Which ions moving in what directions cause the depolarization and repolarization phases of a neuronal action potential?
The correct answer and explanation is:
Correct Answer:
- Depolarization is caused by sodium ions (Na⁺) moving into the neuron.
- Repolarization is caused by potassium ions (K⁺) moving out of the neuron.
300-Word Explanation:
An action potential is the rapid electrical signal that travels along the membrane of a neuron. It is essential for nerve communication and occurs in a sequence of well-coordinated steps involving specific ions—primarily sodium (Na⁺) and potassium (K⁺).
1. Resting State:
Before an action potential begins, a neuron is in a resting state with a membrane potential of approximately –70 mV. This is maintained by the sodium-potassium pump, which actively transports 3 Na⁺ out and 2 K⁺ in, keeping the inside of the cell negatively charged.
2. Depolarization (Na⁺ Influx):
When a stimulus reaches a neuron and the membrane potential reaches a threshold (about –55 mV), voltage-gated sodium channels open. This causes Na⁺ ions to rush into the neuron due to both the concentration gradient and electrical gradient.
- As Na⁺ floods in, the inside of the neuron becomes less negative and eventually positive, peaking around +30 to +40 mV. This phase is called depolarization.
3. Repolarization (K⁺ Efflux):
Shortly after the peak of the action potential, voltage-gated potassium channels open and Na⁺ channels close. Now, K⁺ ions move out of the neuron, down their concentration gradient.
- This loss of positive charge from the inside of the cell causes the membrane potential to return toward the negative resting state. This is called repolarization.
4. After-Hyperpolarization and Reset:
Sometimes, more K⁺ leaves than necessary, causing a brief hyperpolarization. Eventually, the sodium-potassium pump and closing of K⁺ channels restore the original resting potential.
In summary, Na⁺ in = depolarization, K⁺ out = repolarization. This precise ion movement is crucial for fast and accurate nerve signaling.