What Are Action Potentials?

Action potentials are brief, self-regenerating electrical impulses that propagate along the membranes of neurons and other excitable cells, serving as the principal means of transmitting information over long distances in the nervous system. Each action potential is a stereotyped event: a rapid depolarization of the membrane voltage from its resting level of approximately negative 70 millivolts to a peak near positive 40 millivolts, followed by repolarization and a brief undershoot before the membrane returns to rest. The all-or-nothing character of action potentials means that once the membrane voltage crosses a threshold, the full event proceeds regardless of the magnitude of the triggering stimulus. Information is encoded not in the amplitude but in the timing, rate, and pattern of impulses. Action potentials occur in neurons, cardiac muscle cells, skeletal muscle fibers, and certain secretory cells, and their study spans neurophysiology, biomedical engineering, and neural signal processing.

The biophysical basis of the action potential was established by Alan Hodgkin and Andrew Huxley in a series of experiments on the giant axon of the squid, published in 1952 and recognized with the Nobel Prize in Physiology or Medicine in 1963. Their work resolved that the action potential arises from the sequential opening and closing of voltage-sensitive sodium and potassium ion channels in the cell membrane.

Ion Channel Dynamics and Membrane Depolarization

The action potential proceeds in distinct phases governed by the conductance of voltage-gated ion channels. At rest, the membrane maintains a negative potential through the activity of sodium-potassium ATPase pumps and the selective permeability of leak channels. When a local depolarizing stimulus raises the membrane potential above the threshold, voltage-gated sodium channels open rapidly, allowing sodium ions to rush inward along their electrochemical gradient. This inward sodium current further depolarizes the membrane in a positive-feedback cycle that drives the voltage to its peak. The reconstruction of the action potential described in NCBI's Neuroscience textbook details how Hodgkin and Huxley used voltage-clamp measurements to isolate sodium and potassium conductance changes and then reconstruct the full waveform mathematically with remarkable accuracy. Repolarization follows as sodium channels inactivate and delayed-rectifier potassium channels open, allowing potassium ions to flow outward. A brief afterhyperpolarization, during which the membrane is transiently more negative than rest, reflects the slow closure of potassium channels before the resting state is fully restored.

Axonal Propagation and Myelination

Once initiated at the axon initial segment, an action potential propagates along the axon by depolarizing adjacent membrane patches. In unmyelinated axons, propagation is continuous and relatively slow, typically 0.5 to 2 meters per second. In myelinated axons, the myelin sheath produced by oligodendrocytes in the central nervous system (white matter) and Schwann cells in the peripheral nervous system acts as electrical insulation, forcing the action potential to jump between exposed nodes of Ranvier in a process called saltatory conduction. This mechanism increases propagation velocity to 70 to 120 meters per second while reducing the metabolic energy required per impulse. The PMC review of the Hodgkin-Huxley heritage from channels to circuits traces how the original ionic model was extended to explain myelinated conduction, synaptic integration, and network-level firing patterns.

Neural Recording and Computational Modeling

Measuring action potentials from individual neurons or neural populations requires high-bandwidth recording electronics capable of resolving signals in the microvolt to millivolt range at sampling rates of 20 kilohertz or above. Extracellular electrodes, silicon microelectrode arrays, and flexible neural probes are fabricated to record single-unit or multi-unit activity in vivo. The Nature Neuroscience review of the Hodgkin-Huxley theory of the action potential describes how the mathematical framework has been extended into multicompartment neuron models and network simulations used in computational neuroscience.

Applications

Action potentials have applications in a range of engineering and biomedical fields, including:

• Brain-computer interfaces that decode motor intent from recorded spike trains in the motor cortex
• Cochlear and retinal prostheses that electrically stimulate neurons to restore sensory function
• Cardiac electrophysiology and implantable defibrillator design, where action potential propagation determines arrhythmia dynamics
• Neuropathology diagnostics, including nerve conduction studies for detecting demyelinating diseases such as multiple sclerosis
• Neuromorphic computing, where spiking neural network models are inspired by action potential encoding