How is an action potential generated?

A cell's membrane has a potential energy, which simply means that there is a charge difference across the membrane, known as the resting membrane potential. This is because of the difference in concentration of ions diffused within the cell (in the cytoplasm), and outside the cell (in the extracellular fluid), which is kept at different levels via the action of ion pumps, creating a chemical and electrical gradient, creating a potential difference of eg. around -70 mV. The cell membrane does not allow (most of) these ions to simply move across the membrane, but requires channels for ions to properly do this. Most of the channels are specific to some particular ion, and are usually closed. The channels that are relevant for the action potential can be opened by a change in the charge (or 'voltage') across the membrane.Thus, if a sudden increase in voltage causes a large number of voltage-gated sodium channels to open (which are present in nerve cell axons), sodium ions move down their electrical and chemical gradient (in this case, into the cell). As the distribution of the charges across the membrane is changed, the membrane potential changes. In this case, positively charged sodium ions make the membrane potential more positive (for example, from -70 mV to -65 mV). This depolarisation of the cell membrane, can cause other nearby voltage-gated sodium channels to open. This allows a positive feedback mechanism to begin, that is, the change in the membrane potential causes more voltage-gated channels to open, further changing the membrane potential. If sufficient receptors open, and a sufficiently high change in the potential is achieved, known as the threshold potential (often around -55 mV), an action potential is generated, which means, that this process is propagated along a nerve axon.