Explain what is meant by temporal and spatial summation in the context of synapses.

Context: We often think of a linear model of one pre-synaptic neurone linking up with one post-synaptic neurone. In reality, the nervous system is a large network, many pre-synaptic neurones may terminate at the same synapse with one post-synaptic neurone. This allows for more complex and controlled signalling. A pre-synaptic neurone's action potential, allows for the release of a set amount neurotransmitter into the synapse, which may not be enough to trigger an action potential in the post-synaptic neurone; which is where spatial and temporal summation become essential. Spatial summation is where multiple pre-synaptic neurones converge on one post-synaptic neurone. The many pre-synaptic neurones releasing neurotransmitter into the synapse has an additive effect and the increased concentration in the synapse is now high enough to trigger a post-synaptic action potential. Temporal summation occurs when a pre-synaptic neurone has several action potentials in a short time period, leading to a building concentration of neurotransmitter substance in the synapse, eventually enough to trigger an action potential in the post-synaptic neurone.