What Is Bioelectricity? A Complete Beginner’s Guide

In the autumn of 1780, at the University of Bologna, Luigi Galvani was dissecting a frog on a table that happened to be near a static electricity generator. One of his assistants touched a metal scalpel to the frog’s exposed sciatic nerve at the same moment a spark jumped from the machine. The dead frog’s leg kicked. Galvani did not have a word for what he had just seen, so he invented one: he called it animal electricity, a force he believed was generated inside living tissue itself, distinct from the electricity of lightning or static machines.

He was both right and wrong, which is usually how foundational discoveries work. Alessandro Volta, a rival physicist in Pavia, argued that Galvani’s frog leg wasn’t generating electricity at all. It was simply responding to a current produced by two different metals touching moist tissue, a dispute that led Volta to build the first battery in 1800 largely to prove his point. Volta won the argument about the frog. But Galvani turned out to be right about something much bigger: living cells really do generate and use electricity, on their own, without any metal involved. It just took physiology another sixty years to prove it, and biology another two centuries to figure out how far the idea actually extends.

Every Cell Is a Battery, Not Just Neurons

The modern definition of bioelectricity has nothing to do with sparks or nerves firing on command. It starts with a fact that surprises most people outside of cell biology: nearly every cell in the human body, not just neurons and muscle cells, maintains a voltage difference across its outer membrane. Skin cells, liver cells, cells in a developing embryo, all of them sit at a resting electrical potential, typically somewhere between negative 40 and negative 90 millivolts, negative on the inside relative to the outside.

That voltage exists because cell membranes are selectively permeable, meaning they let some charged particles, ions, cross more easily than others. Sodium, potassium, calcium, and chloride ions are all held at different concentrations inside the cell than outside it, a gradient the cell builds and maintains using proteins called ion pumps, chiefly the sodium potassium pump, which burns through a meaningful fraction of the body’s total energy budget just to keep the imbalance in place. Sitting alongside the pumps are ion channels, proteins that open and close like gates, letting specific ions flow down their concentration gradients at specific moments. The combination of pumps that build the gradient and channels that release it is the entire mechanism. There is no third ingredient.

A battery you never plug in still needs constant work to stay charged.

That constant work is the resting potential, and it is not a passive leftover of cell chemistry. It is an actively maintained, energy-consuming state, which is why a cell that runs out of oxygen or glucose loses its voltage within minutes. Emil du Bois-Reymond, working in Berlin in the 1840s, was the first to measure this kind of bioelectric signal directly with an instrument called a galvanometer, showing that an “injury current” flows out of a cut nerve, evidence that the nerve had been electrically charged the whole time, not just during the moment of injury.

The Action Potential Is a Wave of Ions, Not a Current in a Wire

The part of bioelectricity most people learn in school is the action potential, the brief, self-propagating spike of voltage that races down a nerve fiber. It feels intuitive to picture this as electricity moving through a wire, the way current moves through copper. That picture is wrong, and the difference matters.

A nerve fiber does not conduct electrons the way a wire does. Instead, when a neuron’s membrane potential crosses a threshold, usually around negative 55 millivolts, voltage-gated sodium channels snap open at that point on the membrane. Sodium ions flood in, flipping the local voltage from negative to positive in about a millisecond. That local flip is enough to trigger the sodium channels immediately next door to open too, and the process repeats itself down the length of the fiber, like a line of falling dominoes rather than a current flowing through a pipe. Behind the wave, potassium channels open and sodium pumps reset the balance, so the membrane can fire again a few milliseconds later.

Alan Hodgkin and Andrew Huxley worked out the exact mathematics of this process in the early 1950s using the giant axon of a squid, a nerve fiber wide enough, roughly a millimeter, that they could physically insert an electrode down its center at the Marine Biological Laboratory in Plymouth, England. Their 1952 papers described the ionic mechanism in equations still taught in neuroscience courses today, and the work earned them the Nobel Prize in Physiology or Medicine in 1963. In vertebrates, the process is sped up by myelin, a fatty insulating sheath wrapped around the axon that forces the signal to jump between exposed gaps called nodes of Ranvier, a trick called saltatory conduction that lets human nerve impulses travel as fast as 120 meters per second, compared to roughly 1 meter per second in an unmyelinated fiber.

