Neurons are the fundamental communicators within the nervous system, forming an intricate network that orchestrates all bodily functions. These specialized cells generate and transmit information through rapid electrical signals, which are the basis for thoughts, sensations, and movements. Understanding how these electrical signals are generated and regulated is key to understanding nervous system activity. This intricate electrical communication allows for coordinated responses throughout the body, from the simplest reflex to complex cognitive processes.
The Neuron’s Electrical Baseline
A neuron maintains an electrical difference across its membrane when not actively sending a signal, a state known as the resting membrane potential. In this resting state, the inside of the neuron holds a negative charge, typically around -70 millivolts, relative to its outside. This charge difference is established by an uneven distribution of ions across the cell membrane: a higher concentration of positively charged sodium ions (Na+) outside and potassium ions (K+) inside, along with negatively charged proteins and other organic molecules trapped inside.
The cell membrane contains specialized protein structures, including ion channels and the sodium-potassium pump, which help maintain this balance. Leak channels allow some ions to slowly diffuse across the membrane, with more potassium leak channels open than sodium channels, making the membrane more permeable to potassium at rest. The sodium-potassium pump actively counteracts this leakage, pumping three sodium ions out of the cell for every two potassium ions it brings in, using ATP. This maintains the concentration gradients of these ions and the negative resting membrane potential.
Understanding Depolarization
Depolarization is the process where a neuron’s internal electrical charge becomes less negative, or more positive, moving away from its resting state. This shift in voltage is a preliminary step in creating a nerve impulse, moving the neuron closer to a state where it can transmit information.
This change in membrane potential is distinct from other electrical events in a neuron. Repolarization involves the membrane returning to its negative resting charge after a period of depolarization. Hyperpolarization describes a state where the membrane becomes even more negative than its resting potential. Depolarization is the initial positive shift that primes the neuron for action.
The Mechanics of Depolarization
Depolarization typically begins when a neuron receives a stimulus, such as neurotransmitter binding from an adjacent neuron. These stimuli cause voltage-gated ion channels, particularly sodium channels, to open rapidly. The high concentration of positively charged sodium ions outside the cell then rushes inward, following both their concentration gradient and the electrical attraction to the negatively charged interior.
This rapid influx of positive sodium ions causes the inside of the neuron to become more positive. As the membrane potential becomes less negative, it reaches the threshold potential, typically around -55 millivolts. Reaching this threshold triggers a positive feedback loop where more voltage-gated sodium channels open, leading to a rapid increase in the internal positive charge. This rapid and significant change in membrane potential is the core event of depolarization.
Depolarization and Nerve Signal Transmission
The rapid depolarization of a neuron initiates an “action potential,” the all-or-nothing electrical signal that travels along the neuron’s axon. Once the membrane potential reaches the threshold, the influx of sodium ions causes the internal charge to briefly reverse, becoming positive, often reaching about +30 millivolts. This sudden change in voltage constitutes the rising phase of the action potential.
As depolarization occurs in one segment of the axon, the influx of positive sodium ions creates a local electrical current that spreads to the adjacent segment of the membrane. This spread of positive charge depolarizes the neighboring region, quickly bringing it to its threshold potential and triggering the opening of its own voltage-gated sodium channels. This sequential depolarization along the axon allows the nerve signal to propagate, typically moving in one direction, away from the cell body and towards the axon terminals. This mechanism allows neurons to transmit information across distances throughout the body.
Resetting the Neuron
Immediately following the peak of depolarization, the neuron begins “repolarization,” where its membrane potential returns to a negative state. This occurs as the voltage-gated sodium channels, which caused the depolarization, quickly inactivate and close. Simultaneously, voltage-gated potassium channels, which open more slowly in response to depolarization, become active.
With sodium entry halted and potassium channels open, positively charged potassium ions flow out of the cell, driven by their concentration gradient and the now positive internal charge. This outward movement of potassium ions causes the inside of the cell to become more negative. The membrane potential often briefly overshoots the resting potential, becoming slightly more negative, a phase called “hyperpolarization,” before the potassium channels close and the neuron returns to its resting state. The sodium-potassium pump then restores the original ion concentrations across the membrane, preparing the neuron to respond to a new stimulus.