The threshold potential.The value can vary bfrom -90mV to -70mV to -50mV based on several factors. It is the level at which a membrane potential must be achieved( depolarization) in order to initiate an action potential. This means that when a signal is transmitted, it is either strong enough to reach and exceed this value in order to generate a response(action potential) or it is too weak to reach this value, thus no response is generated(no action potential).