How is an action potential generated?

A nerve cell when at rest is negatively charged, this means that on the inside it is more negative than on the outside. The potential difference is around -70mV. The mechanism that helps to achieve that polarisation are sodium potassium pumps found in the cell's membrane. These use energy to actively pump 3 Sodium ions out of the cell for every 2 Potassium ions inside the cell. This creates some polarisation as there end up being more positively charged ions on the outside of the cell than on the inside. Furthermore the cell membrane has some Potassium ion channels through which some Potassium ions escape down their concentration gradient out of the cell, further polarising the cell. Nerve cells have negative molecules such as DNA and proteins, therefore these factors together make the cell negatively charged. When a signal is received some more Sodium ion channels open, for example in a mechanoreceptor - Pascinian corpuscle, Sodium ion channels open in response to pressure when the receptor's lamellae are deformed. Sodium ions rush into the cell down their concentration gradient and down the electrochemical gradient since they are positively charged and the inside of the cell is negatively charged. This is the generator potential - the cell starts depolarising. An action potential is an all or nothing response - it either happens or it does not, its strength is always the same. The cell has to depolarise to about -55mV for an action potential to occur, this is called the threshold value. At -55mV voltage gated Sodium ion channels open and even more Sodium ions flood in to depolarise the cell up to +40mV, which is called the action potential.