along with reconstructing the action potential in the 1950s, alan lloyd hodgkin and andrew huxley were also able to experimentally determine the mechanism behind the threshold for excitation. it is known as the hodgkin–huxley model. through use of voltage clamp techniques on a squid giant axon, they discovered that excitable tissues generally exhibit the phenomenon that 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.