Final answer:
The crossing of the -55mV threshold by the cell's membrane potential initiates an action potential, characterized by the depolarization and repolarization of the neuron through the opening and closing of Na+ and K+ voltage-gated ion channels.
Step-by-step explanation:
When the cell's membrane potential crosses -55mV, an action potential is triggered. This potential fluctuates and leads to the rapid opening and closing of voltage-gated ion channels. Specifically, this involves the opening of Na+ channels allowing sodium ions to rush into the neuron causing depolarization, which changes the membrane potential from -70mV to about +30mV. Following depolarization, to repolarize the neuron K+ channels open allowing potassium to exit the cell. Consequently, Na+ channels inactivate preventing further entry of sodium. As the K+ exits, the neuron's membrane potential is restored towards the resting potential. The action potential is an all-or-nothing event, so the neuron does not fire continuously without any fluctuations nor does it become inactive after firing.