Action Potential Explained: Neuron Firing 101

The resting membrane potential is the foundation of neuronal signaling.Neurons maintain an electrical charge difference across their cell membranes when at rest.This resting membrane potential is typically around negative seventy millivolts, with the inside of the cell more negative than the outside.This electrical gradient is created by an uneven distribution of charged ions across the membrane.This potential is maintained by ion pumps and channels, particularly the sodium-potassium pump, which actively transports sodium ions out of the cell and potassium ions into the cell.The cell membrane is also more permeable to potassium than sodium at rest, allowing potassium ions to leak out more easily, contributing to the negative internal charge.This balance results from multiple forces: chemical gradient, electrical gradient, and the active transport by the sodium-potassium pump.This electrical gradient is the foundation for neuron signaling and must be understood before examining how action potentials occur.Now we'll examine how a neuron transitions from its resting state to firing an action potential through the process of depolarization.The neuron's membrane contains voltage-gated sodium channels that are closed during the resting state.In the extracellular fluid, there's a high concentration of sodium ions, each with a positive charge.Let's track the membrane potential, which starts at around negative 70 millivolts in the resting state.The threshold potential is around negative 55 millivolts. If the membrane potential reaches this threshold, an action potential will be triggered.We can also visualize how the membrane potential changes over time during an action potential.Depolarization begins when the neuron receives stimulation, such as from excitatory neurotransmitters.This causes sodium ions to flow into the cell, making the membrane potential less negative.As the membrane continues to depolarize, it approaches the threshold potential.When the membrane potential reaches the threshold, voltage-gated sodium channels fully activate.This triggers a positive feedback loop: as more sodium channels open, more positive ions enter, causing more depolarization.This massive influx of sodium ions causes the membrane potential to rapidly reverse, reaching a peak of positive 30 millivolts.This rapid change from negative to positive is known as the rising phase of the action potential. Once it reaches its peak, the membrane will begin to repolarize.After the neuron has depolarized, the process of repolarization begins.During repolarization, voltage-gated potassium channels slowly open while sodium channels become inactivated.This causes potassium ions to flow out of the cell while preventing further sodium influx, reversing the membrane potential back toward its negative resting state.The potassium channels remain open briefly, causing more potassium to leave than necessary. This leads to hyperpolarization, where the membrane potential becomes even more negative than the resting state.After an action potential, the neuron enters a refractory period. During the absolute refractory period, the neuron cannot generate another action potential no matter how strong the stimulus.This is followed by the relative refractory period, where another action potential is possible but requires a stronger-than-normal stimulus.This entire process—depolarization, repolarization, and the refractory period—propagates down the axon as an action potential, transmitting the neural signal.The refractory period ensures that action potentials travel in one direction and provides time for the neuron to reset before firing again, which is essential for normal neural function.