Ligand Gated Ion Channels

have you ever wondered how our nervous system like your neurons in our brain can react so quickly well the reason is because of ligand gated ion channels ligand gated ion channels are one type of major membrane receptors the three categories are ligand gated ion channels g-protein coupled receptors and lastly enzyme-linked receptors today we're going to focus on the ligand gated ion channels and these are also called ion channel linked receptors so ligand gated ion channels are transmembrane ion channels that open or close in response to the binding of a chemical messenger like a ligand so like we mentioned very common place to find ligand gated ion channels are an electrically excitable cells like neurons and the reason why is because these ion channels react really quickly to the binding of ligands and so they're very commonly found in cells that need to react very quickly to stimulus when we talk about transmembrane ion channels what we really mean is they are transmembrane or integral proteins that also have a channel or hold through them in which things can move in and out so to start with let's say we have an ion channel that looks something like this let's go ahead and color that in in this particular channel this channel at the moment is closed and right now we have our intracellular environment here this is where our SATA Sol and all of that stuff is and this is our extracellular environment and so as you can see this ion channel has like a weird kink in it and this is this place where a messenger like ligand or a neurotransmitter can bind so let's say we have a ligand that looks like this this is our ligand and this ligand can bind in there and notice that the shape of our ligand is complementary to the shape of our channel and so it fits right in there the only specific ligands can bind to specific channels this is what we call our lock and key or a more updated one as it is called induced fit so once this ligand binds what it'll actually do is it'll cause this closed channel to actually open up so what we'll see is we'll actually see this channel open up kind of like that so now this is our open Channel and so one really interesting thing about ligand gated ion channels is if you look at where this ligand binds the binding site of this ligand is not anywhere near the actual channel and the reason why is because this is what we call allosteric binding and so the ligand binds to what we call an allosteric site this is a place that's away from the ion channel but what happens is once the ligand binds it can control the opening and closing of the ion channel by altering the protein conformation of the entire protein so once it binds a channel opens in a different place and the ion permeability of the entire plasma membrane can quickly change remember this is not just one thing happening when these ligands are binding there are many of these channels scattered throughout these cell membranes so all of these are opening and closing all at once so once this channel opens it'll let irons like potassium sodium chlorine or calcium being the most common move through the open channel and so once these ions are moving in and out this will cause a change in the electrical properties of a cell in other words you'll convert this extracellular ligands signal into an intracellular electrical signal and so once these ions move in or they can also move out you'll have an intracellular electrical response electrical signal happen inside the cell okay so there are two things I'd like to note real quick the first is at the allosteric binding site of the ligands this area that is complementary to the ligand can be intracellular it can be inside the cell but that's considered pretty rare why might that be well we'd have to think about the main purpose of membrane receptors and we have to realize that they generally are meant to respond to extracellular signals to things that are on the outside of the cell and so generally speaking ligand gated ion channels will have the binding site on the extracellular side the second thing to note is that it is possible for there to be multiple allosteric binding sites for lichens so it's possible that there are multiple of these kinks of these complementary shapes that let the ligands bind for each protein now finally I'd like to clear up two quick misconceptions ligand gated ion channels are easily confused with two types of ion channels they are not the same as voltage-gated ion channels voltage-gated ion channels rely on the difference in membrane potential and as we call ligand gated ion channels actually respond to the binding of a ligand the voltage-gated ones only depend on the difference in membrane potential the second one that are easily confused with is what we call stretch activated ion channels now as the name implies stretch activated ion channels depend on what we call the deformation of the cell membrane or the cell membrane stretching and being pushed and being stressed so in summary ligand gated ion channels are one type of membrane receptors they are transmembrane ion channels that open or close in response to the binding of a chemical signal like a ligand so notice that here our channel is closed and once this ligand binds to an allosteric site which is a site that's not on the channel itself ions such as potassium and sodium and so on move through the membrane this will actually cause an intracellular electrical signal so we have one outside lichen and this tool allow us to have an intracellular electrical signal and this will actually tell the cell to do something and ligand gated ion channels are not to be confused with voltage-gated channels which only rely on a difference in membrane potential and they are not to be confused with stretch activated ion channels which are affected by deformation of the cell membrane