Action Potential
What Is an Action Potential?
An action potential is a short-lasting event where the electrical membrane potential of an excitable cell rapidly rises and falls. These events occur in neurons, muscle cells, and endocrine cells, serving as the primary means of transferring information throughout the nervous system(Herbert Sydney Green et al., 1997). In neurons, they are often called nerve impulses or spikes, while in muscle cells, they act as the initial step leading to contraction.
Core Mechanism and Ion Dynamics
The process begins when a stimulus causes the membrane potential to reach a specific threshold value. This triggers voltage-gated sodium channels to open, resulting in a rapid influx of sodium ions that reverses the cell's polarity, known as depolarization(Laura Freberg et al., 2018). Subsequently, sodium channels inactivate and potassium channels allow an outward current of ions to repolarize the membrane, returning it to its resting potential(Joel Michael et al., 2011).
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Propagation and Signal Transmission
Action potentials undergo propagation, maintaining their strength as they travel along the membrane via local current flow (Bryan H. Derrickson et al., 2019). This regenerative process functions like a "wave," where the signal is constantly renewed at adjacent membrane segments (Bryan H. Derrickson et al., 2019). In myelinated axons, saltatory conduction allows the impulse to move much more rapidly (Laura Freberg et al., 2018). Because of the absolute refractory period, the action potential can only propagate in one direction, away from its point of origin (Bryan H. Derrickson et al., 2019).
Variations and Physiological Significance
While animal action potentials rely on sodium and potassium, plant cells achieve depolarization through the release of negative chloride ions. In the heart, cardiac action potentials travel through specialized structures like the bundle of His and Purkinje fibers; anomalies in this process can lead to arrhythmias. Regardless of the cell type, the energy for each impulse is primarily stored across the membrane as an electrochemical gradient (Joseph D. Bronzino et al., 2014)(George Spilich et al., 2023).
