Quick Concept Demonstrations
Jane R. Geuder
Teacher demonstrations to the entire class can be used to introduce a topic or unit, illustrate a concept within the context of a reading, or serve to review or extend the procedures of a lab activity. Some of the following demonstrations have been used as an exploratory lab with student stations having data collection and questions to answer. In some texts these topics may all be in one chapter on the nervous system. Other texts devote separate chapters to one or more of the special senses.
Nerve impulse Polarization
Impulse propagation Depolarization
Physics and Physiology Inhibition
Accommodation Negative Afterimages
These activities may require 5-15 minutes each depending on the activity and the amount of teacher-directed discussion previous to or following the demonstration.
Students should have some basic knowledge of the nervous system. Some demonstrations will help clarify topics as students progress through a chapter.
Most of the materials are already on hand or can easily be made or acquired. The explanation of each demonstration will describe the equipment needed and any procedure required.
The teacher should read through and perform the demonstrations and decide where in his or her curriculum any of them would be useful. Materials needed should then be assembled. The teacher may also want to add to or modify any questions to students that are suggested in some of the demonstrations.
That the action potential is a local event can be shown by laying matches on a sheet of foil with the match heads touching. After one match head flares up, the one next to it will also. If you branch out the match heads while still having them touch, divergence of the impulse as an axon forms collaterals can be shown as well.
Another way to visualize the nerve impulse is with the childs toy, Jacobs Ladder. Sometimes known by other names, it consists of about six 1/2 inch blocks of wood about 4" by 3" in dimension. They are attached end to end by glued ribbon strips so that they flip over, one by one, as the position of the block being held is shifted.
Polarization and Depolarization
Students sometimes struggle with the concept of membrane potential. The ion-maintained difference in charge between the inside and outside of the cell membrane can be seen as a delicate balance that is upset when depolarization occurs as a stimulus is applied. By balancing a coin on edge and blowing it over, this delicate balance can be shown.
An extension of this set-up can address inhibition in which other neurotransmitters cause the inside of the membrane to hyperpolarize and become less likely to discharge an impulse. Stabilizers can be placed around the balanced coin so that a very strong puff is required to tip it over. Other coins lying flat on both sides of the balanced coin could serve as stabilizers.
Modality is that characteristic of a sensation by which we tell it apart from all others. Modality is not determined by the manner in which the sense organ is stimulated; it is due to the area of the brain receiving the receptor impulse. To demonstrate: close eyes; turn eyes sharply to one side; put gentle pressure on the closed eyeball near the outer edge; the luminous circle seen is a phosphene. Ask students to explain what happens in terms of the nervous system.
We are not aware of the presence of our comfortably fitting clothes or wrist watch after they have been on for awhile, because the action potentials have decreased or stopped entirely. Students can use a straight pin to move and hold a single hair on the dorsal surface of their index finger proximal to the hand. If held stationary in a bent state, the sensation subsides and can usually be felt again when the hair is released.
Using a green and red filter on two filmstrip projectors, show that when the two colors are overlapped on the screen, they will reflect yellow to our eyes. These are wavelengths of light, not pigment colors. Next, project only the red on the screen and have students stare at it with one eye only so as to fatigue the retina to red in the fixed area of the staring. Blinking will not affect the outcome, but shifting the gaze will. As staring continues (maybe a minute) blackness fades in and out. Inform students ahead of time that this will happen and that their vision will not be hurt. Meanwhile, outfit the other projector with a yellow filter. Turn off the red projector, turn on the yellow one and immediately have students view the yellow screen first with one eye, then the other. The eye fatigued to red will see the complementary color of green and the eye that had been closed before will see the screen as yellow. Ask students to explain their perceptions.
Hearing Yourself As Others Hear You
Ask students why they sound so different when they hear a recording of their own voice. We hear ourselves speak uniquely: some vibrations of our voice box leave our mouths and enter our ears externally, as is the case for everyone else. But another component for us is the internal vibrations that are conducted directly to the middle ear ossicles. To show the effect of this bone conduction, direct students to repeat a phrase a few times, and, while speaking, hold their hands over their ears.
To show that no cones and few rods are distributed on the outer part of our retina, have students stare ahead and slowly move a colored pencil from in front of them to a position to the side of them where they can no longer see the pencil. Then have them jiggle the pencil. It should then be seen, but not in color. The rods are so sparsely distributed that only movement that causes the pencil image to fall on several receptors will evoke an action potential.
Since each eye receives a separate image from a different angle, one function of the occipital lobe is to fuse these images into a single image of three dimensions ( depth perception ). This is possible because when we focus on an object, its reflection strikes corresponding points on each retina. Have students hold their hands at arms length in front of them with the two longer fingers touching at the tips, but slightly spread apart. Focus on some distant object through the space between the fingers. Have students describe the appearance of their fingers and explain why it happened. We can focus on only one plane at a time. The finger images fall on non-corresponding points of the retina, and therefore an unfused image is perceived by the brain.
In another example of binocular vision, you can also determine which eye is dominant. Hold a pencil in front of you and focus on a distant object beyond the pencil. How many pencils are seen? Now line up one of the two pencils with the distant object. Then, blink one eye, then the other. Whichever eye is open when the object is still lined up, is the dominant eye. The pencil will jump from right to left as the eyes are blinked.
The ability of our eyes to accommodate, to bring a near object into clear focus, involves a change in the size of the pupil as well as a change in the curvature of the lens. As we converge our eyes to allow the image to fall on corresponding points of the retina, an autonomic reflex changes the shape of the lens and the size of the pupil. Have students work in pairs to observe the change in the pupil as the gaze is shifted from a distant object to a near object in line with the distant one. Students should not blink as they refocus. In which instance is the pupil larger - in near or distant vision? The pupil will constrict in near vision to eliminate spherical aberration, a function of the lens.
Morris, Thomas F. et alia. Human Physiology. Holt, Reinhart and Winston, 1977.
About the Author
Jane R. Geuder is a biology teacher at Centennial High School in Ellicott City, Maryland. Jane can be contacted at Centennial High School, 4300 Centennial La., Ellicott City, MD 21042, phone (410) 313-2856,or by e-mail at JRGeuder@aol.com.