Right now, as you read this sentence, thousands of neurons in your visual cortex are firing. Each firing is an action potential — a precisely shaped electrical spike that lasts about one millisecond and travels along the neuron’s axon at speeds ranging from 0.5 to 120 meters per second, depending on the fiber type. The action potential is the fundamental unit of neural communication, and understanding it is the foundation for understanding everything from muscle control to brain-computer interfaces to the mechanism of anesthesia.
The Resting State: A Battery Under Tension
A neuron at rest is not passive. Its membrane actively maintains a voltage difference of about -70 millivolts between the inside and outside of the cell — negative inside relative to outside. This is maintained by ion pumps, primarily the sodium-potassium ATPase, which continuously expels three sodium ions and imports two potassium ions for every ATP molecule consumed. The result is a cell that is perpetually ready to fire, like a cocked spring.
Key Numbers
- →-70 mV — resting membrane potential of a typical neuron
- →-55 mV — threshold voltage; cross this and firing is inevitable
- →+40 mV — peak voltage at the height of the action potential spike
- →1 ms — total duration of a typical action potential
- →120 m/s — maximum conduction velocity in large myelinated fibers
Threshold and the All-or-Nothing Law
When incoming signals push the membrane voltage from -70mV toward -55mV, voltage-gated sodium channels open. Sodium rushes in, depolarizing the membrane further, opening more channels — a runaway positive feedback that drives voltage to +40mV in under a millisecond. Then potassium channels open, sodium channels inactivate, and the membrane repolarizes. The process is all-or-nothing: either the threshold is crossed and a full spike fires, or nothing happens at all.
Why This Matters for Bioelectronic Medicine
Every device in the bioelectronics field — from cochlear implants to retinal prostheses to vagus nerve stimulators — works by either triggering or blocking action potentials. Understanding the action potential is understanding the language these devices speak. As closed-loop systems become more sophisticated, their ability to read and write individual spikes will become the defining measure of how precisely medicine can interface with the nervous system.
Sources: Hodgkin & Huxley (1952) · Kandel, Principles of Neural Science
Credit: Bioscience Image Library by Fayette Reynolds on Unsplash