Explain the concept of 'threshold potential' in the initiation of an action potential

Action potentials are only initiated when depolarization above a certain level occurs, this level is known as the 'threshold potential'. 'Vm' is the resting membrane potential (voltage) and the depolarizing stimulus needs to be of great enough magnitude to cause Vm to become more positive than the threshold value. This occurs when enough Na+ channels open in response to the stimulus, to allow an influx of Na+ into the cell. At a certain point, voltage-gated sodium channels will then also open and elicit the 'all-or-nothing' action potential response.