What happens during the depolarization and repolarization phases of an action potential?

During an action potential, neurons undergo two critical phases: depolarization and repolarization. These phases are essential for the rapid transmission of electrical signals along the neuron.

Depolarization Phase

1. Initiation of Action Potential:
   • Depolarization begins when a stimulus causes the neuron’s membrane potential to reach a threshold level, typically around -55 mV. This threshold is crucial because it determines whether an action potential will occur.
2. Opening of Sodium Channels:
   • Once the threshold is reached, voltage-gated sodium (Na++) channels open rapidly, allowing Na++ ions to flow into the neuron. This influx of positively charged sodium ions causes the membrane potential to become more positive, leading to further depolarization.
3. Positive Feedback Loop:
   • As more Na++ channels open, more sodium enters the cell, creating a positive feedback loop that rapidly increases the membrane potential, often peaking at around +30 mV. This phase is characterized by a swift rise in voltage and is often described as an “all-or-nothing” event—once initiated, it proceeds fully without diminishing.

Repolarization Phase

1. Inactivation of Sodium Channels:
   • Shortly after reaching the peak of depolarization, the Na++ channels begin to inactivate (close), halting the influx of sodium ions. This inactivation is crucial for stopping further depolarization and initiating repolarization.
2. Opening of Potassium Channels:
   • Concurrently, voltage-gated potassium (K++) channels open, allowing K++ ions to exit the neuron. Since there is a higher concentration of K++ inside the cell compared to outside, this efflux of potassium ions contributes to restoring the negative membrane potential.
3. Return to Resting Potential:
   • The outflow of K++ leads to a decrease in membrane potential back toward resting levels (approximately -70 mV). However, potassium channels may remain open slightly longer than necessary, causing a temporary overshoot known as hyperpolarization, where the membrane potential dips below the resting level (around -80 mV) before stabilizing back at rest