How A Neuron Fires The Action Potential Explained?

A neuron fires when its internal electrical charge flips from negative to positive in a fraction of a millisecond. This rapid voltage spike is called an action potential. It is the basic electrical signal that carries information through your nervous system — from a fingertip sensing heat to a brain cell storing a memory.

The process depends on charged particles called ions moving across the neuron’s outer membrane through tiny protein channels. Sodium rushes in, the voltage reverses, potassium flows out, and the cell resets. The whole event lasts about one to two milliseconds in most neurons. Understanding this sequence explains how nerves communicate, why some signals travel faster than others, and what happens when this system breaks down.

How A Neuron Fires The Action Potential Explained

Every neuron maintains a voltage difference across its membrane called the resting membrane potential. In most neurons, this sits at roughly −70 millivolts, meaning the inside is negative relative to the outside. This charge is maintained by differences in ion concentrations and by the selective permeability of the membrane.

Three ions matter most:

• Sodium (Na⁺) — concentrated outside the cell
• Potassium (K⁺) — concentrated inside the cell
• Chloride (Cl⁻) — concentrated outside the cell

The sodium-potassium pump, a protein embedded in the membrane, actively moves three sodium ions out for every two potassium ions it brings in. This pump uses ATP, the cell’s energy currency, and it maintains the concentration gradients that make firing possible. Without it, the gradients would slowly run down and the neuron could not signal.

When a stimulus — say, a pinch or a chemical signal from another neuron — reaches the membrane, it can open sodium channels. If enough sodium channels open, the membrane voltage rises to a critical point called the threshold, typically around −55 millivolts. Once threshold is reached, the process becomes self-reinforcing and an action potential fires.

What Happens During Depolarization and Repolarization?

At threshold, voltage-gated sodium channels snap open. These channels are proteins that change shape in response to voltage changes. When they open, sodium floods into the cell because both the electrical gradient and the concentration gradient push it inward.

The influx of positive sodium ions drives the membrane voltage upward rapidly. This phase is called depolarization. The voltage can overshoot zero and reach about +30 to +40 millivolts at its peak.

Two things then stop the rise:

• The sodium channels have a built-in inactivation gate that closes shortly after opening, even while the channel is still triggered.
• Voltage-gated potassium channels open more slowly and allow potassium to exit the cell.

Potassium leaving the cell removes positive charge from inside, pulling the voltage back down. This is repolarization. The potassium channels stay open slightly too long, so the voltage dips below the resting potential — a phase called hyperpolarization. Then the channels close, and the sodium-potassium pump and passive ion leaks restore the resting state over a few milliseconds.

This entire sequence — depolarization, repolarization, and return to rest — happens in roughly 1 to 2 milliseconds in a typical neuron. Some neurons are faster; some are slower.

Why Does The Action Potential Only Travel One Direction?

An action potential travels from the cell body toward the axon terminals. It does not reverse direction. This one-way flow is not a design choice — it falls out of the biology.

The key is the refractory period. Immediately after a sodium channel opens and inactivates, it cannot reopen for a short time. This means the patch of membrane that just fired is temporarily unable to fire again. The signal can only move forward into fresh membrane that has not yet fired.

There are two refractory phases:

• Absolute refractory period — no new action potential can be generated regardless of stimulus strength.
• Relative refractory period — a stronger-than-normal stimulus can trigger a new signal.

The refractory period also sets a ceiling on how fast a neuron can fire repeatedly. It limits the maximum firing rate, which in turn shapes how information is encoded.

How Do Myelinated Axons Conduct Signals Faster?

Many axons are wrapped in myelin, a fatty sheath formed by specialized cells. Myelin acts as an electrical insulator. It prevents ion flow across the membrane where it is present.

Instead of opening channels along the entire length of the axon, the action potential jumps between gaps in the myelin called nodes of Ranvier. These nodes are densely packed with voltage-gated sodium channels. The signal effectively leaps from node to node — a process called saltatory conduction.

This makes conduction much faster and more energy-efficient than in unmyelinated axons. It is why myelinated fibers can carry signals at speeds well over 100 meters per second in large human axons, while thin unmyelinated fibers conduct at less than 1 meter per second.

This speed difference matters in daily life. The sharp, immediate pain you feel when you touch something hot travels along myelinated fibers. The slower, dull ache that follows travels along thinner unmyelinated ones.

What Is The All-or-None Principle?

An action potential either fires completely or not at all. There is no such thing as a “half” action potential. Once the membrane reaches threshold, the spike reaches the same peak amplitude every time.

This is the all-or-none principle. It means the strength of a stimulus is not encoded in the size of the action potential. Instead, the nervous system encodes stimulus intensity in two other ways:

• Firing rate — a stronger stimulus triggers more action potentials per second.
• Number of neurons activated — a stronger stimulus recruits more sensory neurons.

This is a common point of confusion. People often assume a bigger signal means a bigger action potential. It does not. The brain reads intensity from how frequently a neuron fires and how many neurons are firing, not from the voltage of any single spike.

What Happens At The Synapse After Firing?

When the action potential reaches the axon terminal, it triggers the release of neurotransmitters. These are chemical messengers that carry the signal to the next neuron, a muscle cell, or a gland.

The arriving electrical signal opens voltage-gated calcium channels at the terminal. Calcium enters the cell and causes vesicles — small membrane-bound packages — to fuse with the membrane and release their neurotransmitter contents into the synaptic gap.

The neurotransmitter crosses the gap and binds to receptors on the receiving cell. Depending on the neurotransmitter and receptor type, this can excite or inhibit the next cell. If excitation is strong enough to bring that cell to threshold, a new action potential fires there. The signal continues.

This is where the electrical signal becomes chemical and then electrical again. The conversion happens at every synapse, and it allows the nervous system to modulate, amplify, or dampen signals as they travel.

What Disrupts Normal Action Potential Firing?

Because action potentials depend on ion channels working correctly, anything that interferes with those channels can disrupt signaling. Several well-documented examples exist.

Local anesthetics such as lidocaine block voltage-gated sodium channels. They prevent the membrane from reaching threshold, so no action potential fires. This is why you feel no pain during a dental procedure — the sensory neurons in that area cannot send signals.

Toxins can target the same channels. Tetrodotoxin, found in pufferfish, blocks sodium channels and can cause paralysis. Saxitoxin, produced by certain algae, does the same and can contaminate shellfish.

Demyelinating conditions damage the myelin sheath. When myelin is lost, saltatory conduction fails. Signals slow down or fail to propagate entirely. This can cause weakness, numbness, and coordination problems. Multiple sclerosis is the most well-known condition in this category, though other diseases can also damage myelin.

Electrolyte imbalances affect the concentration gradients that make firing possible. Abnormally low or high blood levels of sodium or potassium can alter neuronal excitability. This is one reason electrolyte disturbances are taken seriously in clinical settings.

Channel mutations can alter how ion channels open, close, or respond to voltage. Some inherited conditions affecting muscle or nerve excitability are linked to specific channel gene mutations. These are relatively rare but well documented.

How Does This Compare Across Different Cell Types?

Not all excitable cells fire the same way. Neurons, muscle cells, and cardiac cells all use action potentials, but with differences in timing and ion channel composition.

The long cardiac action potential is worth noting. Its extended plateau phase means the heart muscle cannot be stimulated again while it is contracting. This built-in pause is essential — it prevents the heart from locking up in a sustained contraction, which would be fatal. The difference comes from calcium channels that stay open much longer in cardiac cells than in neurons.