Properties of Graded and Action Potentials - Online Tutor, Practice Problems & Exam Prep

Graded potentials are variable strength signals occurring when ion channels open or close in response to stimuli, typically on dendrites and the cell body. In contrast, action potentials are brief, identical depolarization events that propagate along axons, initiated only when the membrane reaches a threshold of approximately -55 mV. Graded potentials can be depolarizing or hyperpolarizing, while action potentials are always depolarizing. Understanding these differences is crucial for grasping neuronal communication and the all-or-none principle of action potentials.

Graded and Action Potentials

Okay. So this video is going to be about the properties of graded and action potentials. Now, we're going to 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. 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. The transition from graded to action potential takes place at the initial segment of the axon. And you guys remember the initial segment, we'll 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 is 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.

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. Okay? 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 graded potential graded potentials can be depolarizing or they can be hyperpolarizing. That will just depend on the type of signal that our neuron received. So remember, depolarizing means our membrane is getting more positive and hyperpolarizing 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 action potentials work very differently. These will only initiate at a threshold of approximately negative 55 millivolts. So sometimes you'll see, 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 the 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, an 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. Alright. 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.

Properties of Graded and Action Potentials Example 1

Okay. 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 the threshold for an action potential is -55millivolts. 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 -70millivolts, in order for our neuron to initiate communication, we don't want to have to get this membrane potential way up here. So if this is like 0 over here, we're at 30millivolts positive. Jumping from -70 millivolts all the way to 0 or all the way to positive 30, any kind of number like that, that would be a massive shift. Right? But going from -70millivolts to -55millivolts, 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, 20millivolts. So the action potential threshold has to be fairly close to the 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 -55millivolts, and our answer is b. There you go.

___________ 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.

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.

Here’s what students ask on this topic:

What are the main differences between graded potentials and action potentials?

Graded potentials are variable strength signals that occur when ion channels open or close in response to stimuli, typically on dendrites and the cell body. They can be depolarizing or hyperpolarizing and vary in strength based on the stimulus. In contrast, action potentials are brief, identical depolarization events that propagate along axons. They are initiated only when the membrane reaches a threshold of approximately -55 mV and are always depolarizing. Action potentials follow the all-or-none principle, meaning they either occur fully or not at all, regardless of the stimulus strength once the threshold is reached.

How do graded potentials and action potentials differ in terms of signal strength and distance traveled?

Graded potentials vary in strength depending on the magnitude of the stimulus and are designed for short-distance travel, typically occurring on dendrites and the cell body. In contrast, action potentials are all identical in strength and are designed for long-distance travel along axons. The strength of an action potential does not vary with the stimulus once the threshold is reached, adhering to the all-or-none principle.

What is the threshold value for an action potential, and why is it important?

The threshold value for an action potential is approximately -55 mV. This value is crucial because it determines whether an action potential will be initiated. If the membrane potential at the initial segment of the axon reaches this threshold due to depolarization from graded potentials, an action potential will fire. This all-or-none phenomenon ensures that action potentials are consistent and reliable signals for neuronal communication.

Can graded potentials be both depolarizing and hyperpolarizing? Explain.

Yes, graded potentials can be both depolarizing and hyperpolarizing. Depolarizing graded potentials make the membrane potential more positive, moving it closer to the threshold for an action potential. Hyperpolarizing graded potentials make the membrane potential more negative, moving it further from the threshold. The type of graded potential depends on the nature of the stimulus and the specific ion channels that open or close in response.

Why are action potentials described as an all-or-none phenomenon?

Action potentials are described as an all-or-none phenomenon because they either occur fully or not at all. Once the membrane potential at the initial segment of the axon reaches the threshold of approximately -55 mV, an action potential will always fire with the same magnitude. This ensures that the signal is consistent and reliable, regardless of the strength of the initial stimulus, as long as the threshold is reached.