Discovery
Initial experiments revolved around the concept that any electrical change that is brought about in neurons must occur through the action of ions. Nernst applied this concept to discovering nervous excitability, and concluded that the local excitatory process through a semi-permeable membrane depends upon the ionic concentration. Also, ion concentration was shown to be the limiting factor in excitation. If the proper concentration of ions was attained, excitation would certainly occur. This was the basis off of which the threshold value was discovered.
Along with reconstructing the action potential in the 1950s, Hodgkin and Huxley were also able to experimentally determine the mechanism behind the threshold for excitation. Through use of voltage clamp techniques, they discovered that excitable tissues generally exhibit the phenomenon where a certain membrane potential must be reached in order to fire an action potential. Since the experiment yielded results through the observation of ionic conductance changes, Hodgkin and Huxley used these terms to discuss the threshold potential. They initially suggested that there must be a discontinuity in the conductance of either sodium or potassium, but in reality both conductances tended to vary smoothly along with the membrane potential.
They soon discovered that at threshold potential, the inward and outward currents were exactly equal and opposite. As opposed to the resting membrane potential, the threshold potential's conditions exhibited a balance of currents that were unstable. Instability refers to the fact that any further depolarization activates even more voltage-gated sodium channels, and the incoming sodium depolarizing current will overcome the delayed outward current of potassium. At resting level, on the other hand, the potassium and sodium currents are equal and opposite in a stable manner, where a sudden, continuous flow of ions should not result. The basis is that at a certain level of depolarization, when the currents are equal and opposite in an unstable manner, any further entry of positive charge will generate an action potential.
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