We are juiced. From head to toe, we contain miles of infrastructure flush with impulses of electricity. Mild charges pour in from the receptors that translate our five senses into electric signals, through the central processor in our head, through the wiring to every muscle cell. Evolution built the system, its smart variation on the way electricity flows in the rest of nature.
Neurons appeared in the earliest jellyfish and sponges because, unlike plants, those swimmers moved around. They avoided threats and searched for food, so they gained loose nets of nerves for input about their environment and the coordination of their swimming.
Before the animals, two billion years ago in the first fully developed cells, membranes seem to have been the place where neurons originated. A membrane is “a selective barrier; it allows some things to pass through but stops others” (Wikipedia). Such membranes probably helped cells manage the changing salt levels in the early oceans. And since the salts of sodium, potassium and calcium easily become electrically charged, membranes began to evolve with pores that opened and closed quickly to let charged atoms pass through.
As animals became larger, such membranes lengthened into neurons with conductive axons, the “wire” of the cell. In us, the longest axon runs down the length of each leg, branching as it goes. The shortest axons, fractions of a millimeter, fill our heads by the billions.
The axons don’t carry an electric charge in the way that a wire carries electricity or a lightning bolt of electrons crashes to the ground. Instead, think of the wave at a sports stadium, where groups of fans stand up, throw their hands in the air, and sit down in a spontaneous sequence that moves through the seats. An impulse moves down the axon in the same way, charged atoms crossing through opened pores from one side of the membrane to the other and then quickly back again while the “wave” of the electric charge moves along.
The impulse never varies in strength. It is either on or it is off, moving or only ready to move. There are no drops in the current, no power failures, no biological surge protectors needed. If a muscle must contract to move a load, the nerve signal, always at the same strength, simply repeats rapidly enough so that the muscle cells remain contracted.
At the ends of the axon, where the impulse begins and terminates, clever devices translate between the electrical charge and other structures, depending on the specialty of the neuron. In our eyes, light causes molecular changes that trigger the impulses to the brain to form the image we recognize as a chair. In the ear, sound waves cause small hairs to vibrate and set off the impulse that we hear as “hello.” Where a neuron terminates at a muscle cell, the final “wave” triggers chemicals that start the muscle’s contraction.
But we barely notice all this electric action. Compared to the breath that we can feel and the blood that we can see, the electronic wizardry in our bodies is invisible to us. Unless you have been badly shocked or suffer from numbness or from irregular heartbeats, you may not pay much conscious attention to your internal circuitry at all.
But there is one way you are aware of it already, at least unconsciously. Take a moment to notice the slight tingle that is always present in your limbs and head. It’s a sense of animation, a knowing that you can move a muscle or think a thought whenever you want. Those tingles are, essentially, your neurons ready to go. And they are what remind you that you’re alive.