How is an action potential fired?

When a neurone is at rest, the outside of the neurone is more positive than the inside. This is due to the sodium-potassium pump which actively pumps 3 Na+ outside the cell, whilst at the same time pumping 2 K+ inside the cell. This creates a potential difference of -70mV, known as the resting potential. If a receptor is stimulated, there is a change in the permeability of the membrane, as Na+ channels open, allowing many Na+ ions to move into the cell, down a concentration gradient. This is called depolarisation, and causes the inside of the cell to become more positive than the outside. If the potential difference reaches about -50mV, an action potential will be fired. This is known as the threshold value. The potential difference keeps increasing until it reaches about +40mV. At this point, the Na+ channels close, and K+ channels open allowing K+ to leave the cell. The cell becomes repolarised, as the potential difference decreases and approaches the resting potential. However, the potential difference goes slightly below the resting potential, as there is a slight delay before the K+ channels close. This is called hyperpolarisation. The potential difference then returns to the resting potential.