An action potential is a rapid, transient, and self-propagating change in membrane voltage that occurs in excitable cells — primarily neurons, muscle cells, and cardiac cells — as a result of the sequential opening and closing of ion channels across the membrane. It constitutes the fundamental unit of electrical signaling in the nervous system and muscle tissue, forming the physiological basis of information transmission, muscle contraction, and cardiac rhythm.
Resting Membrane Potential
Understanding the action potential first requires grasping the resting state. In an unstimulated neuron, the membrane potential is approximately -70 mV, with the intracellular environment being negative relative to the extracellular space. This electrochemical equilibrium is maintained by three principal mechanisms.
The sodium-potassium pump (Na⁺/K⁺-ATPase) expels 3 Na⁺ ions from the cell while importing 2 K⁺ ions with each cycle. This asymmetric transport maintains both the ion concentration gradients and the negative intracellular voltage.
Potassium leak channels remain partially open at rest, allowing K⁺ ions to diffuse outward down their concentration gradient, thereby sustaining the negative interior voltage.
Large intracellular anions — including proteins and organic phosphates — carry negative charges and are unable to cross the membrane, remaining trapped within the cell and contributing to the negative resting potential.
Phases of the Action Potential
The action potential unfolds across five distinct sequential phases.
Subthreshold depolarization and threshold: When a stimulus reaches the cell, the membrane potential begins to rise. Once this rise reaches the threshold value of approximately -55 mV, voltage-gated Na⁺ channels open rapidly and the action potential is initiated. Stimuli that fail to reach threshold do not generate an action potential — this is the cellular basis of the all-or-none principle.
Rapid depolarization: The opening of voltage-gated Na⁺ channels allows Na⁺ ions to rush into the cell along their electrochemical gradient. The membrane potential rises sharply from -70 mV to approximately +30 to +40 mV. This phase is completed within a matter of milliseconds.
Repolarization: Upon reaching approximately +30 mV, Na⁺ channels inactivate and voltage-gated K⁺ channels open. K⁺ ions flow outward, rapidly driving the membrane potential back toward its resting value.
Hyperpolarization (afterhyperpolarization): Since K⁺ channels remain open briefly beyond what is necessary for full repolarization, the membrane potential transiently undershoots the resting value, reaching approximately -80 to -90 mV. This period of excess negativity constitutes part of the refractory period.
Recovery: The coordinated action of the Na⁺/K⁺ pump and leak channels restores ion concentrations to their resting values, returning the membrane to its baseline potential and rendering the cell ready for the next stimulus.
The All-or-None Principle
One of the most fundamental properties of the action potential is the all-or-none principle. Any stimulus that reaches the threshold value generates an action potential of standard amplitude and shape, regardless of the strength of the stimulus. Stronger stimuli do not produce larger action potentials — they produce action potentials at a higher frequency. The nervous system therefore encodes stimulus intensity not through the magnitude of individual action potentials but through the rate of firing — the number of action potentials generated per unit time.
Refractory Periods
Following an action potential, the cell is transiently resistant to generating a new one.
The absolute refractory period corresponds to the interval during which Na⁺ channels are in their inactivated state and no stimulus, however strong, can initiate a new action potential. This phase guarantees the unidirectional propagation of action potentials — a signal can only travel forward along the nerve fiber, never backward.
The relative refractory period coincides with the hyperpolarization phase. During this interval, a new action potential can be generated, but only in response to a stimulus of greater than normal strength, sufficient to overcome the excess negativity of the membrane.
Propagation Mechanisms
Once initiated, the action potential propagates along the nerve fiber by two distinct mechanisms.
In unmyelinated fibers (continuous conduction), depolarization spreads sequentially from one membrane segment to the adjacent one in a continuous wave. Conduction velocity is relatively slow and is directly proportional to the diameter of the fiber.
In myelinated fibers (saltatory conduction), the action potential leaps from one node of Ranvier to the next, skipping across the insulating myelin sheath. This mechanism dramatically increases conduction velocity; in large myelinated fibers, propagation speeds can reach up to 120 m/s. Saltatory conduction is not only faster but also substantially more energy-efficient, as ion exchange occurs only at the nodes rather than along the entire membrane surface.
Action Potentials in Different Cell Types
Although the fundamental principles of the action potential are universal, significant variations exist across different excitable cell types.
In neurons, the classical Na⁺/K⁺ mechanism predominates. The action potential duration is approximately 1 to 2 milliseconds, and firing rates can reach hundreds of impulses per second.
In skeletal muscle cells, the action potential triggers Ca²⁺ release from the sarcoplasmic reticulum, initiating the excitation-contraction coupling cascade that results in mechanical contraction.
In cardiac cells, the action potential is dramatically prolonged — lasting approximately 200 to 400 milliseconds — and this duration prevents tetanic contraction, thereby preserving the rhythmic pumping function of the heart. Voltage-gated Ca²⁺ channels play a critical role during the characteristic plateau phase.
In pacemaker cells such as those of the sinoatrial node, the resting membrane potential is not stable but instead undergoes slow spontaneous depolarization known as the pacemaker potential, which drives rhythmic automatic firing without external stimulation.
Clinical Significance
Disruptions in action potential physiology underlie a broad spectrum of clinical conditions.
Local anesthetics such as lidocaine and bupivacaine block voltage-gated Na⁺ channels, halting action potential propagation and interrupting pain signal transmission.
Epilepsy is characterized by excessive and synchronous neuronal firing; the majority of antiepileptic drugs act by stabilizing Na⁺ or Ca²⁺ channels to reduce neuronal excitability.
Multiple sclerosis involves destruction of the myelin sheath, slowing or completely blocking action potential conduction along affected nerve fibers.
Cardiac arrhythmias arise from abnormal firing of pacemaker cells or conduction system disturbances; antiarrhythmic drugs are classified precisely according to the ion channels they target.
Myasthenia gravis impairs the initiation of muscle action potentials at the neuromuscular junction due to autoantibodies directed against acetylcholine receptors.
Virtually all pharmacological interventions in the domains of local and general anesthesia, chronic pain management, and cardiac electrophysiology act by directly targeting the ion channel mechanisms that govern action potential generation and propagation.