We understand enough about how a biological neuron works to know that single signals do not provoke a diminished response after the hysterisis period, which is

We understand enough about how a biological neuron works to know that single signals do not provoke a diminished response after the hysterisis period, which is not modeled by either a memristor or a sigmoid. Sigmoids don't have hysterisis, and you have to backflow a memristor for it to recover which is not true of biological nuerons. (The activation function is the neuron('s mathematical model) - we elide everything that happens internal to the neuron and model it as only its inputs and outputs.)