Resting and Action Potential — Core Principles

Core Principles

The electrical activity of neurons is governed by the membrane potential, which is the voltage difference across the cell membrane. The resting potential is the stable, negative charge (around $-70,\text{mV}$) maintained by a neuron when inactive.

This is established by the differential permeability of the membrane to ions, primarily potassium (K$^+$) through leak channels, and the active transport of ions by the sodium-potassium pump (Na$^+$/K$^+$ ATPase), which pumps 3 Na$^+$ out and 2 K$^+$ in, maintaining concentration gradients.

When a neuron receives a sufficient stimulus, it reaches a threshold potential (around $-55,\text{mV}$), triggering an action potential. This involves a rapid sequence of events: depolarization (inside becomes positive, due to rapid influx of Na$^+$ through voltage-gated Na$^+$ channels), followed by repolarization (inside returns to negative, due to efflux of K$^+$ through voltage-gated K$^+$ channels), and sometimes a brief hyperpolarization (undershoot).

After an action potential, the neuron enters a refractory period (absolute and relative), preventing immediate re-firing and ensuring unidirectional signal propagation. This 'all-or-none' electrical signal is the basis of nerve impulse transmission.

Important Differences

vs Action Potential

Aspect	This Topic	Action Potential
Definition	Resting Potential: The stable, negative electrical potential across the membrane of an excitable cell when it is not actively transmitting a signal.	Action Potential: A rapid, transient, and self-propagating reversal of membrane potential that serves as the nerve impulse.
Membrane Potential Value	Resting Potential: Typically around $-70, ext{mV}$ (negative inside).	Action Potential: Rapidly changes from negative to positive (e.g., $+30, ext{mV}$ to $+50, ext{mV}$) and then back to negative.
Ion Channels Involved	Resting Potential: Primarily K$^+$ leak channels, some Na$^+$ leak channels, and the Na$^+$/K$^+$ pump.	Action Potential: Voltage-gated Na$^+$ channels (depolarization) and voltage-gated K$^+$ channels (repolarization).
Ion Movement	Resting Potential: Net outward diffusion of K$^+$, inward diffusion of Na$^+$, balanced by Na$^+$/K$^+$ pump.	Action Potential: Rapid Na$^+$ influx (depolarization), followed by rapid K$^+$ efflux (repolarization).
Energy Requirement	Resting Potential: Requires ATP for the Na$^+$/K$^+$ pump to maintain gradients.	Action Potential: Primarily passive ion movement down electrochemical gradients; no direct ATP consumption for the rapid phase.
Nature of Signal	Resting Potential: A stable, baseline electrical state.	Action Potential: A dynamic, 'all-or-none' electrical signal that propagates without decrement.
Refractory Period	Resting Potential: Not applicable.	Action Potential: Followed by absolute and relative refractory periods, preventing immediate re-firing.

Resting potential represents the neuron's stable, inactive electrical state, maintained by ion gradients and leak channels, primarily involving K$^+$ efflux and the Na$^+$/K$^+$ pump. It's a baseline voltage. In contrast, an action potential is a dynamic, transient, and 'all-or-none' electrical impulse, triggered by reaching a threshold. It involves rapid, sequential opening and closing of voltage-gated Na$^+$ and K$^+$ channels, leading to depolarization (Na$^+$ influx) and repolarization (K$^+$ efflux). While the resting potential is energy-dependent due to the pump, the action potential itself is a passive flow of ions down gradients.