A nerve racking model
Dominos taped to a ruler with masking tape hinges.
The propagation of a nerve impulse down an axon can be modeled by a row of falling dominos.
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.Once you have the hinge in place put second piece of masking tape across the hinge at a right angle to help hold it down. Optionally you can run two more pieces of masking tape beside all dominos from one end of the ruler to the other. The extra masking tape will make the domino fall exhibit operate longer before it needs to be refurbished.
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.
Whats 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
The nerve impulse does not lose energy as it propagates.
The dominos cannot fall again until they are
After a nerve impulse has propagated down the axon, there is a "refractory period" during which the nerve cannot fire again.
You can also use this activity to ask the visitors, "what is moving from one end of the ruler to the other?" Each domino merely falls over, no domino moves from one end to the other. Many people say that energy moves from one end to the other, this is correct, but we might ask "what kind of energy?" To which the answer is kinetic energy, the energy of motion. So at the base what moves is motion itself, each domino falls but the falling of the dominos moves from one end to the other.
I usually say "Motion moves".
You can measure the speed,s, of the domino wave by dividing the distance, L, the wave travels by the time, t, it takes to travel that distance. s = L/t.
Try different domino spacings and see how the speed depends on the spacing. When the dominos are close together or far apart the speed of the wave of falling is slower than when they have some intermediate spacing. The speeds are close enough however that it is useful to use a row of dominos that is as long as possible, perhaps a meter long.
Measure the domino speed several times and average your results.
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
30 May 2000