An action potential is a rapid, temporary change in the electrical voltage across a cell membrane that allows a nerve cell called a neuron to transmit information. It occurs when voltage-sensitive ion channels open and close in a coordinated sequence, producing an electrical impulse that can travel along an axon without progressively losing its strength.
Action potentials are responsible for much of the rapid communication between the brain, spinal cord, muscles, and sensory organs. They connect the microscopic behavior of cell membranes with larger functions studied in human anatomy, including movement, sensation, and reflexes.
Unlike an ordinary electrical current traveling through a wire, an action potential is regenerated by ion channels along the membrane. Understanding how it begins, travels, and ends helps explain how nervous tissue communicates reliably over long distances.
Why Action Potentials Are Important
An action potential is a rapid electrical signal that travels along a neuron’s axon, allowing nerve cells to communicate with each other and with muscles or glands. These signals provide a mechanism for carrying information from one location to another.
Action potentials help the nervous system perform several functions:
- Sensory detection: Carrying information about touch, pain, temperature, and other stimuli.
- Muscle control: Delivering commands that activate muscle fibers.
- Reflex responses: Supporting rapid communication through spinal and brainstem circuits.
- Automatic regulation: Helping coordinate breathing, cardiovascular activity, and digestion.
- Neural processing: Allowing interconnected neurons to exchange information involved in perception, learning, and memory.
Neural communication also depends on whether incoming signals excite or inhibit a neuron. The balance between these effects, explained in Inhibitory vs Excitatory Signaling, helps determine when action potentials are generated.
How Neurons Generate Electrical Signals
Neurons maintain a difference in electrical charge across their cell membranes. This difference is known as the resting membrane potential.
In many neurons, the resting membrane potential is approximately -70 millivolts (mV), meaning the inside of the cell is electrically negative relative to the outside. The exact resting voltage varies among cell types.
This voltage results from unequal ion concentrations and the membrane’s selective permeability. Two particularly important ions are sodium (Na+) and potassium (K+).
- Sodium: Normally has a higher concentration outside the neuron than inside it.
- Potassium: Normally has a higher concentration inside the neuron than outside it.
- Ion channels: Allow selected ions to cross the membrane when the channels are open.
- Sodium-potassium pump: Uses ATP to transport three sodium ions outward and two potassium ions inward per cycle, helping maintain the concentration gradients.
The sodium-potassium pump maintains the conditions needed for electrical signaling over time, but the rapid voltage changes during an individual action potential are produced mainly by ions moving through channels.
The Five Stages of an Action Potential
A typical neuronal action potential follows a sequence of resting conditions, threshold activation, depolarization, repolarization, and recovery. These stages describe changes in membrane voltage rather than the physical movement of an electrical charge along the entire length of the axon.
1. Resting State
Before an action potential, the neuron maintains a relatively stable resting membrane potential. Potassium leak channels and other membrane properties help establish this voltage.
The neuron is electrically polarized, but it is not inactive. Ion channels, transport proteins, and synaptic inputs continue operating while the membrane remains near its resting voltage.
2. Threshold and Initiation
Incoming signals can depolarize the membrane, making its voltage less negative. If depolarization reaches the threshold for action potential initiation, enough voltage-gated sodium channels open to produce a self-reinforcing electrical response.
A commonly used illustrative threshold is approximately -55 mV, although the actual threshold depends on the neuron and its physiological conditions. In many neurons, action potentials begin near the axon initial segment.
3. Depolarization
During the rapid rising phase, voltage-gated sodium channels open and sodium ions enter the neuron. This inward positive current causes the membrane voltage to rise sharply.
The voltage may briefly become positive relative to the outside of the cell, often reaching approximately +30 mV in a textbook neuronal example. This reversal of membrane polarity is called the overshoot.
4. Repolarization
As the action potential approaches its peak, many voltage-gated sodium channels become inactivated. Meanwhile, delayed voltage-gated potassium channels contribute to outward potassium movement.
These changes cause the membrane voltage to move back toward negative values. Repolarization restores the electrical polarity of the membrane, although the cell may not immediately return to its exact resting voltage.
