What happens during an action potential?

Action potentials are how nerve impulses travel along nerve cells/neurones. At rest a neurone has a membrane potential of -70mV which is maintained by the action of sodium potassium pumps in the plasma membrane. These pumps transport 3 Na+ ions out of the neurone for every 2 K+ ions transported into the neurone which leads to a net negative charge inside the neurone. The membrane of the neurone is said to be polarised. When a stimulus is detected, Na+ ion channels open and Na+ ions diffuse into the neurone down their concentration gradient. This is known as depolarisation. The membrane potential becomes less negative, eventually reaching -50mV which is known as the threshold potential. Once the threshold potential has been reached, an action potential of the same magnitude will always occur. At the threshold potential, voltage-gated Na+ channels open and there is a further influx of Na+ ions which increases the membrane potential until it eventually reaches +40mV. This is an example of positive feedback. At +40mV, the Na+ ion channels close and K+ channels open. K+ ions diffuse out of the neurone down their concentration gradient through these channels and this decreases the membrane potential in a process known as repolarisation. The membrane potential becomes negative again but slightly overshoots the resting potential of -70mV. This is known as hyperpolarisation. Following hyperpolarisation, the resting potential is restored via the action of the sodium potassium pumps in the plasma membrane. The period during which this occurs is known as the refractory period and during this time no further action potential can be produced.