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.
