How are action potentials generated in a neuron?

A stimulus at the receptor end of the neuron causes the membrane to become more permeable to Na+ ions. Because there is a higher concentration of Na+ on the outside of the cell the flood into the cell down the concentraion gradient. The resting potential (voltage across the membrane at rest) is usually -70mV meaning it is 70mV more negatively charged on the inside but this influx of Na+ ions causes it to become more positive. If the stimulus is great enough it reaches a level called the threshold potential which is around 45mV. This causes an action potential to be triggered and is also why it is called an "all-or-nothing" response. Voltage gated Na+ channels open at this point (Na+ channels sensitive to voltage across the membrane) and a massive sudden influx of Na+ into the cells occur. This causes the voltage to reach a massive +40mV. This is the peak of the action potential.

At the peak the Na+ channels close and voltage gated K+ channels open and because there is a higher concentration of K+ inside the cell than ouside it rushes out of the cell. This causes the voltage to become more negative again (as positive charge is leaving the cell) and it "repolarises" (returns towards resting potential). Hyperpolarisation actually occurs at this point as the K+ channels are slow to close and the volatge overshoots and goes to around -75/80mV. A protein pump in the membrane called a sodium-potassium-ATPase-pump moves 3Na+ out and 2K+ into the cell simultaneously to return it to normal resting potential (-70mV).