We are juiced. From head to toe, electric signals pour into the brain from eyes, ears, nose, mouth, and skin as the raw data for what we see on a screen, hear across the room, feel at our fingertips. In the other direction, more charges pour from the brain out through the nervous system to the muscles that move our eyes, mouth our words, or type out the s e l e t t e r s .
How and when did such a refined system for bodies – all bodies, plants included – get started? It began at our beginning, when living things were no more than single cells floating through the ocean, encased in a cell membrane. In Wikipedia’s words, a membrane is “a selective barrier; it allows some things to pass through but stops others.” Such a membrane was essential in helping a cell maintain the salt levels inside the cell as it floated in the salty water. And since the salts of sodium, potassium and calcium consist of atoms with a positive or negative charge, the pores in membranes also became gates that opened and closed to control the electrical potential across the membrane itself.
By about 500 million years ago, in jellyfish and other sea creatures, these membranes had evolved into loose nets of nerves that responded both to light and to the touch of other creatures that might be a meal or might be a predator. As animals evolved, such nerves lengthened into neurons with conductive axons, the “wire” of the nerve cell. In humans, 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.
But axons don’t carry an electric charge in the way that a copper wire carries electricity or a spark of lightening splits the air. 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 rows. A nerve impulse moves down the axon in a similar way; the charged atoms crossing through opened pores from one side of the membrane to the other and then back again form the “wave” of the electric charge. .
The impulse never varies in strength. It is either on or off, either 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 both ends of the axon, where the impulse begins and ends, devices of various kinds translate data into and then out of the electrical “wave.” In the ear, for example, in-coming sound waves cause small hairs to vibrate and set off the wave impulses that our brain takes as “hello.” Or, when the brain tells my right big toe to move, the wave message gets to the end of the long leg-axon where it triggers chemicals that start the muscle’s contraction.
We barely notice all this wizardry. Compared to our breathing that we can feel and our blood that we can see, our circuitry seems inconspicuous, barely detectable. But if we’ve felt a jolt from a faulty toaster or felt numbness or irregular heartbeats, we’ve glimpsed what can go wrong.
In another way, though, we know full well how electric we are. Notice the faint tingle in your limbs and head. It’s a sense of animation, a potential, an ability to move a muscle whenever you choose to, to look around or think a thought at any time. That tingly readiness is, essentially, neurons at the ready. It’s being alive.