The neuron is the basic operational unit of the nervous system, communicating through rapid electrical impulses known as action potentials. An action potential is a swift, temporary reversal of the electrical charge across the neuron’s membrane, which allows a signal to travel long distances along the cell. Understanding the sequence of events that triggers this electrical spike is essential for grasping how the nervous system processes and transmits information.
Maintaining the Neuron’s Resting State
Before a neuron can fire an action potential, it must establish a baseline electrical condition called the resting membrane potential (RMP). This resting state is characterized by a negative voltage inside the cell compared to the outside, typically around -70 millivolts (mV). The negative charge, or polarization, is maintained by the uneven distribution of positively charged ions, specifically sodium (\(\text{Na}^{+}\)) and potassium (\(\text{K}^{+}\)).
The sodium-potassium pump, a protein embedded in the cell membrane, works to keep this ion imbalance in place. For every three sodium ions it pumps out of the cell, it brings two potassium ions in, using energy in the process. This exchange helps maintain a high concentration of sodium outside the cell and a high concentration of potassium inside the cell. The membrane also contains potassium “leak” channels that allow positive potassium ions to passively exit the cell, contributing to the negative internal charge at rest.
Receiving Input Graded Potentials
A neuron begins to receive input from other cells, usually in the form of chemical neurotransmitters released across a synapse. These chemical signals bind to receptors on the receiving neuron’s dendrites or cell body, opening specialized ion channels. The resulting local changes in the membrane voltage are known as graded potentials because their magnitude varies directly with the strength of the incoming stimulus.
These potentials can be either excitatory (Excitatory Postsynaptic Potentials or EPSPs) or inhibitory (Inhibitory Postsynaptic Potentials or IPSPs). EPSPs occur when positive ions, such as sodium, enter the neuron, making the cell interior slightly less negative, or depolarized. Conversely, IPSPs usually involve the entry of negative ions like chloride or the exit of positive ions like potassium, making the cell more negative. This process is called hyperpolarization.
Graded potentials are localized and quickly fade as they travel away from the point of stimulation. For a signal to progress, these small electrical events must be combined at the axon hillock, often called the neuron’s trigger zone. This process of combining is called summation, occurring in two main ways: spatial and temporal. Spatial summation involves the simultaneous combination of graded potentials arriving from different locations on the cell. Temporal summation involves rapid, successive inputs from a single source that add up before the previous input has faded completely.
Reaching the Activation Threshold
The event that activates a neuron and begins an action potential is the moment the accumulated graded potentials, summed at the axon hillock, reach the threshold potential. This threshold is a specific voltage level, typically ranging between -55 mV and -50 mV, which represents the “point of no return” for the neuron. It is the exact voltage necessary to trigger the massive, coordinated opening of voltage-gated sodium channels concentrated in this trigger zone.
Before this moment, all the electrical changes were mediated by ligand-gated channels or leak channels. Reaching the threshold, however, induces a critical conformational change in the structure of the voltage-gated sodium channels. These channels are highly sensitive to the membrane voltage, and once the threshold is crossed, their activation gates snap open almost instantaneously.
This sudden opening commits the neuron to firing a signal. The threshold potential acts as an electrical barrier; any stimulus that falls short results only in a decaying graded potential that returns the cell to rest. Only by overcoming this barrier is the positive feedback loop initiated, where the opening of a few sodium channels causes enough local depolarization to open many more.
The Rapid Depolarization Phase
The immediate consequence of reaching the threshold is the rapid and massive influx of positively charged sodium ions into the neuron’s interior. This movement is driven by both the electrical attraction to the negative interior and the high concentration gradient of sodium outside the cell. The flood of positive charge causes the membrane potential to swing dramatically from its negative resting state to a positive value, often reaching a peak of about +40 mV.
This swift upward spike in voltage is known as depolarization, constituting the rising phase of the action potential. This powerful electrical event is governed by the “all-or-none” principle. Once the threshold is met, the action potential fires completely and regenerates itself fully, or it does not fire at all. The intensity of the original stimulus does not change the amplitude of the resulting action potential, ensuring reliable transmission down the length of the axon.