welcome biologists this session where we're going to take a look at the roles of synapses this is part two of two videos so there are different roles of a synapse and this is questioned quite a lot in an exam and students tend to get mixed up here and not quite understand the question that's being asked of them so let's take a look at some of these roles the first role is summation and there are two types of summation you need to know about this temporal summation and spatial summation now temporal summation as you can see here is where i've got one presynaptic membrane feeding into one postsynaptic membrane and this is really good because it allows you to filter out background or low-level stimuli so you can see here in this image here when i've got one stimulus in the presynaptic membrane i haven't released enough neurotransmitter to trigger a postsynaptic depolarization of that membrane um but in this example here because i've got several stimuli in my presynaptic membrane i've got enough neurotransmitter being released into my synaptic cleft for the threshold value to be reached here and an action potential to be established in my postsynaptic membrane so therefore i've only got a low level stimuli temporal summation allows that to be filtered out so for example things like wearing clothes having clothes on your back is background stimuli i don't constantly feel them all the time if it's filtered out eventually the next one is um spatial summation and this is where i have several presynaptic membranes going into one postsynaptic membrane and this allows convergence where i've got impulses from more than one neuron to be passed into a single neurons as you can see here if i have a b and c triggering at different times i'm not going to get enough neurotransmitter to be released to trigger an action potential in my postsynaptic memory but if all are triggered at the same time of all release neurotransmitter at the same time allows convergence of those signals so that my i'm going to get depolarization of my postsynaptic membrane it also again allows you to filter out background or low-level stimuli because if only one of these membranes was to release neurotransmitter i'm not going to get an action potential in my postsynaptic memory you've also got things like convergence and divergence which synapses allow you don't need to know too much about that you just need to know that that is possible we can also have inhibitory and excitatory synapses so for example an excitatory synapse is where the membrane's potential would increase towards a threshold value however here you can see i've got an example of an inhibitory synapse where i would actually get the membrane potential decreasing away from the threshold value and these can cause um an action potential would be triggered or not in the postsynaptic membrane and again that's just one thing that you need to be aware of that can happen now there are several roles uh in the synapse and all of these are taken directly from the mark scheme so self signaling a big thing here and that's linking into our as topic as well lots of that going on insure transmission in one direction because i've only got um for example calcium channels on my presynaptic membrane i've only got vesicles of neurotransmitter in my presynaptic knob i've only got my receptors for my acetylcholine or my neurotransmitter or my postsynaptic membrane so these all these factors allow transmission in one direction only allows convergence which we've talked about on divergence which we've mentioned filtering out a low level or background stimuli prevents fatigue and over stimulation it can it allows many low-level stimuli to be amplified which we talked about there in spatial summation and inhibitory and stimulatory synapses and the last one it permits memory and learning and decision making now this is what i mentioned just then in terms of why synapses only allow transmission in one direction and there are several things here again this is taken directly from the mark scheme so it is in a red box and that is really all we need to know here about the roles of synapses there is quite a lot of information there but bear in mind that those bits in the red boxes are what you need to know for the math scheme and for your exam guys all the best with your exams and please try and include as much of that detail as possible all the best good luck