The signal isn’t current flowing through the nerve. It’s a wave of ion channels opening in sequence.

Voltage Patterns Organize Tissue Before Any Signal Ever Fires

For most of the twentieth century, bioelectricity was treated as a specialty of neuroscience and cardiology, relevant to nerves and heart muscle and not much else. Harold Saxton Burr, a Yale anatomist working from the 1930s through the 1950s, pushed back on that framing early, measuring steady electrical fields around growing plant embryos, salamander eggs, and even human tumors, and arguing that these fields, which he called L-fields, correlated with and possibly directed the shape of the tissue growing inside them. Burr’s work was controversial and, for decades, largely set aside.

It resurfaced in a more rigorous form starting in the 2000s in the lab of Michael Levin, now at Tufts University and the Allen Discovery Center for Regenerative Biology. Levin’s research shows that non-neural cells, ordinary skin and muscle and epithelial cells with no capacity to fire an action potential, still communicate with each other electrically through voltage gradients across sheets of tissue, using gap junctions that let ions and small molecules pass directly from one cell into its neighbor. During embryonic development, and during regeneration in animals like the flatworm planaria and the axolotl, these voltage patterns appear to function as a kind of positional map, telling a cluster of cells where an eye should form, or how many heads a regenerating flatworm should grow. Levin’s group has shown that altering the bioelectric pattern with ion channel drugs, without touching a single gene, can cause a planarian to regenerate two heads instead of one.

Development is not only chemistry. It is a voltage map with a memory.

That finding reframes what bioelectricity even means. It is not a special trick that nerves and muscles evolved late. It is closer to the reverse: nerves and muscles took an ancient, cell-wide electrical communication system, one that predates neurons by hundreds of millions of years, and specialized it for speed. Every cell can hold a charge. Only some cells learned to fire it fast.

From Frog Legs to Pacemakers: One Continuous Idea

The line from Galvani’s twitching leg to a modern implanted medical device is more direct than it looks. Willem Einthoven, a Dutch physiologist, built the first practical electrocardiogram in 1903, showing that the rhythmic electrical activity Galvani had glimpsed in a single nerve could be recorded from the intact, beating human heart. Wilson Greatbatch built the first fully implantable cardiac pacemaker in 1958, using the same underlying fact, that heart muscle cells depolarize and repolarize on a predictable voltage cycle, and that an external pulse of current, delivered at the right moment, can force that cycle back into rhythm when the heart’s own pacing cells fail.

Every piece of modern bioelectronic medicine, from cochlear implants to deep brain stimulators to experimental vagus nerve stimulators for inflammatory disease, rests on the same two-hundred-year-old observation Galvani stumbled into with a scalpel and a spark: cells run on voltage, that voltage can be read from outside the body, and it can be changed from outside the body too. What has changed since 1780 is resolution. Du Bois-Reymond needed a whole nerve to detect a signal. Hodgkin and Huxley needed a squid axon a millimeter wide. Today’s neural interfaces read and write to individual neurons, and Levin’s tissue-level experiments manipulate voltage across entire sheets of cells that never fire an action potential at all.

What Galvani saw in 1780 was real, just narrower than he understood it to be. He had found one visible symptom of a property that turns out to belong to essentially every living cell: a standing voltage, actively maintained, capable of being read and capable of being changed. The frog leg twitched because a nerve fired. The deeper truth, the one that took two more centuries and a shift from neuroscience to developmental biology to surface, is that the twitch was never the whole story. The voltage was there before the spark arrived, and it would have stayed there, quietly organizing the cell’s behavior, even if no one had ever touched it with a scalpel.