Resting and Action Potential — Definition

Definition

Imagine a tiny battery inside every nerve cell, constantly maintaining a charge. This 'charge' is what we call the membrane potential. When a nerve cell, or neuron, is just sitting there, not sending any signals, it maintains a steady electrical difference across its outer boundary, the cell membrane.

This steady state is known as the resting potential. Think of it like a stretched spring, ready to be released. Inside the neuron, it's typically more negative compared to the outside, usually around $-70,\text{mV}$ (millivolts).

This negative charge is crucial and is maintained by a delicate balance of different ions – charged particles like sodium (Na$^+$), potassium (K$^+$), and chloride (Cl$^-$) – moving across the membrane, and a special protein pump called the sodium-potassium pump.

This pump actively pushes three sodium ions out of the cell for every two potassium ions it brings in, using energy, which helps keep the inside negative.

Now, when a neuron receives a strong enough stimulus, like a touch or a chemical signal, this 'stretched spring' gets released, and the electrical charge across the membrane rapidly changes. This rapid, dramatic shift in membrane potential is called an action potential, often referred to as a 'nerve impulse.

' It's like flipping a switch from negative to positive and then back to negative very quickly. This happens because specific channels in the membrane, called voltage-gated ion channels, open up. First, sodium channels open, allowing Na$^+$ ions to rush into the cell, making the inside temporarily positive (this phase is called depolarization).

Then, these sodium channels close, and potassium channels open, allowing K$^+$ ions to rush out, bringing the inside back to its negative resting state (this is repolarization). Sometimes, it even goes a little more negative than the resting potential for a brief moment (this is hyperpolarization) before settling back.

This entire process is incredibly fast, lasting only a few milliseconds, and it travels down the length of the neuron like a wave, transmitting information throughout our nervous system. It's an 'all-or-none' event, meaning once the stimulus reaches a certain 'threshold,' an action potential will fire with its full strength, or it won't fire at all.