ACHY, BREAKY NERVE
Jeff OShields and Cathy Williamson
The degeneration of myelin, the fatty sheath that covers the neurons, is responsible for several neurological disorders, such as multiple sclerosis and adrenoleukodystrophy. In this activity students will develop models to understand the function of myelin as it applies to the normal and abnormal transmission of nerve impulses. In addition, the students will also have the opportunity to apply their knowledge through a service learning project.
This activity should take two 55 minute class periods to introduce and plan the model and one 55 class period for model presentation.
Compared to invertebrates, vertebrates have a more highly developed nervous system. Part of this complexity rests in the fact that most vertebrates have myelinated axons which serve to increase the rate of nerve impulse propagation. The complex chemical composition of the myelin sheath which consists of multilayered lipids and proteins contributes to its ability to conduct impulses much more rapidly than unmyelinated sheaths. These faster conduction rates permit depolarization to be followed by ion movement and current flow. This process is called saltatory conduction. Like a well-insulated wire, myelinated nerve fibers are able to conduct efficient impulses without sacrificing electric potential to their surrounding. Myelination enhances the transmission of action potentials which involves sodium ions entering and potassium ions exiting the neuron in the unmyelinated gaps of the axon which are called nodes of Ranvier. Myelination prevents the exiting of potassium ions from leaky potassium channels along the internode of the axon creating a sustained impulse.
An interesting result of the understanding of the seepage of potassium ions from the internode of the neuronal membrane is the use of potassium channel blockers in the treatment of multiple sclerosis. One of the blockers that has been used for this purpose is 4-aminopyridine.
The sample model explained in this lesson graphically represents the advantages of a myelinated nerve over an unmyelinated nerve. Imagine that the connection between the aquarium tubing and the pump is the initiation of the action potential. The holes in the side of the tubing represent leaky potassium-gated channels found along the internode. When there is no myelin sheath, air bubbles which represent the electrochemical impulse are released into the extracellular environment; less air flows out the end of the tube which represents the node of Ranvier. If the nerve is covered with myelin, symbolized by the packing tape, there is an observable increase in the air flow from the end of the tube because air in not allowed to escape along the side of the tube (see Figure 1). For comparison, it is important that the two simulated nerves (tubing) are placed side by side. The overhead makes it easy for the students to see the difference in air flow by noting the number and size of bubbles that exit the tubes.
"Demyelinating diseases" such as adrenoleukodystrophy (ALD), multiple sclerosis (MS) and Tay-Sachs result in the degeneration of myelin with consequent loss of function of the nerve fiber. Technological advances in biomedical research have enabled scientists to study the demyelination process more closely and to discover the regenerative capability of myelin. One way that scientists study such biological problems is through the use of scientific models. This modeling activity will provide your students the opportunity to compare and contrast myelinated and unmyelinated nerve fibers while also strengthening their creative scientific skills.
You will need to have materials available for students to construct models of myelinated and unmyelinated nerves. For this reason, it might be better to have students plan one day and construct the models the following day. You may also require the students to bring their own materials.
You may want to construct a sample model for the students. This example can serve as an explanation for their model assignment or as a culmination after the students models have been presented.
The following materials will be needed for the sample model:
two pieces of aquarium tubing approximately 30 cm in length
flame heat source such as a Bunsen burner
dissecting needle or similar metal object
two graduated polyethylene pipettes
an aquarium pump
long clear dish approximately the length of the overhead projector (a Pyrex baking dish would work well)
brown package mailing tape
aquarium gang valve with dual outlets that regulates the air flow
a short piece of aquarium tubing.
1. As an introduction you may want to use the videodisc "Lorenzos Oil". (You may use the video tape but the videodisc makes it easier to access segments.) The following segments will heighten student awareness and understanding of the problem and consequences of diseases that demyelinate the nerves and the importance of models in understanding science.
Chapter 1 "Cleverest Boy in the World"
The objective of this segment is for the students to see an unaffected Lorenzo.
Chapter 3 "The Mystery Disease"
This segment gives background about adrenoleukodystrophy.
Chapter 10 (near the end of the chapter) "The Education of Augusto and Michaela"
Michaela uses a model to explain why Lorenzos trial diet has not reduced his blood lipid content.
Chapter 18 "The Chain" and Chapter 19 "The Same Enzyme" (use only the parts that pertain to models)
Augusto uses a model to explain that the same enzyme is used for the synthesis of monunsaturated C-18 and saturated C-22 fatty acids.
Chapter 16 "Setbacks"
This segment shows the degenerative effects of ALD on Lorenzo.
Chapter 29 "Remyelinating Some Puppy Dogs"
This segment describes the possibility of future research to remyelinate nerves and the implications this will have on victims of MS and ALD.
