Action Potentials Explained- How Neurons Send Electrical Signals
What Is an Action Potential?
An action potential is a rapid, self-propagating electrical signal that travels along a neuron's axon. It's the fundamental unit of communication in the nervous system. When a neuron "fires," it's generating an action potential.
These signals are all-or-nothing events. Either the neuron fires a complete action potential, or it doesn't fire at all. There's no partial firing, no halfway signal.
Your brain generates roughly 70 billion action potentials per second across all your neurons. That's just background noise for your nervous system.
The Setup: Resting Membrane Potential
Before any signal fires, the neuron sits at its resting membrane potential—typically around -70 millivolts. This negative charge comes from:
- More potassium ions inside the cell than outside
- More sodium ions outside the cell than inside
- The sodium-potassium pump actively moving 3 sodium out and 2 potassium in
The cell membrane is selectively permeable. It lets potassium leak out freely but restricts sodium from coming in. This imbalance creates the negative interior.
How an Action Potential Actually Works
Step 1: Depolarization Begins
When a stimulus reaches the neuron, sodium channels open. Sodium ions rush into the cell because there's more sodium outside than inside. This makes the interior less negative.
If the depolarization reaches about -55 millivolts (the threshold), the action potential triggers. Nothing happens below this point.
Step 2: Rapid Depolarization
Once threshold is hit, voltage-gated sodium channels open all at once. The membrane potential shoots up to roughly +30 millivolts in about 1 millisecond. Sodium floods in. The inside of the cell goes from negative to positive.
Step 3: Repolarization
At the peak, sodium channels close. Potassium channels open. Potassium ions rush out of the cell. The interior swings back toward negative values.
This happens fast—also about 1 millisecond.
Step 4: Hyperpolarization
Potassium channels stay open slightly longer than needed. The membrane potential dips below the normal resting level to about -80 millivolts. This is the refractory period—another signal cannot fire during this time.
Step 5: Return to Resting
The sodium-potassium pump restores the original ion distribution. The neuron is ready to fire again.
The All-or-None Law
Action potentials don't get stronger with stronger stimuli. A threshold stimulus produces the same action potential as a stimulus ten times stronger.
What changes with stimulus strength is frequency, not amplitude. Stronger stimuli cause neurons to fire more rapidly, not bigger signals.
Signal Propagation
The action potential doesn't travel directly down the axon. Each segment of axon triggers the next one, like dominoes falling. The signal regenerates at each point, which prevents it from weakening over distance.
Myelinated Axons: Faster Conduction
Some axons have a myelin sheath—fatty insulation made by glial cells. This insulation forces the action potential to jump between gaps called nodes of Ranvier.
This is called saltatory conduction (from the Latin word for "jumping"). It increases conduction speed up to 10 times compared to unmyelinated axons.
Your motor neurons that control movement are heavily myelinated. That's why you can react quickly.
Factors That Affect Conduction Speed
- Axon diameter—larger diameter means faster conduction
- Myelination—myelinated axons conduct much faster
- Temperature—warmer speeds up conduction, which is why mammals evolved warm bodies
- Node spacing—closer nodes in myelinated axons increase speed
Comparing Neural Conduction Types
| Type | Speed | Example Location | Characteristics |
|---|---|---|---|
| Unmyelinated | 0.5–2 m/s | Visceral organs | Simple, low-energy, slow |
| Myelinated (small) | 10–50 m/s | Autonomic nervous system | Moderate speed, energy efficient |
| Myelinated (large) | 80–120 m/s | Motor neurons, sensory neurons | Fast, critical for reflexes |
Getting Started: Studying Action Potentials
If you want to understand action potentials practically:
- Learn the ion concentrations first—sodium outside high, potassium inside high. Everything else follows from this.
- Memorize the voltage values—resting (-70mV), threshold (-55mV), peak (+30mV), hyperpolarization (-80mV)
- Trace the ion movements—sodium in causes depolarization, potassium out causes repolarization
- Watch video animations—the timing of channel opening is hard to grasp from text alone
- Practice with sample traces—identify which phase is shown based on the voltage
The Hodgkin-Huxley model from 1952 is the foundational paper. Alan Hodgkin and Andrew Huxley won a Nobel Prize for describing the ionic basis of the action potential in squid giant axons.
Why This Matters
When this system breaks, you notice. Local anesthetics like lidocaine block sodium channels, preventing pain signals from reaching the brain. Anti-epileptic drugs work by stabilizing sodium channels to prevent the abnormal synchronized firing seen in seizures.
Multiple sclerosis destroys myelin, dramatically slowing conduction. This causes the motor and sensory deficits patients experience.
The action potential is not abstract physiology. It's the mechanism behind every thought, movement, and sensation you have.