Every thought you have, every muscle you move, every heartbeat — all of it runs on the same basic electrical event: a spike of voltage that lasts about one millisecond and travels down a nerve fiber at speeds between 1 and 120 meters per second. This event is called an action potential, and it is the universal language of biology. Every nervous system on Earth, from the diffuse nerve net of a jellyfish to the 86-billion-neuron human brain, uses some version of it.
Understanding action potentials isn’t just neuroscience basics — it’s the foundation for understanding bioelectronic medicine, brain-computer interfaces, cardiac devices, and the emerging field of bioelectric developmental biology. Here is how they actually work.
The Neuron at Rest
A neuron sitting quietly has a voltage difference of about -70 millivolts across its membrane — inside negative relative to outside. This is called the resting membrane potential, and it’s maintained by ion pumps that constantly move sodium ions out and potassium ions in, against their concentration gradients, burning ATP to do it. The cell is, in a real sense, a charged battery waiting to fire.
The Action Potential in Numbers
- →−70 mV — resting membrane potential
- →−55 mV — threshold voltage that triggers firing
- →+40 mV — peak of depolarization during the spike
- →1–2 ms — total duration of a single action potential
- →120 m/s — maximum conduction velocity in myelinated human neurons
The All-or-Nothing Principle
When a stimulus pushes the membrane voltage above the threshold of about -55 millivolts, voltage-gated sodium channels snap open. Sodium floods in, the inside of the cell rapidly becomes positive — swinging to around +40 millivolts — and this depolarization propagates down the axon like a wave. The cell either fires completely or not at all. There is no such thing as a half-strength action potential.
Why This Matters Beyond Neuroscience
The same channel types that generate action potentials in neurons appear in non-neural cells too — including stem cells, cancer cells, immune cells, and developing embryos. Researchers are now finding that resting membrane potential variations in non-excitable cells influence gene expression, cell division, and even organ patterning. The action potential may be the most visible manifestation of a much broader bioelectric code that runs throughout living tissue.
Source: Hodgkin & Huxley (1952) · Levin Lab, Tufts University · Bhattacharya et al., Cell (2024)
Credit: Jose Antonio Rodriguez Davia on Unsplash