in the Nervous System
1991 Woodrow Wilson Biology Institute
- To determine the speed of nervous transmission in an integrated nervous system.
- To understand the differences between signal transmission in a single neuron and in a nervous system.
- To create a little group rapport.
I usually do this at the beginning of a class just after the one in which we discuss the mode of signal transmission in axons. In that class one of the issues that gets discussed is how fast signals are transmitted in neurons. This differs for myelinated and unmyelinated neurons. Depending on the level of your class, you can describe the speed of transmission in a variety of ways, but it is helpful to have the actual number for such transmission. Biology by Curtis and Barnes (5th Edition, Worth Publishing, 1989) gives a figure of 200 meters/second for the speed of some large myelinated nerves. The purpose of this exercise is actually to measure the speed of transmission -- with a twist.
- A bicycle horn (the kind with a squeeze bulb)
- A stop watch
- A room in which all of your students can stand forming a circle with joined hands
- A meter stick
Have all the students stand in a circle with hands joined. Join the circle and give the person to your left the bicycle horn to hold in his or her right hand. Hold the stop watch in your left hand, behind your back. Simultaneously squeeze the hand of the person to your right and start the stopwatch. That person is then to squeeze the hand of the person to his or her right, and so on around the circle until the last person feels the squeeze. He or she is then to squeeze the bicycle horn. Stop the stop watch when you hear the horn.
Usually I do this once or twice just for practice and then do it "for real." Record the time it takes for the signal to pass around the circle.
Measure the span from tip of right hand to tip of left for several people in the class and figure out the average distance the signal traveled per person.
Count the number of people in the circle.
Given the number of people and the length per person, calculate the distance the signal travelled.
Divide the distance travelled by the time the signal travelled to determine the speed in meters per second.
Compare this to the speed of transmission of large myelinated axons like the ones in the circuit we've been studying.
It develops that the speed we get is markedly slower than the sped of signal transmission in an axon. Sometimes it will be at least an order of magnitude longer. Why?
Think of all the things the signal has to do besides just travel along an axon -- synapses, integration time as it moves from one side of the body to the other, time for muscles to squeeze and pressure receptors to recognize the squeeze. How you structure this thinking will depend on the level of sophistication of your group. The point is that signal transmission in integrated systems involves much more than just axonal transmission. That's why it's a system!
This can lead to a discussion of levels of organization and how the properties of systems differ from those of the parts of the system. So it is possible to go very far afield from the original question, once again showing how issues in human ecology can emerge from a wide variety of areas of biology, even neurobiology.
Enjoy the exercise too. It generates laughter and togetherness, and that is worth generating whenever possible.