Action potentials are the units of transmission across nerve cells. Action potentials were first observed by Hodgkin & Huxley in a squid’s giant axon. Through experimentation and mathematics, they hypothesized a namesake model that suggested that there were gated channels controlling the rise and fall in membrane potential. As technology developed, this model was vindicated. Through the application of suction to a microscopic region of the axon, ion channels could be isolated and their current measured. When current was run through the axon, it was shown that the channels produced a short-term inward current that stops until the membrane returns to below resting membrane potential. Later studies using tetrodotoxin and other poisons yielded information about the structure of these channels. Today, the canonical information taught about action potentials is that there is a resting membrane potential around -65 mV, an action potential threshold of -40 mV, a rising phase, an overshoot above 0 mV, a falling phase, an undershoot under -65 mV, and a return to resting potential. The action potential is often referred to as an all-or-nothing response, as a neuron will fire once the membrane potential reaches threshold. Whether or not this occurs within a neuron is a complicated process that often involves thousands of computations. However, once the action potential reaches threshold, voltage-gated sodium channels open, sodium floods into the axon due to the concentration gradient and the electrical gradient, causing membrane depolarisation. Then, voltage-gated potassium channels open approximately 1 ms later - a system known as a delayed rectifier - allowing potassium to go down its electrochemical gradient to rush out of the axon, causing hyperpolarisation. The sodium channels lock preventing re-depolarisation, forcing the depolarisation to travel down the axon in a chain reaction of opening and closing voltage-gated channels. Then the sodium/potassium pump returns the membrane to resting potential, allowing for the next action potential to fire.
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