As a neuroscientist with a focus on cellular and molecular neuroscience, I can tell you that the action potential is a critical mechanism for the propagation of signals in neurons. It's initiated when a stimulus alters the membrane potential of a cell to a level that triggers a rapid, self-propagating depolarization. The threshold for an action potential is typically reached when the membrane potential of a neuron reaches a level slightly more negative than the resting potential, which is around -55 mV. This value can vary slightly depending on the neuron type and the species. Once this threshold is crossed, voltage-gated sodium channels open, leading to a rapid influx of sodium ions, which causes the membrane potential to become positive and initiates the action potential. read more >>

The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold.read more >>