between electrode pair #3 and pair #6. Convert 30 mm to micrometers.

2. Velocity = Distance/Time. Calculate velocity for each trial. Include a sample calculation for determining velocity in meters/second and miles/hour below:

Trial Velocity (m/s) Velocity (miles/hr)

3. Find the average velocity for each column. Compare the electrotonic potential velocity with that of the known action potential velocity.

Figures 2 and 3 are based on original design by Dr. Charlie Drewes.

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