Action Potential in Neurons: Step-by-Step Mechanism

Our brains are incredible electrical networks, and the "action potential" is the fundamental signal enabling neurons to communicate. It's a rapid, brief reversal of electrical charge across a neuron's membrane, allowing information to zip through the nervous system.

It begins at the **resting potential**, where the neuron's interior is negatively charged (around -70 millivolts) compared to the outside. This state is maintained by an unequal distribution of sodium (Na+) and potassium (K+) ions, actively pumped and balanced by leak channels, keeping the neuron poised.

When sufficient stimulation occurs, the membrane depolarizes, becoming less negative. If this reaches a critical **threshold** (e.g., -55mV), an action potential is triggered in an all-or-none fashion.

This threshold opens voltage-gated sodium channels, allowing a rapid influx of positively charged Na+ ions. The interior quickly becomes positive (up to +30mV) – this is the **depolarization phase**.

Almost immediately, these sodium channels inactivate, and slower voltage-gated potassium channels open. K+ ions rush *out* of the cell, restoring the negative charge inside in the **repolarization phase**.

As K+ channels are slow to close, the membrane briefly dips even more negatively than the resting potential, known as **hyperpolarization** or the "undershoot." This creates a **refractory period**, preventing immediate re-firing and ensuring one-way signal propagation. Finally, ion pumps restore the membrane to its resting potential, ready for the next signal.