Properties of Graded and Action Potentials - Video Tutorials & Practice Problems
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concept
Graded and Action Potentials
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OK. So this video is going to be about the properties of graded and action potentials. Now we're gonna have a whole bunch of videos going into each of these in great detail for you coming up. But the purpose of this video is to just understand their definitions and do a little compare and contrasting of their properties. So let's dive right in now, graded potentials are variable strength signals. And what I mean by that is they can literally vary in strength and these are going to occur when ion channels open or close in response to a stimulus. Now, basically what's happening here is our neuron is receiving some kind of stimulus and as a response to that, it has a graded potential. Now, action potentials are brief depolarization events that are propagated along the axon of a neuron. Now the transition from graded to action potential takes place at the initial segment of the axon. And you guys remember the initial segment will scooch down to our neuron. Here is right where the axon meets the cell body. And what I mean by transition is that when our neuron receives this signal and has a graded potential, it's going to travel down the cell body. And when it gets to that initial segment, if it's strong enough, it's going to trigger an action potential so broadly, that's what's happening here. Now, let's dive into that little compare and contrast I mentioned earlier. So in terms of what type of signal each of these is as I just alluded to graded potentials are input signals, this is our neuron receiving some kind of signal. In contrast, action potentials are an output signal that is our neuron sending a signal. And so I would bet just based on that language receiving versus sending, you can already guess where these are going to be taking place on the neuron. So graded potentials typically take place on dendrites, dendrites receive, right. But they can also take place on the cell body and axons send signals, right. So that is where our action potentials will be taking place on axons. Now, graded potentials are designed for short distance travel. OK. And that makes sense, right? Because dendrites and the cell body are pretty small, whereas axons can be very long, remember, they can be up to like a meter long. And so action potentials are designed for long distance travel, they have to be able to go down an entire axon no matter how long it is. In terms of the type of signal that we're dealing with. Graded potentials can be depolarizing or they can be hyper polarizing that will just depend on the type of signal that our neuron received. So remember, depolarizing means our membrane is getting more positive and hyper polarizing would be our membrane getting more negative. So they can do either one of those. Whereas in contrast, as that definition, stated, action potentials are always depolarizing. So they're always making our membrane more positive. Now, in terms of the signal strength, as the definition kind of told us graded potentials can vary in strength and that will depend on the magnitude of the stimulus that came in. So a strong stimulus will make a strong graded potential and a weaker stimulus will make a weaker or a smaller one. Now, in contrast to that action potentials are all identical. So every single action potential, regardless of how strong an incoming signal is, will always be the same magnitude, they're all identical. They're all the exact same. Now, in terms of if we have some kind of minimum threshold of membrane potential that has to be reached for one of these events to happen for graded potentials, the answer is no, as I stated, these will respond even to a very weak stimulus. So if a small stimulus comes in, it's just going to create a small graded potential. Whereas potentials work very differently, these will only initiate at a threshold of approximately negative 55 millivolts. So sometimes you'll see um just the term threshold or threshold value get tossed around. If you see that they're saying what they're trying to say is negative 55 millivolts usually. And what that means is. So our initial segment down here, it's going to start off at resting potential rate at negative 70. And if those graded potentials come down and they manage to depolarize that initial segment enough to get it up to negative 55 millivolts at that threshold and action potential will happen. Now, action potentials are sometimes called an all or none phenomenon and they're called that because if that initial segment hits that threshold value of negative 55 millivolts, an action potential will always fire. They can't go backwards, they can't slow down and again, they're all going to be identical. It does not matter how strong that incoming stimulus is. And so that's why they're called this all or none phenomenon. All right. So that is kind of graded potentials and action potentials in a nutshell. As I said, we'll be diving into each of these in even more detail in the upcoming videos. So I'll see you there.
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example
Properties of Graded and Action Potentials Example 1
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OK. So let's go into an example here. Action potentials are an all or none phenomenon, meaning that once the membrane potential reaches approximately blank, they will always happen. So what this is asking us for is basically what is the threshold of an action potential? What's that threshold value? And we know that threshold for an action potential is negative 55 millivolts. So our answer is b now if you're having some kind of conceptual trouble with this, I would encourage you to think about it this way. So if we are at rest here at negative 70 in order for our neuron to initiate communication, we don't want to have to get this membrane potential way way up here. So if this is like zero over here, we're at positive 30 jumping from negative 70 all the way to zero or all the way to positive 30 any kind of number like that, that would be a massive shift, right? But going from negative 70 to negative 55 that's a totally doable thing, right? We can just have some sodium come on in and bada bing bada boom, we're up you know 20 mia volts. So the action potential threshold has to be fairly close to resting potential because we don't want our membrane to have to make some kind of massive massive spike, it needs to be fairly close. So our action potential threshold again is negative 55. And our answer is b there you go.
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Problem
Problem
___________ are all identical, whereas __________ can be larger or smaller depending on the level of stimulation.
A
Action potentials; graded potentials.
B
Graded potentials; action potentials.
C
Depolarizations; hyperpolarizations.
D
Repolarizations; depolarizations.
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Problem
Problem
Which of the following statements about graded potentials is FALSE?
A
Graded potentials act as short distance signals.
B
The magnitude of a graded potential is proportional to the magnitude of the stimulus.
C
Graded potentials occur mainly in axons.
D
Graded potentials can be depolarizing or hyperpolarizing.
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