An action potential is a rapid, temporary spike in electrical voltage across a cell membrane that carries a signal from one place to another. Unlike a steady current in a metal wire, it is an all-or-nothing pulse triggered only when a cell reaches a specific threshold. These electrical pulses allow your nervous system to communicate, muscles to contract, and certain glands to release hormones.
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An action potential is not a continuous current. It is a discrete, all-or-nothing spike of energy. When a neuron hits a specific voltage threshold, millions of microscopic gates swing open in its membrane.
Charged atoms, mostly sodium, rush inside, creating a temporary reversal of the cell's electrical charge. That, right there, is the signal.
We cracked this code in the 1930s using the Atlantic squid. Human nerves are microscopic, but squid have a giant axon nearly a millimeter thick, large enough to see with the naked eye.
By sticking a wire into that massive nerve, researchers Alan Hodgkin and Andrew Huxley recorded the very first electrical pulse of a living thought.
After firing, the cell must reset. It pumps the sodium back out, a phase called the refractory period. This reset is why you cannot feel two rapid touches as distinct events; your nerves are physically busy recharging.
To bypass this delay, your body wraps axons in myelin, letting the signal skip across gaps like a stone on water.
A resting animal cell membrane sits at a negative voltage of about -70 mV. Action potentials occur because voltage-gated ion channels, which are specialized proteins embedded in the fatty cell membrane, open and close in response to electrical changes.
When the cell membrane reaches a precise threshold voltage, voltage-gated sodium channels rapidly open. Sodium ions rush into the cell interior, reversing the electrical charge across the membrane in an explosive chain reaction. Once the spike peaks, sodium channels quickly shut, and potassium channels open to let potassium ions flow out, restoring the resting negative balance.
Following the sudden electrical peak, the membrane enters a brief dip below its resting voltage, known as afterhyperpolarization. The cell actively transports sodium back out to reset its electrochemical balance during this refractory period.
Because of this reset phase, individual spikes remain separate rather than blurring together into a single signal. In neurons, the resulting wave of depolarization travels along the axon until it reaches synaptic terminals at the end, triggering signals to adjacent nerves, muscle cells, or glands.
Action potentials are not unique to neurons. They occur in many excitable cells, including skeletal and cardiac muscle cells, pancreatic beta cells that release insulin, certain cells in the anterior pituitary gland, and even some plant cells.
Different cells rely on different ions and timeframes. While a sodium-based nerve impulse in a mammalian neuron lasts only a fraction of a millisecond, calcium-based action potentials can last 100 milliseconds or longer. In heart muscle cells, a fast sodium spike acts as a primer for a longer calcium spike that drives contraction.
When an action potential reaches threshold, what starts the voltage spike?
Sodium rushes into the cell. Threshold opens membrane gates, letting sodium enter and briefly reverse the cell’s charge. The spike is triggered rather than a gradual voltage increase.
Why might two rapid touches feel like one during an action potential?
The nerve may still be resetting after firing. After firing, a nerve enters a refractory period while it resets, limiting how quickly it can fire again.
How does an action potential travel along a neuron compared to household electricity?
As a wave of chemical gate openings. Action potentials rely on charged atoms rushing through membrane gates rather than free-flowing electrons, making them discrete biological pulses instead of standard currents.
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Duration depends on the cell type. In mammalian neurons, a typical sodium-driven action potential lasts under one millisecond, whereas calcium-driven action potentials in other cells can last 100 milliseconds or more, and plant action potentials can last several seconds.
An action potential starts when the electrical charge at an excitable section of the membrane, such as the axonal initial segment near the axon hillock, reaches a specific threshold voltage. This shift forces voltage-gated sodium channels to swing open.
No, the overall concentrations on either side of the membrane remain relatively unchanged. Only a tiny fraction of ions needs to cross the membrane during a spike to completely flip the electrical potential.
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A thought is electricity, right up until the last millimeter.
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