Under low frequency stimulation, responses of neurons tend to be stable. However under non-stable conditions, the way we respond to stimulation can change. How we respond to a new stimulus different than how we would when it gets customary, or. Nervous system has evolved in a way that allows this adaptation to occur: adapt themselves based on prior inputs, shape responses based on past experience. Change in the behaviour of synapses that occurs in a time frame of milliseconds to tenths of seconds. Presynaptic membrane potential spike often causes a response in the post-synaptic membrane potential: however sometimes the size of the postsynaptic potential can change if the two occur in a very short time frame. Two occur very close together as a way of ensuring that the firing is not random noise, which is more likely to occur with just one firing than it is with two successive firings.