The nervous system relies on specialized cells called neurons to transmit information throughout the body. Neurons communicate by generating rapid, controlled electrical signals that travel along their length. Depolarization is the foundational electrical event that transforms a resting neuron into an active signaling unit. This temporary shift in electrical charge across the neuron’s membrane allows a nerve impulse, or action potential, to be created and propagated.
The Neuron’s Electrical Baseline
A neuron that is not actively transmitting a signal maintains a stable electrical imbalance known as the resting membrane potential. This baseline state typically registers around \(-70\) millivolts (\(\text{mV}\)), meaning the inside of the cell is negatively charged relative to the outside environment. This negative charge is maintained by an unequal distribution of ions across the cell membrane. Positively charged potassium ions (\(\text{K}^+\)) are highly concentrated inside the neuron, while sodium ions (\(\text{Na}^+\)) are heavily concentrated outside.
The sodium-potassium pump, a protein embedded in the cell membrane, works constantly to preserve these concentration gradients. This pump expends energy to move three \(\text{Na}^+\) ions out of the cell for every two \(\text{K}^+\) ions it brings in. The membrane is also more permeable to \(\text{K}^+\) ions than \(\text{Na}^+\) ions in this resting state, allowing \(\text{K}^+\) to leak out and further establish the negative charge inside the cell.
Initiating the Signal: The Depolarization Event
Depolarization is the rapid electrical shift where the neuron’s interior becomes less negative, moving its membrane potential toward a positive value. This event is triggered when the neuron receives a stimulus strong enough to change its membrane voltage, causing specialized voltage-gated sodium (\(\text{Na}^+\)) channels to open.
The opening of these channels creates a rush of positively charged \(\text{Na}^+\) ions into the cell, driven by both electrical attraction and the concentration gradient. This sudden influx of positive charge causes the membrane potential to swing sharply upward, a process known as the rising phase of the action potential. The change in charge distribution is so significant that the membrane potential briefly reverses polarity, peaking at a positive value, often around \(+30\) to \(+40\) \(\text{mV}\).
For depolarization to occur, the initial stimulus must push the membrane potential past a specific point called the threshold potential, typically around \(-55\) \(\text{mV}\). If the voltage fails to reach this level, the \(\text{Na}^+\) channels close, and the event fades away. If the threshold is met or exceeded, the full action potential is generated, adhering to the “all-or-nothing” principle regardless of the strength of the stimulus.
Completing the Cycle: Repolarization and Refractory Period
The spike in voltage during depolarization is immediately followed by repolarization, which restores the neuron’s negative resting charge. This phase begins when the voltage-gated \(\text{Na}^+\) channels quickly inactivate, stopping the flow of positive ions into the cell. Simultaneously, a different set of voltage-gated potassium (\(\text{K}^+\)) channels open.
Since the cell interior is now positive and \(\text{K}^+\) concentration is high inside, these positive \(\text{K}^+\) ions are driven out. This rapid efflux of positive charge causes the membrane potential to fall steeply, bringing the voltage back down toward the negative resting potential. The \(\text{K}^+\) channels are slow to close, causing a brief “undershoot” where the membrane potential becomes slightly more negative than the \(-70\) \(\text{mV}\) resting value, a state called hyperpolarization.
The period during and immediately after the action potential is known as the refractory period, which prevents the neuron from firing a second signal too quickly. The absolute refractory period occurs when the \(\text{Na}^+\) channels are inactivated and cannot be reopened. Following this is the relative refractory period, where a second action potential can be triggered only by a much stronger-than-normal stimulus.
How the Electrical Signal Travels
The depolarization event must be transmitted along the entire length of the neuron’s axon to communicate with the next cell. The influx of positive \(\text{Na}^+\) ions at the site of depolarization creates a flow of current that spreads a short distance to the adjacent, resting section of the axon. This spreading current quickly raises the voltage in the neighboring area, causing its own voltage-gated \(\text{Na}^+\) channels to open and trigger a new, full action potential.
In unmyelinated axons, this process, known as continuous conduction, happens sequentially, with the action potential being regenerated at every point along the membrane. This method is comparable to a line of dominoes falling. Conversely, in myelinated axons, the signal travels much faster through a process called saltatory conduction.
Myelin, a fatty insulating sheath, covers most of the axon, preventing ion movement across the membrane. The depolarization signal effectively “jumps” from one exposed gap to the next, called the Nodes of Ranvier, where the voltage-gated \(\text{Na}^+\) channels are highly concentrated. This jumping mechanism accelerates the transmission speed, allowing signals to travel up to 100 meters per second in large, myelinated fibers.