What is an Action Potential?
An action potential is a brief electrical signal that travels along the membrane of a neuron. It allows communication between neurons and between neurons and muscles or glands. It occurs due to rapid changes in the membrane potential caused by the movement of ions (mainly Na⁺ and K⁺) across the neuronal membrane.
Key Concepts Before the Action Potential
Resting Membrane Potential
When the neuron is not sending a signal, it is at rest.
The inside of the neuron is negatively charged compared to the outside.
Typical value: −70 mV
Caused by:
Na⁺/K⁺ pump (pumps 3 Na⁺ out, 2 K⁺ in)
More K⁺ leak channels than Na⁺ leak channels
Large negatively charged proteins inside the cell that cannot leave
Phases of the Action Potential
Resting State
Voltage-gated Na⁺ and K⁺ channels are closed.
The membrane is at resting potential (~−70 mV).
Depolarization
A stimulus (like neurotransmitter binding) causes a slight depolarization.
If the membrane reaches a threshold (~−55 mV), voltage-gated Na⁺ channels open.
Na⁺ rushes into the cell (due to concentration and electrical gradient).
Membrane becomes more positive — up to +30 mV.
Repolarization
At peak depolarization, Na⁺ channels close (inactivate).
K⁺ channels open, and K⁺ flows out of the cell.
The inside of the cell starts returning to a negative charge.
Hyperpolarization (Undershoot)
K⁺ channels are slow to close, so extra K⁺ leaves.
Membrane potential goes below resting level (e.g., ~−80 mV).
This makes it harder to fire another action potential immediately.
Return to Resting State
K⁺ channels close.
The Na⁺/K⁺ pump restores the original ion distribution.
Resting membrane potential is re-established (~−70 mV).
All-or-None Principle
Once threshold is reached, the action potential happens fully — always the same size.
If threshold is not reached, there is no action potential.
Propagation of Action Potential
The depolarization at one section causes nearby voltage-gated Na⁺ channels to open.
This continues like a wave along the axon.
In myelinated neurons, the impulse jumps from node to node (Nodes of Ranvier). This is called saltatory conduction, and it makes conduction faster.
Refractory Periods
Absolute Refractory Period
No new action potential can start.
Na⁺ channels are inactivated.
Relative Refractory Period
A stronger-than-normal stimulus can cause an action potential.
The membrane is still hyperpolarized.
Importance of Action Potentials
They allow neurons to send signals quickly and reliably.
Essential for muscle contraction, sensation, thought, movement, and reflexes.
