Each action potential triggers the release of the same amount of neurotransmitter into the synaptic cleft.Only if the amount of neurotransmitter is sufficient will an action potential be triggered in the post-synaptic neurone so there is a threshold.If a number of presynaptic neurones release enough neurotransmitter so that collectively there is enough to trigger an action potential, this is called spatial summation. Temporal summation occurs when a single presynaptic neurone releases neurotransmitter in response to a number of action potentials and this builds up to a sufficient amount to trigger an action potential in the post synaptic neurone.