Graded Potentials - Video Tutorials & Practice Problems
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1
concept
Postsynaptic Potential
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5m
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So in this video, we're going to be talking about graded potentials. So you can see up here in the corner in our neuron, we are working in the SOMA and the dendrites because that's where our graded potentials take place. And quick disclaimer. Before we dive right in, we're going to be talking about graded potentials in the context of chemical synapses, which as you guys may remember are the most common type of synapse in the human body. So this is how most A and P textbooks will be talking about them. But I do want to be clear about that distinction before we, we move forward. So when we have communication via chemical synapses, we end up with post synaptic potentials and post synaptic potentials are just changes in the membrane potential at the post synaptic terminal of a chemical synapse, which is a bit of a mouthful. But you guys know all of those words. So if we look down here and we have our presynaptic neuron, he's the neuron sending us a signal and our post synaptic neuron, that's just the neuron receiving the signal, right? And when this post synaptic potential comes in, it's going to change our membrane potential, change the voltage of the neuron receiving that signal. So that's all that, that means I get that out of our way. Now, post synaptic potentials are all graded potentials. And there are two types of them. The first type is an excitatory postsynaptic potential or an epsp for short, and these make our membrane more positive. So in other words, the are deep polarizing events and they literally excite our membrane, they get our neuron excited and they make it more likely that our neuron will fire an action potential. I always picture this as like the neuron talking to us being like fire, fire, fire, we have to send a message. So he's kind of just like yelling at us and getting us all excited. Now, we can also have inhibitory postsynaptic potentials or IP SPS. And these make our membrane more negative. In other words, these are hyper polarizing. So this is kind of like that neuron adjacent to us being like, don't fire, don't fire, don't send the message, right? And these literally inhibit our neuron, they make it less likely that our neuron will be sending out an action potential. So those are our two types of post synaptic potentials. Now, we're gonna go over what the sequence of a depolarizing graded potential would look like. So in other words, what does it actually look like when our neuron receives an epsp? So just to kind of orient you to our figure here, like I had just showed you this is the axon terminal of a neuron that is talking to us our pre synaptic neuron and this is our neuron over here. He is receiving that signal, the post synaptic neuron, right? And so what's gonna happen here is that this neuron is gonna send out some kind of stimulus. It's probably some neurotransmitters, right? So the first step is that our gated sodium channels are going to open up in response to this stimulus. So some neurotransmitters get tossed into that into that synapse. Bota bing bada boom. Our channel opens up. And when our sodium channel opens up, sodium ions are going to come rushing into our cell following their electrochemical gradient, right. So now positively charged sodium ions are rushing into our cell and all those positive ions are going to depolarize our cell. So the inside of our cell depolarizes OK. So it's getting more and more positive. So if we look at our graph over here, we can see if we start at resting potential at negative 70 as we start depolarizing, that membrane potential will get more and more positive, let's say up to about negative 60. So that would be a change of about 10 millivolts. We'll put that here just as an example. Now that deep polarization is going to be strongest right here at that initial site of stimulation where all that sodium is rushing into our cell. So the biggest change, the biggest depolarization is gonna happen right there. Now, what's gonna happen is that depolarization is going to now spread in a local current. So it's gonna start spreading down the membrane down the dendrite down the SOMA toward that initial segment. However, as it is spreading in this local current, what's happening is that our membrane is covered in leak channels. And so some sodium is getting lost throughout this process. And so as sodium is getting lost, our current gets weaker and weaker. So if we had a change of about 10 millivolts right here at that initial site, by the time it gets out here, we might be down to a change of five millivolts by the time it gets out here, a change of maybe two. And by the time we're halfway down the SOMA that entire signal is lost and the current has dissipated. So on our graph over here would look, it would look like this, we hit negative 60 that was kind of the peak of this depolarization and then it kind of got lost and we ended up back at resting potential. So that is how a depolarizing graded potential would look, we're not going to go over a hyper polarizing one just for the sake of time, but just to give you kind of some kind of context, it would look very similar, very similar steps, but it would involve different types of channels and different types of ions entering or leaving the cell. So that's kind of what that would look like. Now, if you were, were kind of watching this and thinking like, OK, well, if this little epsp just dissipates and can't get all the way down the neuron, how do we ever depolarize the initial segment over here enough to have an action potential? And that is a great train of thought. You are totally on the right track. And in our next video, we'll be talking about how multiple EP SPS can work together to trigger action potentials. So I'll see you there.
2
example
Graded Potentials Example 1
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1m
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All right. So this example asks us when voltage gated sodium channels open in response to a stimulus, what effect does it have on the neuron? So let's go through our options and see what we have. So, A reads that potassium will rush out of the cell and right away, we know that that's not correct, right? Because potassium is not moving through voltage gated sodium channels. So A is out B reads that sodium will enter the cell. And that is correct, right. When voltage gated sodium channels open up sodium ions will follow their electrochemical gradient and they will begin to enter the cell. So B is correct. Let's check the other ones just in case. So C reads the influx of positive ions causes depolarization. And that is also correct, right. So sodium is a positively charged ion and when it starts rushing into our negatively charged cell, it's gonna make the cell become more positive and that is depolarization. So B and C are both correct, which means our answer is D and there you go.
