Final answer:
An early influx of Na+ into a neuron produces a transient inward current, which is part of the depolarization phase of an action potential and constitutes a positive feedback loop.
Step-by-step explanation:
An early influx of Na+ into the neuron produces a transient inward current, leading to a process called depolarization. This occurs when a nerve impulse opens voltage-gated sodium channels, and as a result, Na+ enters the cell, which changes the membrane's potential. Initially, this is a positive feedback loop as more sodium influx causes further opening of sodium channels. Eventually, the influx of Na+ ceases as channels close and repolarization begins, often involving K+ ions leaving the cell to restore the negative membrane potential.
Answering the multiple choice question presented, option A (early; transient) is correct because the influx of sodium occurs shortly after the nerve impulse and is not sustained. The depolarization phase is quick and leads to a sharp rise in the membrane potential, but it is then followed by repolarization or the recovery of the resting membrane potential.