Hi everyone! Let's go over temporal and spatial summations. Excitatory post-synaptic potentials, or EPSPs, are voltage fluctuations in the post-synaptic neuron that promote membrane depolarization. In other words, EPSPs gradually increase the membrane potential so that threshold can be more easily met.
Inhibitory postsynaptic potentials, or IPSPs on the other hand, are voltage fluctuations that prevent depolarization. They make the membrane potential more negative, so that it's more difficult to reach threshold potential. Both EPSPs and IPSPs are caused by neurotransmitters that are released from the presynaptic neuron.
The most common excitatory neurotransmitter is glutamate. Glutamate increases post-synaptic membrane permeability to both sodium and potassium ions, which makes membrane potential more positive. The most common inhibitory neurotransmitter is GABA, which increases membrane permeability to negative chloride ions. This influx of chloride ions makes membrane potential more negative. causing threshold potential to be harder to achieve.
Usually a single APSP is not enough to achieve threshold potential and stimulate an action potential. This is where summation comes into play. Summation is the total effect that various potentials have all together.
It takes the voltage changes caused by two or more potentials and adds them. It occurs at the axon hillock, which is located here. indicated by this dotted line.
Unlike action potentials, EPSPs and IPSPs decrease in voltage as it travels from the synapse to the axon hillock. Therefore, an EPSP that might result in this amount of voltage change at the synapse might only account for this amount of voltage change once it arrives at the axon hillock. Now let's look at the two types of summations with EPSPs. Temporal summation occurs when multiple action potentials travel down a presynaptic neuron one after another within a short time frame, causing them to all summate at the axon hillock of the postsynaptic neuron.
Here there are three action potentials traveling down this presynaptic neuron in the middle. Because they were right after one another, the EPSPs add up. The compound EPSP reaches threshold potential at the axon hillock, which successfully initiates an action potential. You can see how the amplitude on this axon hillock graph is smaller than that on the synapse graph. This is because of the loss of voltage as the EPSPs travel across the cell body.
Temporal summation is time dependent. The action potentials have to be in succession within a short time frame in order for them to be summated. For example, two action potentials occurring in succession but not within a short time frame will not summate because the first EPSP would have already faded before the second EPSP would have the chance to add on to it. In this case, the threshold potential will not be reached. Spatial summation on the other hand occurs when action potentials on several presynaptic neurons arrive at the postsynaptic neuron simultaneously to result in EPSPs.
So here these three presynaptic neurons may each have an action potential traveling down their axon that synapse simultaneously at the postsynaptic neuron. The resulting EPSPs will summate into a large compound EPSP at the axon hillock. In this case, The resulting voltage is enough to reach threshold potential and trigger an action potential.
So here is a comparison between these two types of summation. You can see how temporal summation involves several successive action potentials in a single presynaptic neuron. If they occur within a short time frame, they can add on top of each other and summate at the axon hillock to create a large compound EPSP.
Spatial summation involves several presynaptic neurons synapsing on a single postsynaptic neuron. Action potentials on each presynaptic neuron arrive simultaneously at the postsynaptic neuron, resulting in a large compound EPSP at the axon helix.