Postsynaptic Potentials and Summation

by Jason Amores Sumpter
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the binding of neurotransmitters will either increase the likelihood of an action potential or decrease the likelihood of an action potential occurring in the post synaptic cell. Now this is as a result of changes in the membrane potential of the post synaptic cell, and the's are going to be due to what are called excited Torrey Post synaptic potentials and inhibitory post synaptic potentials. So if there is a deep polarization of the post synaptic membrane, what we have is an excited Torrey Post synaptic potential or an E p S P. Because, boy, is that a mouthful. And this is a deep polarization of the membrane, which increases the chance of an action potential occurring now inhibitory post synaptic potentials or i ps ps will hyper polarized the membrane and decrease the chance of an action potential. Now remember, this could be happening through I on a Tropic or Motaba tropic receptors, but because it's probably a little easier to think about this in terms of I on a tropic receptors. If a nyongo tropic receptor allows positively charged ions in, we're going to have deep polarization, so it's going to be causing E P s ps if a ni on a tropic receptor allows negatively charged giants are an ions in It's going to cause a hyper polarization or a night PSP. Now e P s ps can actually be summed together. They can add to each other and d polarize the membrane potential up to the threshold of an action potential. So you can add E P s PS together to cross the threshold and cause an action potential. Right again, it's the E P s PS. Aren't going thio generate the action potential, They're going to get the celled across the threshold and then it will generate the action potential on its own. Now the sodium channels that will actually trigger the official action potential are located in the acts on hillock. So these excited Torrey post synaptic potentials have to carry their deep polarization from the Dent, writes to the axon hillock in orderto actually cause the action potential. Now there are two ways that you can combine thes post synaptic potentials. We think of them as temporal summations or spatial summations. Now, temporal summations are essentially when a bunch of these post synaptic potentials occur in quick succession and add together to become a larger deep polarization. So here we have. This is our pre synaptic cell, right? Here's are synapse right here. That's the synapse. And here is our post synaptic cell. So what's happening? Basically, if a high frequency of action potentials come in so that's like one. And then right after that, another and then right after that, another and so on and so forth that's going to lead to a lot of neurotransmitter release. And that's going to cause, ah, bunch of little deep polarization zones. Right? So let's say that's action potential number one And then this one was a result of action potential number two, and so on and so forth. Eventually these action potentials will some together and cross the threshold potential. And when they cross the threshold potential, it's action potential time, right? Boom were happening. Go through the whole process. So essentially with temporal summation, you have a quick succession of action potentials that lead to a quick succession of deep polarization that add together to cross the threshold. Now, spatial summation is basically when things happen at the same time, but in close proximity. So we have simultaneous action potentials, right, coming in together to the same post synaptic sell. They're gonna release their neurons. We're gonna get an E p s p over here and an E p s p over here too excited, Torrey Post synaptic potentials. And those will some together, as you can see here, here's number one Here's number two, and that will get them to cross the threshold or get the membrane potential to cross the threshold leading to an action potential. So these air two ways that post synaptic deep polarization can add together to actually lead the post synaptic sell to generate an action potential of its own. But remember that post synaptic potentials can also be inhibitory and can decrease the likelihood of an action potential. With that, let's go ahead and flip the page.