Describe the events that occur leading to an action potential, and briefly explain why it is described as "all or nothing"?

Across the membrane of a neuron, the membrane potential at rest is approximately -70mV. When a stimulus of some kind is detected by sensory receptors (for example the nociceptors on the skin surface which detect pain), this causes the membrane to become depolarised, increasing in mV. This increase occurs as sodium ions flow rapidly into the nerve cell. If the depolarisation increases to -40mv, then an action potential is fired, increasing the potential to up to +40mv. If the depolarisation does not increase to -40mV, then the action potential is not fired, hence why it is described as “all or nothing”.  Once depolarisation has occurred, there is then a period of repolarisation, with the membrane potential even decreasing to a point where it is described as ‘hyperpolarised’ at -80mV or even -90mV. This has happened because potassium ions begin to move out of the neuron. The neuron then undergoes a period described as the ‘refractory period’ which is essential for one-way transmission as it allows the membrane to acclimatise back to the resting potential of -70mV from which another potential could be fired if depolarisation where to start again.