The Action Potential

Before a signal fires, a neuron sits at a Resting Membrane Potential of approximately $-70\text{ mV}$. This charge is maintained by the sodium-potassium pump, keeping more $Na^+$ outside and more $K^+$ inside. For a nerve to 'fire,' a stimulus must be strong enough to shift the voltage to the Threshold of $-55\text{ mV}$. This is the 'point of no return.' If the stimulus is too weak (e.g., reaching only $-60\text{ mV}$), nothing happens. This binary nature is known as the All-or-None Principle: a neuron either fires a full-strength impulse or it does not fire at all. There are no 'weak' action potentials; intensity is instead coded by the frequency of firing.

Once the threshold is reached, the membrane undergoes three rapid phases: 1. Depolarization: Voltage-gated $Na^+$ channels burst open. $Na^+$ rushes into the cell, driven by concentration gradients, flipping the internal charge to $+30\text{ mV}$. 2. Repolarization: $Na^+$ channels close and voltage-gated $K^+$ channels open. $K^+$ rushes out of the cell, restoring the negative internal charge. 3. Hyperpolarization: The $K^+$ channels stay open slightly too long, causing the voltage to drop below the resting state (e.g., $-90\text{ mV}$). This creates a Refractory Period, ensuring the signal only travels in one direction.

In unmyelinated axons, the signal moves like a slow-burning fuse. However, many human neurons are wrapped in Myelin, a fatty insulating layer. This insulation is interrupted by gaps called Nodes of Ranvier. Instead of traveling the whole length, the action potential 'jumps' from node to node. This is called Saltatory Conduction (from the Latin saltare, to leap). This process increases conduction velocity by up to 100 times while conserving energy, as the neuron only needs to pump ions at the nodes rather than across the entire axonal surface.