5. After-Hyperpolarization and Recovery
Some potassium channels remain open briefly after the main falling phase. As potassium continues leaving the cell, the membrane may become more negative than its resting potential.
This temporary after-hyperpolarization ends as membrane conductances return toward their resting conditions. Sodium channels recover from inactivation, allowing the neuron to become ready for another action potential.
Action Potential Stages Compared
The following table summarizes the major electrical events in a typical neuronal action potential. Voltage values are representative examples, not fixed values for every neuron.
| Stage | Typical Voltage | Main Electrical Event |
|---|---|---|
| Resting state | About -70 mV | Stable membrane voltage maintained by ion gradients and permeability |
| Threshold | About -55 mV | Regenerative opening of voltage-gated sodium channels begins |
| Depolarization | Rises toward +30 mV | Sodium enters rapidly |
| Repolarization | Falls toward resting voltage | Sodium channels inactivate and potassium exits |
| After-hyperpolarization | Temporarily below resting voltage | Elevated potassium conductance persists briefly |
| Recovery | Returns toward -70 mV | Membrane conductances and channel availability recover |
Worked Example: Calculating the Voltage Change
Consider a simplified neuron with a resting membrane potential of -70 mV, an action potential threshold of -55 mV, and a peak voltage of +30 mV.
- Calculate the threshold change: -55 mV minus -70 mV equals a 15 mV depolarization.
- Calculate the rise from threshold to peak: +30 mV minus -55 mV equals 85 mV.
- Calculate the total rise: +30 mV minus -70 mV equals 100 mV.
- Interpret the result: The membrane voltage changes by 100 mV from rest to peak, then falls during repolarization.
These values demonstrate the size of a typical voltage change. They do not mean that all neurons have identical thresholds or peaks. An action potential involves a rapid change in voltage across the membrane, not a reversal of all the sodium and potassium concentration differences inside and outside the cell.
How Action Potentials Travel Along Axons
When one region of an axon generates an action potential, local electrical currents spread into neighboring membrane regions. If those regions reach threshold, their voltage-gated sodium channels open and generate another action potential.
This repeated regeneration allows the impulse to propagate without the progressive loss of amplitude that would occur with passive electrical spread alone.
Under normal physiological conditions, action potentials generally travel forward along axons because the membrane immediately behind the advancing impulse is temporarily refractory. It cannot immediately generate another action potential in the usual way.
Absolute and Relative Refractory Periods
During the absolute refractory period, another normal action potential cannot be initiated because many voltage-gated sodium channels are inactivated.
During the relative refractory period, another action potential may occur, but it generally requires stronger stimulation because the membrane is recovering and potassium conductance may remain elevated.
These refractory periods limit how frequently neurons can fire and contribute to the normal direction of impulse propagation.
The All-or-None Principle
Action potentials follow the all-or-none principle. When the membrane reaches the required threshold, regenerative ion channel activity produces a full action potential. If the threshold is not reached, that regenerative event does not occur.
A stronger stimulus does not normally produce a proportionally taller action potential in the same axon. Instead, stronger stimulation may increase the frequency of impulses or recruit additional neurons.
Action potential amplitude can vary under certain physiological conditions, but the all-or-none principle remains a useful description of how individual neuronal impulses are initiated.
What Determines the Speed of Action Potentials?
Action potential conduction speed depends on several properties of the nerve fiber, especially axon diameter and myelination.
- Axon diameter: Larger axons generally conduct impulses faster because they offer less internal resistance to electrical current.
- Myelin: Insulation reduces current leakage and allows electrical signals to spread efficiently between exposed membrane regions.
- Nodes of Ranvier: These gaps between myelin segments contain concentrated voltage-gated channels that regenerate the action potential.
- Temperature: Temperature influences ion channel behavior and conduction speed.
In myelinated axons, action potentials are regenerated mainly at nodes of Ranvier, producing saltatory conduction. The electrical signal spreads rapidly beneath myelinated segments between nodes rather than literally jumping through empty space.
Some large myelinated nerve fibers can conduct at approximately 120 meters per second, while small unmyelinated fibers may conduct at only a few meters per second or less. For a closer comparison, see Myelination and Signal Conduction Speed.
