After neurotransmitters are released from the cell, they bind to receptors on the next cell.
In this video, Leslie explains how the two different types of receptors – the ionotropic and metabotropic receptors – work to bring about various responses in the cell.
Transcript of Today’s Episode
Hello and welcome to Interactive Biology TV, where we’re making biology fun! My name is Leslie Samuel. In this episode, Episode 17, I’m going to be talking about 2 types of receptors. We’ve been talking about the nervous system, we’ve been looking at neurons, and we’ve seen how the action potential starts at the axon hillock, the signal travels all the way down the axon, down to the axon terminals. In Episode 16, we looked at how the neurotransmitters are released from the axon terminals, and they bind to receptors on the next cell.
What we’re going to be doing is looking at those receptors because there are 2 basic types of receptors:
What we’re going to do is we’re going to look at the ionotropic receptors first. With ionotropic, these are very fast-acting receptors. What I’m going to do is I’m going to attempt to draw one now. Let’s say here we have a receptor, and this is a cell membrane. We have the signal that comes along the axon of the preceding cell, and it releases neurotransmitters. I’m going to say these little dots here are neurotransmitters, and they’re in the synaptic cleft.
What’s going to happen if it’s an ionotropic receptor, the neurotransmitter is going to come and it’s going to bind to the receptor. The way these receptors are set up is relatively simple. When the neurotransmitter binds to the receptor, that causes the channel to open. So, I’m going to draw this showing that now there’s an open space. And then, if there are ions that are outside the cell that are specific to that channel, those ions can then enter the cell. So it’s very fast-acting. The neurotransmitter binds to the receptor, and then the channel opens so that the ions can travel inside the cell. Once again, these are ionotropic receptors.
Now, of course, there are going to be different types of neurotransmitters and different types of receptors that are going to act in this way. I’m going to take the example of acetylcholine as a neurotransmitter. So we’re going to start with ACh, and that’s for acetylcholine. We’re going to call these neurotransmitters acetylcholine, and the receptor that’s the ionotropic receptor for acetylcholine is called the nicotinic receptor. The reason it’s called nicotinic is because this is the receptor that nicotine acts on, and we’re going to talk about that in a later episode.
So, acetylcholine comes, and actually 2 acetylcholines bind to the nicotinic receptor, and then that causes sodium ions to rush in. And now you know that sodium ions are going to have a positive charge, so what do you think that’s going to do to the membrane of the cell? Well, of course, that’s going to make it more positive. So I’m going to look at it here. Let’s say I’m looking at voltage or membrane potential on the Y-axis, and I’m going to have time on the X-axis. This is the resting membrane potential.
When something like this happens that causes sodium to come in, that can cause the membrane potential to get this little bump here. So it increases a little from that sodium rushing into the cell. Because this is becoming more positive, we’re going to call this an excitatory (it’s getting it excited) post-synaptic potential. EPSP, excitatory post-synaptic potential. Because it’s acetylcholine binding to the nicotinic receptor, that’s going to cause sodium ions to rush in, causing an excitatory post-synaptic potential.
Now, there’s another type of neurotransmitter, 2 examples would be GABA and glycine (forgive my writing there, but I think you get it.) When these bind, let’s say this is GABA or glycine, what that is going to do is it’s not going to cause not sodium ions, but chloride ions, and let’s say this is chloride, Cl-, to rush into the cell.
If a negative ion rushes into the cell, what is that going to do? Well, you probably guessed it. Instead of causing an excitatory post-synaptic potential, that’s going to cause an inhibitory post-synaptic potential, or an IPSP. So if it’s a positive ion rushing in, you get an EPSP. If it’s a negative ion rushing in, you’re going to get an IPSP. This is a really fast-acting process: neurotransmitter binds, channel opens, ion rushes in.
Let’s go to the next type of receptor, and that’s called the metabotropic receptor. This is going to be a little more complicated, because what we have here, just like before, we have a receptor in the membrane. And just like before, we have neurotransmitters that are outside the cell. But what’s different here, is that inside the cell, associated with this receptor, we have a G protein.
What happens is this neurotransmitter comes and it binds to the cell, just like before, and instead of opening a channel, what that does is it activates the G protein. And then this G protein then goes on to activate a second messenger system where there can be multiple processes that are happening, causing a certain response on the inside of the cell.
So this is a slower process in that there are multiple processes happening, and it causes a different type of response. That response can be a number of different things, and we’re going to talk about that a little later.
An example of a metabotropic receptor would be the muscarinic receptor. With the muscarinic receptor, acetylcholine is still the neurotransmitter, so ACh, and that binds to the receptor that activates a G protein. When it activates a G protein, a number of processes happen that cause multiple responses, depending on the type of muscarinic receptor we’re dealing with. One of the features that we have here is for every neurotransmitter that binds, that can activate a G protein, and whatever process this is can happen multiple times, and then this process this is can happen multiple times, so that we get a greater response on the inside.
For example, I’m just going to take a random number. Let’s say here we activate 1 G protein, and this process can happen 10 times, and each one of those can cause this next process to happen 10 times. So this second messenger system can result in a significant amount of amplification, so that we can get a significantly greater response.
Those are the 2 types of receptors: we have the nicotinic receptor and we have the muscarinic receptor. If you have any questions about this, you can leave them in the comment section below, or you can just leave a comment letting me know what you think about the format of what I’m doing, and even give suggestions for future episodes. That’s it for this video, and I’ll see you on the next one.