Neurons are the fundamental units of the nervous system, specialized cells for communication. These cells transmit information through rapid, temporary shifts in their electrical charge, known as action potentials. This electrical event is a brief reversal of the electrical polarization across a neuron’s membrane, allowing for widespread signal transmission.
The Neuron’s Resting State
A neuron at rest maintains an electrical difference across its membrane, known as the resting membrane potential, around -70 millivolts (mV). This negative charge inside the cell, relative to the outside, is established by varying concentrations of ions across the cell membrane. Specifically, there is a higher concentration of sodium ions (Na+) outside the neuron and a higher concentration of potassium ions (K+) inside.
This potential relies on the sodium-potassium pump, a protein embedded in the cell membrane. This pump actively transports three sodium ions out of the cell for every two potassium ions it brings in, using energy to move these ions against their concentration gradients. This contributes to the net negative charge inside the neuron. Additionally, the membrane has more “leak” channels for potassium ions than for sodium ions, allowing potassium to leak out more readily, further contributing to the negative resting potential.
The Spark: Initiating an Action Potential
A neuron transitions from its resting state when it receives a sufficient stimulus, such as from another neuron or a sensory input. This stimulus causes localized changes in the membrane potential, making the inside of the neuron slightly less negative. For an action potential to be triggered, this change must reach a specific level called the threshold potential, around -55 mV.
Upon reaching this threshold, voltage-gated sodium channels in the neuron’s membrane rapidly open. This opening leads to a swift influx of positively charged sodium ions into the cell. The rapid entry of sodium ions causes the inside of the neuron to become progressively more positive, marking the initial rise in membrane potential known as depolarization. This process illustrates the “all-or-none” principle: if the threshold is reached, an action potential will fire completely; otherwise, it will not fire at all, regardless of the stimulus strength.
The Electrical Wave: Phases of an Action Potential
Once the threshold potential is met, the action potential unfolds through distinct electrical phases. The initial rapid influx of sodium ions continues during the depolarization or rising phase, causing the membrane potential to swiftly become positive, reaching a peak of approximately +30 mV. This surge is due to the continued opening of voltage-gated sodium channels, making the inside of the cell highly positive compared to the outside.
Following depolarization, the repolarization or falling phase begins as voltage-gated sodium channels rapidly inactivate and close. Simultaneously, voltage-gated potassium channels, which open more slowly, become fully active. The outward movement of positively charged potassium ions through these open channels causes the membrane potential to quickly return to a negative state.
A brief period of hyperpolarization, or undershoot, follows repolarization. During this phase, potassium channels remain open slightly longer, causing the membrane potential to temporarily become even more negative than the resting potential, reaching around -80 mV. The membrane then gradually returns to its resting potential as these potassium channels close.
An action potential is followed by a refractory period, which has two subphases: absolute and relative. The absolute refractory period, overlapping depolarization and early repolarization, makes it impossible to trigger another action potential, ensuring unidirectional signal propagation. The relative refractory period follows, during which a stronger-than-usual stimulus can trigger another action potential.
Action Potential Propagation and Significance
Once an action potential is generated, it travels along the neuron’s axon, a process known as propagation. The depolarization at one point on the axon triggers the opening of voltage-gated sodium channels in the adjacent membrane segment, causing the action potential to propagate down the axon. This sequential activation ensures the electrical signal is transmitted efficiently along the neuron.
In many neurons, the axon is covered by a fatty insulating layer called the myelin sheath, which is interrupted at regular intervals by gaps known as Nodes of Ranvier. Myelin prevents ion leakage, allowing the depolarization to spread rapidly and passively underneath the sheath until it reaches a Node of Ranvier. At these nodes, the action potential is regenerated by voltage-gated sodium channels, causing the signal to “jump” from node to node in a process called saltatory conduction. This jumping mechanism increases the speed of nerve impulse transmission, allowing for faster communication within the nervous system, with speeds ranging from 1 to 100 meters per second.
Action potentials are essential for nervous system functions. They transmit sensory information to the brain and control muscle movement, enabling voluntary actions and reflexes. Beyond basic functions, action potentials support complex brain activities like thought, memory, and emotion, by facilitating rapid communication between neurons. Their consistent, all-or-none nature ensures reliable long-distance signal transmission for the coordinated functioning of the entire body.