Action Potentials and Synaptic Communication
An action potential traveling along an axon can trigger communication with another neuron, muscle fiber, or gland cell when it reaches a specialized terminal.
At a typical chemical synapse, the arriving action potential opens voltage-gated calcium channels. Calcium enters the terminal and helps trigger the fusion of neurotransmitter-containing vesicles with the cell membrane.
Released neurotransmitters diffuse across the synaptic cleft and bind to receptors on the receiving cell. Depending on the receptors involved, this may excite, inhibit, or otherwise modify the target cell’s activity.
The receiving neuron does not automatically generate an action potential. Its response depends on the combined effects of incoming signals and whether its membrane reaches threshold. These steps are examined in Synaptic Transmission and Neurotransmitters.
How Action Potentials Support Movement and Sensation
Imagine touching a hot surface. Specialized sensory nerve endings detect potentially damaging heat, generating electrical signals that can trigger action potentials in sensory neurons.
These impulses travel toward the spinal cord, where neural circuits can activate motor neurons involved in withdrawing the hand. Other signals reach the brain and contribute to conscious awareness of the painful stimulus.
At a neuromuscular junction, an action potential in a motor neuron causes acetylcholine release. This activates receptors on the muscle fiber and can trigger a muscle action potential, leading to contraction through excitation-contraction coupling.
Action potentials also occur in other excitable cells, including skeletal and cardiac muscle cells. Their electrical mechanisms differ in important ways from typical neuronal action potentials, but they share the general principle of voltage-dependent ion movement.
Common Misconceptions About Action Potentials
Misconception 1: Stronger Stimuli Produce Larger Action Potentials
Individual neuronal action potentials are generally all-or-none events. Stronger stimuli are commonly represented by higher firing frequencies or recruitment of additional neurons rather than larger impulses.
Misconception 2: The Sodium-Potassium Pump Directly Creates Each Action Potential
The sodium-potassium pump maintains ion gradients over time. Rapid action potential depolarization and repolarization occur primarily through voltage-gated sodium and potassium channels.
Misconception 3: Action Potentials Travel by Moving the Same Ions Along the Entire Axon
An action potential propagates because local currents trigger neighboring membrane regions to generate new electrical responses. Individual sodium ions do not need to travel from one end of the axon to the other.
Misconception 4: Every Neuron Conducts Action Potentials at the Same Speed
Conduction speed varies with axon diameter, myelination, temperature, and other factors. Some nerve fibers transmit impulses much faster than others because their structures differ.
Frequently Asked Questions
What Is an Action Potential in Simple Terms?
An action potential is a brief electrical impulse generated by an excitable cell. In neurons, it allows information to travel along an axon through rapid changes in membrane voltage.
What Triggers an Action Potential?
An action potential begins when membrane depolarization reaches the threshold needed to activate regenerative voltage-gated ion channel activity. In typical neurons, opening of voltage-gated sodium channels drives the rapid rising phase.
What Are the Main Phases of an Action Potential?
The main phases are depolarization, repolarization, and often after-hyperpolarization. These occur against the background of a resting membrane potential, with threshold activation initiating the impulse.
Why Do Action Potentials Travel in One Direction?
In normal axonal signaling, the membrane immediately behind the advancing impulse is refractory because sodium channels are temporarily inactivated. This makes backward regeneration unlikely while the impulse continues into excitable membrane ahead.
How Long Does an Action Potential Last?
A typical neuronal action potential lasts approximately 1–2 milliseconds, although duration varies by neuron type and conditions. Action potentials in other excitable cells, particularly cardiac muscle cells, can last considerably longer.
An action potential converts changes in membrane voltage into a rapidly propagating electrical signal through coordinated ion channel activity. Its threshold, all-or-none behavior, and refractory periods allow neurons to transmit information reliably, while myelin and axon structure influence conduction speed. Together with synaptic communication, these impulses enable neural circuits to coordinate sensation, movement, and many automatic body functions.
This article is for general education and is not medical advice. Talk to a qualified health professional about any medical concern.
Sources: OpenStax, Anatomy and Physiology 2e; Kandel et al., Principles of Neural Science; Purves et al., Neuroscience; Guyton and Hall, Textbook of Medical Physiology.
