Introduction

The propagation of a nerve impulse down an axon can be modeled by a row of falling dominos.

Material

• Dominos, at least 8
• minimum, a smooth, hard, flat surface
• advanced a ruler and masking tape

Assembly

Stand the dominos on their smallest faces in a row of at least 8 dominos.

It will be easier to do the activity if you make a hinge out of masking tape and attach the domino to the flat side of a ruler. The hinge will allow the domino to fall over in one direction. Make a line of hinged dominos so that when one falls over it will knock over its neighbor.

To Do and Notice

Push over the first domino so that it falls and hits the second which knocks over the third etc. Observe the propagating wave of falling dominos.
The falling dominos model a nerve impulse.

To reset the dominos simply lift the far end of the ruler and they will all return to their original position ready to be knocked down again. There is something inherently satisfying about knocking over dominos.

What’s Going On?

There are several ways in which the falling dominos model a nerve impulse.
The first domino will not fall until it is pushed beyond a critical angle.
A nerve impulse will not be triggered until the nerve is excited beyond its firing threshold. (Nerves also occasionally fire spontaneously, just as the domino sometimes fall over without triggering, as when someone jiggles the table.)

Once the pulse of falling dominos begins to propagate, it moves at a constant speed independent of the size of the starting push.
The speed of propagation of a nerve impulse is independent of the size of the triggering signal.

The domino pulse does not lose energy as it propagates, gravitational potential energy is added as each domino falls.
The nerve impulse does not lose energy as it propagates.

The dominos cannot fall again until they are reset.
After a nerve impulse has propagated down the axon, there is a "refractory period" during which the nerve cannot fire again.

Etc

You can estimate the speed of a nerve impulse by having a row of ten people hold hands. Squeeze the hand of the first person and ask the row to pass the squeeze along. Time how long it takes for the nerve impulse to move through the line of people, t. Measure the length of the line, L, and calculate the speed of the nerve impulse s = L/t

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Scientific Explorations with Paul Doherty

© 1999

30 May 2000

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