Final answer:
In a neuron, the influx of Na+ ions during the first phase of an action potential triggers a positive feedback loop, which is further enhanced by the opening of additional voltage-gated sodium channels, leading to an increase in membrane depolarization.
Step-by-step explanation:
In the first phase of triggering an action potential in a neuron, Na+ ions flow into the neuron and trigger a positive feedback loop. This occurs because as the sodium ions enter the neuron, it causes the voltage across the neuron's membrane to become more positive. This depolarization then leads to the opening of additional voltage-gated sodium channels, allowing more Na+ ions to flow into the neuron, which further depolarizes the membrane. This is known as a positive feedback loop because the initial change (the influx of Na+ ions) leads to further changes in the same direction (more Na+ ions entering the cell).
The correct answer to the question is B. 'only trigger a positive feedback loop'. This continues until enough sodium has entered the cell and the voltage-gated sodium channels inactivate. Following this, potassium channels open, allowing K+ ions to flow out and repolarizing the neuron to its resting state. The sodium-potassium pump then works to restore the original ionic concentration gradients, but it is not directly activated upon the initial influx of sodium during the action potential. The sodium-potassium pump requires energy in the form of ATP to move ions against their concentration gradients to maintain the neuron's resting potential.