Graded Potentials - Online Tutor, Practice Problems & Exam Prep

Graded potentials occur in the soma and dendrites of neurons, primarily through chemical synapses, resulting in excitatory postsynaptic potentials (EPSPs) that depolarize the membrane and inhibitory postsynaptic potentials (IPSPs) that hyperpolarize it. Summation of these potentials, either temporally or spatially, determines if the membrane reaches the threshold of -55 mV to trigger an action potential. Understanding these processes is crucial for grasping neuronal communication and the overall function of the nervous system.

Postsynaptic Potential

So in this video, we're going to be talking about graded potentials. 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 anatomy and physiology textbooks will be talking about them, but I do want to be clear about that distinction before we move forward. So when we have communication via chemical synapses, we end up with postsynaptic potentials. And postsynaptic potentials are just changes in the membrane potential at the postsynaptic 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 postsynaptic neuron, that's just the neuron receiving the signal, right? And when this postsynaptic 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. Let's get that out of our way.

Now postsynaptic 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, these are depolarizing 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 the 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 IPSPs, and these make our membrane more negative. In other words, these are hyperpolarizing. 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 postsynaptic potentials.

Now, we're going to 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 presynaptic neuron, and this is our neuron over here. He is receiving that signal, the postsynaptic neuron. Right? And so what's going to happen here is that this neuron is going to 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 synapse. Bada 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. Okay? 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 depolarization 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 going to happen right there. Now what's going to happen is that depolarization is going to now spread in a local current. So it's going to 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 5 millivolts. By the time it gets out here, a change of maybe 2, 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, 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 hyperpolarizing 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 kind of watching this and thinking like, okay. 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 EPSPs can work together to trigger action potentials. So I'll see you there.

Graded Potentials Example 1

Alright. So this example asks us, when voltage-gated sodium channels open in response to a stimulus, what effect does it have on the neuron? Let's go through our options and see what we have. Option a reads that potassium will rush out of the cell. Right away, we know that that's not correct, because potassium is not moving through voltage-gated sodium channels. So a is out. Option b reads that sodium will enter the cell, and that is correct. 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. Option c reads, the influx of positive ions causes depolarization, and that is also correct. Sodium is a positively charged ion, and when it starts rushing into our negatively charged cell, it's going to 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.

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.

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.

Summation

Okay. So we saw in that last video how individual EPSPs and IPSPs are actually kind of tiny. 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 100, if not thousands of other neurons. And so all of those neurons sending us postsynaptic potentials are going to have a cumulative effect on our membrane potential, and that is the process of summation. So summation is adding multiple postsynaptic potentials at the initial segment. So all of those synapses are sending us EPSPs, IPSPs. It's going to 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 2 types of summation. The first type is temporal summation and temporal summation is the summation of graded potentials at 1 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 EPSPs. 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 membrane, 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 fast is what's happening there.

Now, we can also have spatial summation and this is the summation of multiple graded potentials that are happening in close proximity. So this is basically when we have a bunch of synapses that are 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 up. 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 EPSPs. They could all be sending IPSPs, or like we have depicted here, we could be getting a combination of EPSPs and IPSPs. So, you can see here just for simplicity's sake, we have 10 neurons talking to us and some of them are sending us EPSPs and they're saying fire, fire and they're depolarizing our membrane and then some of them are sending us IPSPs 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 going to happen is they're going to 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.

Graded Potentials Example 2

Alright. So this example asks us to imagine 15 neurons synapse onto 1 postsynaptic neuron. At the trigger zone, 13 of the neurons produce EPSPs of 2 millivolts each, and the other 2 produce IPSPs of 3 millivolts each. The threshold for the postsynaptic cell is negative 55. We know the threshold, right? And in this scenario, we are asked whether an action potential would be produced. And then they just kind of remind us that we are currently at the resting potential at negative 70. So, as we've been discussing summation, let's do some adding:

• We've got 13 neurons that are sending us EPSPs of 2 millivolts each. Remember, EPSPs are positive. They're going to make our membrane more positive, and IPSPs are going to make our membrane more negative.
• These 13 EPSPs of 2 millivolts each are going to get us a total change of 26 millivolts, positive 26 millivolts.
• These 2 IPSPs that are 3 millivolts each are going to get us a total of negative 6 millivolts.

So we just add those together: 26 minus 6, which gives us a net change from all 15 of these of 20 millivolts, and that is positive. Now, we're starting at rest, so we're at negative 70. If we add 20 millivolts to negative 70, we are going to end up with negative 50. Now the threshold is negative 55, and we have gotten even more positive than that; we have surpassed the threshold. In this case, an action potential would indeed happen. We have hit and even surpassed the threshold. So there you go.

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.

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.

Here’s what students ask on this topic:

What are graded potentials and where do they occur in a neuron?

Graded potentials are changes in the membrane potential that occur in the soma and dendrites of neurons. These changes are typically the result of chemical synapses, where neurotransmitters released from a presynaptic neuron bind to receptors on a postsynaptic neuron. This binding can lead to excitatory postsynaptic potentials (EPSPs), which depolarize the membrane, or inhibitory postsynaptic potentials (IPSPs), which hyperpolarize it. Graded potentials are crucial for the initial stages of neuronal communication and can summate to trigger an action potential if the membrane potential reaches the threshold of -55 mV.

What is the difference between EPSPs and IPSPs?

Excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) are two types of graded potentials. EPSPs depolarize the membrane, making it more positive and increasing the likelihood of triggering an action potential. This occurs when positively charged ions, such as Na⁺, enter the neuron. In contrast, IPSPs hyperpolarize the membrane, making it more negative and decreasing the likelihood of an action potential. This typically happens when negatively charged ions, such as Cl^-, enter the neuron or positively charged ions, such as K⁺, leave the neuron.

How do temporal and spatial summation differ in graded potentials?

Temporal summation occurs when multiple graded potentials from a single synapse overlap in time, effectively adding together to influence the membrane potential. This happens when a presynaptic neuron fires rapidly, sending multiple EPSPs or IPSPs in quick succession. Spatial summation, on the other hand, involves multiple graded potentials from different synapses that are close in proximity on the neuron's membrane. These potentials can be a mix of EPSPs and IPSPs, and their combined effect determines whether the membrane potential reaches the threshold to trigger an action potential.

What role do leak channels play in the propagation of graded potentials?

Leak channels play a significant role in the propagation of graded potentials by allowing ions to passively diffuse across the neuron's membrane. As a graded potential spreads from its initial site of stimulation, some of the ions responsible for the change in membrane potential can leak out through these channels. This leakage causes the graded potential to weaken as it travels, meaning that the depolarization or hyperpolarization effect diminishes over distance. Consequently, graded potentials are typically strongest at their point of origin and decrease in strength as they move away from this site.

Why do graded potentials dissipate over distance?

Graded potentials dissipate over distance primarily due to the presence of leak channels in the neuron's membrane. As the graded potential spreads, ions responsible for the change in membrane potential can leak out through these channels, causing the signal to weaken. Additionally, the cytoplasm of the neuron offers resistance to the flow of ions, further contributing to the reduction in signal strength. This means that the depolarization or hyperpolarization effect of a graded potential is strongest at its point of origin and decreases as it moves away from this site.