To control the response of our nervous and muscular systems we use summation. It may require more than one impulse to reach a synapse before enough neurotransmitter is released to open enough ion channels in the post-synaptic membrane to generate a big enough action potential to start the self propagating process needed for a nerve impulse to be generated. Sometimes a certain number of impulses must reach the presynaptic membrane within a limited time to release the required volume of neurotransmitter... this is temporal summation.

Sometimes there are a number of nerve cells all converging onto the same post-synaptic membrane and here it may require a certain number of simultaneous impulses to release the required amount of neurotransmitter....this is spatial summation.

Both of these forms of summation allow our nervous circuits to limit the passage of impulses along the circuit, allowing responses to occur only if the level of stimulation is great enough. For example I pick up a warm cup, it would be nonsensical if my hand released the cup and was rapidly withdrawn in a heat defence response, I only want that to happen when there are so many nerve impulses shooting up my arm that I know it’s sensible to protect my hand from a red-hot object!