Hey Turtle!
This is a good idea. Are you thinking of doing past papers in preparation too, there are some tricky questions on that we could look at too.
One of the textbooks (sherwoood or silverthorn) goes through it pretty well. I think I used silverthorn. Presynaptic inhibition is when an inhibitory neuron directly synapses on say, one axon termial of a presynaptic neuron. So say there are 3 axon terminals, one is modulated such that it will not release a neurotransmitter thus there will be no response in the target cell and the other 2 since they have not been directly affected can still release neurotransmitter and there is a response for those target cells. That's the way I understand it but i'm not sure if it's entirely correct. Postsynaptic inhibition is to do with summation of the signals generated by inhibitory and excitatory neurons. So here there is a postsynaptic cell which an inhibitory presynaptic neuron and an excitatory neuron both synapse to. The IPSP produced by the inhibitory neuron exerts the dominant effect such that the EPSP produced by the excitatory neuron (remembering it's a gp) is not of high enough amplitude by the time it reaches the axon hillock to fire an action potential. Thus there is no action potential reaching the axon terminals which would cause it to release neurotransmitter. The target cells in this case for each axon terminal are all inhibited equally, and we would see no response in the target cell.
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