Domino Effect

1. Use the ruler to measure the length of one of your dominoes. Record the measurement.
2. Cut sixteen pieces of masking tape, each about the same length as a domino. (They don’t need to be exact.)
3. Place the first domino near the end of the ruler. Use a piece of masking tape to make a hinge connecting the back of the domino to the ruler (see photo below).
   
4. Use another piece of tape to reinforce the hinge. Wrap the piece of tape around the ruler and the base of the tape hinge, securing the domino in place (see photo below).
   
5. Place the second domino on the ruler about three-quarters of a domino’s length from the first domino. Connect the second domino to the ruler with a hinge of tape, as you did in Step 3, and reinforce the hinge as you did in Step 4.
6. Attach the remaining six dominoes in the same way (see photo below).
   

Place the ruler flat on a table and set all the dominoes upright.

Flick the first domino with your finger to make it fall. What happens?

What do you have to do before you can make the dominoes fall again?

Make the dominoes fall several more times and closely observe them as you do. Do they all fall at the same speed? Can you make them fall in the reverse direction?

Reset the dominoes so they’re upright. Barely touch the first domino with your finger and notice what happens. Do this again several times, using slowly increasing amounts of force, pushing it slightly farther forward each time until you get the dominos to fall.

Remove one domino from the middle of the ruler. Reset the remaining dominoes so they’re upright. Flick the first domino. What happens?

Your domino model is a simple but useful tool for understanding the function of nerve cells, or neurons.

Neurons make up the information highways of the body. Their job is to pick up signals from neighboring neurons and transmit them either to other neurons or to target cells. Signals are transmitted along the length of a neuron’s axon (a long projection from the cell body) in the form of charged ions moving across the cell membrane. Axons can be quite lengthy: The neuron that reaches from your big toe to the base of your spinal cord, for instance, is a single cell about a meter long.

In a neuron at rest, there’s a higher concentration of negative ions inside the cell than outside. This difference in charge creates a small voltage, known as the membrane potential. A sufficiently strong stimulus to the neuron initiates a nerve impulse. The impulse begins as an exchange of ions across a localized area of the axon’s plasma membrane, which reverses the polarity of the membrane potential there.

This polarity reversal is an all-or-nothing response, meaning it doesn’t vary in intensity as it cascades down the length of the membrane. The impulse doesn’t lose energy as it travels because it’s continually regenerated at each new site along the membrane. When the impulse reaches the end of the neuron’s axon, it releases chemicals, called neurotransmitters, which pass along the signal to other neurons or target cells.

Your domino model has a lot of similarities with a firing neuron. For starters, the first domino will not fall until it is pushed beyond a critical angle. In a similar way, a nerve impulse will not be triggered until the nerve is excited beyond its firing threshold. The threshold phenomenon can be seen in the operation of our sensory nerves. We can’t hear very faint sounds, for example, because the stimulus is not strong enough to excite the auditory nerve.

Once the first domino is toppled, it begins a chain reaction that travels down the line. Just like a firing neuron, the pulse of falling dominoes is an all-or-nothing event. The pulse moves at a constant speed without losing energy as it travels, and it travels in one direction only. In the same way, the speed of a nerve impulse in your body is independent of the size of the triggering signal, and nerve impulses can travel only one way—from the cell body to the end of the axon.

It takes energy to reset the dominoes between each trial. In the same way, the nerve cell requires energy to redistribute ions and reestablish the resting state after a nerve impulse has propagated down the axon. The nerve cannot fire again until this ionic reset occurs.

The removal of a domino mimics the effects of a severe nerve or spinal-cord injury. A nerve impulse cannot propagate past the site of the injury, just as the pulse of falling dominoes is stopped by the missing domino.

There are sounds you can’t hear because they aren’t strong enough to trigger the neurons in your inner ear. In the same way, there are tastes you can’t taste, aromas you can’t smell, and touches you can’t feel. How strong does a stimulus have to be to trigger the neurons that allow you to experience touch, sight, sound, taste, and smell? Design some experiments to test the threshold level of the stimuli for one or more of your senses.