Action Potential | How Do Neurons Fire and Communicate?

An action potential is a rapid, temporary change in the electrical potential across a neuron's membrane. It functions as the fundamental unit of communication between neurons. For an action potential to occur, an initial stimulus must be strong enough to reach a specific threshold voltage, typically around -55 millivolts. If this threshold is not reached, no action potential is generated. This is known as the 'all-or-none' principle. The neuron either fires a complete, full-strength action potential or it does not fire at all; there is no such thing as a 'weak' or 'strong' action potential. The intensity of a sensation, such as pain or heat, is not coded by the magnitude of individual action potentials but by their frequency. A more intense stimulus will cause neurons to fire action potentials more frequently, not with greater amplitude.

The entire process unfolds in a sequence of stages. It begins with the resting potential, where the neuron is polarized, meaning the inside is negative relative to the outside. When a stimulus reaches threshold, voltage-gated sodium channels open, allowing an influx of positive sodium ions. This phase, called depolarization, causes a rapid spike in the membrane potential. Following the peak, repolarization begins as sodium channels inactivate and voltage-gated potassium channels open, allowing potassium ions to exit the cell. This efflux of positive ions causes the membrane potential to fall. Often, the potential briefly becomes more negative than the resting potential, a phase known as hyperpolarization, before returning to its resting state.

Ion channels are specialized proteins in the neuron's membrane that act as gates, controlling the flow of specific ions like sodium (Na+) and potassium (K+). During an action potential, voltage-gated ion channels are critical. They open and close in response to changes in the membrane's electrical potential. The depolarization phase is driven by the opening of fast-acting Na+ channels, while the repolarization phase is driven by the opening of slower K+ channels and the closing of Na+ channels. The precise timing and function of these channels are essential for the propagation of the nerve impulse.

The action potential propagates along the axon, a long projection of the neuron, without diminishing in strength. In many neurons, this process is accelerated by the myelin sheath, a fatty insulating layer that covers the axon. This sheath has small gaps called nodes of Ranvier. The electrical signal "jumps" from one node to the next, a process called saltatory conduction. This method is significantly faster and more energy-efficient than continuous conduction along an unmyelinated axon, allowing for rapid communication throughout the nervous system.

The action potential is an electrical signal, but communication between neurons typically occurs via a chemical signal. When an action potential reaches the end of an axon, called the axon terminal, it triggers the release of chemical messengers known as neurotransmitters. These chemicals are released into a tiny gap called the synaptic cleft, located between the first neuron and the next. The neurotransmitters then bind to receptors on the subsequent neuron, causing a new electrical change in that cell. This change can either excite the next neuron, making it more likely to fire its own action potential, or inhibit it, making it less likely to fire.