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Problem
Problem
A graded potential is strongest at the __________:
A
Initial zone of the axon.
B
Location on the membrane with the most voltage-gated potassium channels.
C
Site of stimulation.
D
Axolemma.
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Problem
Problem
An excitatory postsynaptic potential (EPSP) is __________.
A
the same as a nerve impulse along an axon.
B
a result of a stimulus strong enough to produce threshold.
C
a graded depolarization produced by the arrival of a neurotransmitter.
D
an action potential complying with the all-or-none principle.
5
concept
Summation
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3m
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OK. So we saw in that last video how individual EP SPS and IP SPS are actually kind of tiny. Huh. And by themselves, they're probably not going to have a super significant impact on our membrane potential. But lucky for them, they are rarely by themselves because remember our neuron has connections synapses with hundreds if not thousands of other neurons. And so all of those neurons sending us post synaptic potentials are going to have a cumulative effect on our membrane potential. And that is the process of summation. So summation is adding multiple post synaptic potentials at the initial segment. So all of those synapses are sending us EP SPS, IP SPS. It's it's gonna be changing our membrane potential and that current will travel down our cell body. And when it gets to the initial segment, all that change literally is going to get added up. And if it adds up to a depolarization that gets us to negative 55 millivolts threshold, we will have an action potential. And if it doesn't quite make it there nothing's going to happen right now. There are two types of summation. The first type is temporal summation and temporal summation is summation, upgraded potentials at one synapse. So one neuron talking to our neuron overlapping in time. So if you look at this graph here, this is depicting temporal summation. And this is basically just one neuron and he is very excited. He is just yelling at us, fire, fire, fire, he's sending us ep SPS. We can tell because it's depolarizing our neuron, right? And what's happening here is that he's sending those signals so fast that they're getting added together. So he's sending us an EPSP, it's depolarizing our memory. And then before we can start losing that current boom, he sent another one before we can lose current. Boom, he sent another one. So they're basically having this additive effect on each other because they're happening so close in time. So temporal summation is basically just one neuron talking to our neuron very, very fast is what's happening there. Now, we can also have spatial summation and this is summation of multiple graded potentials that are happening in close proximity. So this is basically when we have a bunch of synapses that are basically really close to each other on one chunk of our membrane. And because they're all so close when they're having this change on our membrane potential, when they're affecting it in some way, and that current is dispersing, it's going to kind of start bleeding together and getting basically summed summation, right? It's gonna get summed together. And so this is our graph depicting spatial summation and spatial summation is a bit unique because what can be happening is all of those neurons could be sending EP SPS. They could all be sending IP SPS or like we have depicted here, we could be getting a combination of EP SPS and IP SPS. So you can see here, just kind of simplicity sake. We have 10 neurons talking to us and some of them are sending us EP SPS and they're saying fire, fire and they're depolarizing our membrane. And then some of them are sending us IP SPS. And they're saying, don't fire, don't fire. And they're trying to hyperpolarize that membrane. And you can see these are coming in at different strengths, different intensities. Some of them are pretty big. Some of them are kind of little, but basically what's gonna happen is they're gonna get summed together because they're all happening so close in proximity. And when we get down to that initial segment again, if we hit threshold, if we have depolarized up to negative 55 millivolts, we will have our action potential, which is the topic of our next video. So I'll see you there.
6
example
Graded Potentials Example 2
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1m
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All right. So this example asks us to imagine 15 neurons synapse onto one post synaptic neuron at the trigger zone. 13 of the neurons produce ep SPS of two millivolts each and the other two produce IP SPS of three millivolts each. The threshold for the post syn cell is negative 55 we know threshold, right? And in they're asking us in this scenario, would an action potential be produced and then they just kind of remind us that we are currently at resting potential at negative 70. So we've been talking about summation. Let's do some adding. So we've got 13 neurons that are sending us ep SPS of two millivolts each. Remember, ep SPS are positive, right? They're gonna make our membrane more positive and IP SPS are gonna make our membrane more negative. So these 13 ep SPS of two millivolts each are gonna get us a total change of 26 millivolts positive 26 millivolts. And these two IP SPS that are three millivolts each are going to get us a total of negative six millivolts. So we're just gonna add those together. So you've got 26 minus six, which gets us a net from all 15 of these of 20 millivolts. And that is positive right now. We're starting at rest. So we're at negative 70. And if we add 20 millivolts to negative 70 we are going to end up with negative 50. Now, threshold is negative 55 and we have gotten even more positive than that, we have surpassed threshold. And so in this case, an action potential would indeed happen. We have hit an even surpassed threshold. So there you go.
7
Problem
Problem
When a second EPSP arrives at a single synapse before the effects of the first have disappeared, what results?
A
Temporal summation.
B
Spatial summation.
C
Hyperpolarization.
D
Inhibition of the impulse.
8
Problem
Problem
The EPSPs from two different synapses occur at the same time and cause a larger depolarization than either one alone can cause. This is an example of:
A
Presynaptic inhibition.
B
Postsynaptic melding.
C
Temporal summation.
D
Spatial summation.
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