2. A discussion should take place to explain what myelin does to improve nerve transmission. See background information.
1. Students should be given the job of designing a model that demonstrates the effects of myelin and unmyelinated nerves on nerve impulses. Cooperative learning groups would work well for this activity.
2. The teacher model may be shown at this time. These steps should be performed in advance of the demonstration.
a. Using a metal dissecting needle which has been heated in a flame, put three holes every three centimeters along the length of the tube, starting two centimeters from the end. Repeat this procedure on the second piece of tubing.
b. Cut a graduated pipette five centimeters from the tip.
c. Insert the pipette tip into the end of the tubing closest to the first holes.
The following steps should be done for the class as part of the model presentation.
d. Fill the baking dish half full of water and place on the overhead.
e. Attach the short piece of tubing to the aquarium pump.
f. Attach the dual aquarium gang valve to this short piece of aquarium tubing.
g. Connect the two 30 centimeter pieces of tubing to the double valve. Adjust the air flow so that the tubes have an equal amount of air passing through them.
h. Have the students observe the results.
i. Disconnect one 30 centimeter piece of tubing and dry it off. Cover this piece of tubing with brown packing tape. (Note to teacher: This represents the myelinated nerve fiber.)
j. Reconnect the tubing and have the students observe the results.
5. Once students have completed their models, these should be shared with the class.
6. Have the students answer the questions on the activity sheet. Questions 1-3 relate to the teacher model.
7. The videodisc "Lorenzos Oil" can be purchased from most video stores for approximately $40.00.
The above activity lends itself well to a service learning extension. As defined by the Corporation on National and Community Service, service learning is "a method through which citizenship, academic subjects and values are taught." Service learning must include three components: 1) sufficient preparation including objective setting, 2) the performance of service, and 3) reflection on and discussion of the performed service.
Service learning can be incorporated into this module by having students plan and operate a health awareness fair with a focus on diseases of the nervous system, keeping in mind that this decade has been dedicated as "the decade of the brain". Students can display models and compile information in the form of brochures, videos, etc. at this fair. Myelin related diseases that could be researched might include amyotropic lateral sclerosis, autism, Charcot-Marie-Tooth disorder, Tay-Sachs, hypomyelination, adrenoleuko-
dystrophy, multiple sclerosis, paraplegia, and primary lateral sclerosis. In addition, representatives from various health organizations can be available to answer questions. Students can also obtain assistance and resources for the fair by contacting the following agencies and/or utilizing the world-wide web: The MS Foundation, 6350 N. Andrew Avenue, Ft. Lauderdale, FL 33309 (telephone : 1-800-441-7055 or internet address http://www.icanect.net/msf/) and The Myelin Project Headquarters, Suite 950, 1747 Pennsylvania Ave., N.W., Washington, D.C. 20006 (telephone 202-452-8994 or internet address firstname.lastname@example.org). Incorporating a service component into this module will increase the citizenship skills of students of any age or background.
Aanonsen, Lin, professor of biology, Macalester College. Interterview, July 17, 1996.
Asimov, Isaac. The Human Brain: Its Capacities and Functions. Penguin Books, 1994.
Drews, Charles, professor of zoology and genetics, Iowa State University. Interview, July 11, 1996.
Lorenzos Oil, Laserdisk. MCA Universal Home Video, 1992.
Tortora, Gerald J. Principles of Anatomy and Physiology. Harper Collins, 1996.
van Diemen, H. A, et. al. "4-Aminopyridine induces functional improvement in multiple sclerosis patients:a neurological study," Journal of Neurological Science, 116: pp220-226, June, 1993.
About The Authors
Jeff OShields is a biology teacher at Spring Valley High School in Columbia, South Carolina. Jeff can be contacted at Spring Valley High School, 120 Sparkleberry Lane, Columbia, South Carolina 29223 or by fax at 803-699-3541.
Cathy Williamson is a biology teacher at C. E. Byrd High School in Shreveport, Louisiana. Cathy can be contacted at C. E. Byrd High School, 3201 Line Avenue, Shreveport, Louisiana 71104 or by e-mail at email@example.com.
NAME: _____________________ DATE: ______________ PERIOD: ___________
Achy, Breaky Nerve
Teacher Model Questions
1. Describe what happened when the two pieces of tubing were attached to the pump and placed in the water.
2. Compare and contrast the activities of the wrapped and unwrapped tubing.
3. Explain how the above observations relate to nerve impulses.
4. Explain how your model demonstrates the effects of myelin on nerves.
5. In your lab notebook summarize each groups model as they do their presentation.
6. Of all the models presented, which model best explains the action of myelinated verses unmyelinated nerves. Why? Be very specific in your answer.