Final answer:
The Na+ influx through one channel that results from a depolarizing stimulus current opens the voltage-gated Na+ channels, leading to an action potential if the membrane potential reaches the threshold of -55 mV.
Step-by-step explanation:
The voltage change caused by the Na+ influx through one channel provides the stimulus to open the voltage-gated Na+ channels. When a depolarizing stimulus current, such as a nerve impulse, causes some Na+ channels to open, Na+ ions enter the cell, which depolarizes the membrane. If the membrane potential reaches the threshold of -55 mV, it results in the opening of a large number of voltage-gated Na+ channels. This initiates an action potential, which is a rapid and temporary change in the electrical charge across the cell membrane, allowing the nerve impulse to propagate along a neuron.