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Current time:0:00Total duration:9:56

Video transcript

last video we looked at how light from the Sun can enter the eye and this is the eye so it enters the eye hits the back of the eye and somehow through a set of steps eye gets converted into neural impulse and the impulse gets sent to the brain for you to make sense of the information so we looked at something called the retina in the last video and we talked about how the retina is made up of a bunch of different cells and the two main cells are rods and cones so we're going to look at just the rods in this video so the rod which is shaped like this let's give him a little smiley face because he's happy and what he does is that as soon as light is is presented to him he basically takes that light and then converts it into a neural impulse so normally he is turned on when there's no light the rod is turned on but when light is present it actually turns him off so with light he's turned off so how does this happen so this occurs through this thing called the phototransduction cascade cascade so the phototransduction cascade is this set of steps that occurs at the molecular level that basically takes this rod and turns him off and in turning them off he's actually able to turn on a bunch of other cells that eventually let the brain know hey there's like here so it allows the brain to realize that there's light and allows it to comprehend what's going on and make sense of the world so let's go ahead and examine this phototransduction cascade so let's make let's give ourselves a little bit of space here just look at it in a little bit more detail so let's look at just a part of the rod so I'm going to redraw the rod over here and let's just look at this part of the rod just is very top a little bit of the rod now I'm going to draw it a lot bigger I'm going to draw a lot bigger so that we can really make sense of what's going on here so inside the rod which we just made much bigger there are a bunch of these little disks so there are really thin little disks and they're just stacked on top of one another and there are hundreds of them and they basically they're like this they just go just stacked on top of one another and they fill up the entire rod same thing in cones but we're just going to look at the rods here so inside of these little disks there are a whole bunch of different proteins all interspersed throughout the the disks so this protein that I'm drawing in red let me just go ahead and draw it a lot bigger so we're going to go ahead and blow this protein up and it's basically this multi Merrick protein so it consists of a bunch of different sub subunits there are actually seven subunits so there are five six and seven so there are all these subunits that make up this little protein right here so this protein as a whole is called rhodopsin and it's called rhodopsin because it's in a rod if it were in a cone it would be called cone opsin but it's basically the same protein and sitting inside of this protein is a small molecule and it's it's just inside here let's go ahead and just kind of just zoom in a little bit so we can make a little bit more sense of what we're looking at here let's go ahead and zoom in over here so this little little molecule is just sitting inside of rhodopsin and it's kind of bet you can see here how I drew it it's just a little bit bent so this little molecule is called retina rats and now and in this conformation in this bent conformation we call it 11 sis retinal and so what happens is light comes in from the Sun goes in through the pupil hits the retina and then hits this little rod some of the light will actually hit this molecule so it'll hit rhodopsin and it'll actually hit this molecule so here's the light it comes in and we just hits the molecule right there so an interesting thing happens when the light hits the molecule right at this region and what it actually causes is it causes the retina to actually change conformation so it actually causes the light provides enough energy that the retina goes from this bent conformation and it causes it to look more like this so I'll draw it over here draw a little carbon ring so it becomes straight so it went from being bent to being straight so the light does this and basically it goes from being 11 cysts right now to all trans right now so when the retina is changes shape it actually causes the rhodopsin molecule to also change shape so the two are really closely linked so when the retinal changes shape the rhodopsin change shape so let's go ahead and pretend that you know it looks like maybe it looks like this so this region that I'm shading in right here is what the new rhodopsin looks like after the retina has changed shape after the light hit it so rhodopsin now changes shape and that basically begins this this big cascade of events so what happens next is there's a molecule and I'm going to draw that molecule right here in green and it's made up of three different parts so there's this alpha subunit there's a beta subunit and there is a gamma subunit and this molecule as a whole is called transducin so transducin um sorry so trains juice Doosan so transducin basically as soon as retinal changes shape and causes rhodopsin to change shape transducin breaks away from rhodopsin and the alpha subunit actually comes over here to another part of the disk and binds to a protein called called phosphodiesterase which I'll just draws this little box over here so this little little protein is called cyclic GMP phosphodiesterase or PDE for short so PDE basically what it does let me just go ahead and zoom out a little bit what PDE does when it's activated is it takes cyclic GMP which is floating all around the cell it's a little molecule it's tiny and it's a it basically takes the cyclic GMP and converts it into just regular GMP so this basically reduces the concentration of cyclic GMP and increases the concentration of GMP and the reason that this is important is because there's another channel over here so there's a whole bunch of these sodium channels so a whole bunch of sodium channels sodium channels and they're all over the cell so they're just a whole bunch of them and basically what they let the cell to do is they allow the cell to take in calcium take in sodium from the outside so let's just say it is a little sodium ion and it allows it to come inside the cell so in order for this calcium in order for the sodium channel to be open it actually needs cyclic GMP to be bound to it so as long as cyclic GMP is bound the channel is open but as the concentration of cyclic GMP decreases because of the phosphodiesterase it actually causes sodium channels to close so now we have a closing of sodium channels and now we basically have less sodium entering the cell and as less sodium enters the cell it actually causes the cell to hyperpolarize and turn off so as the sodium channels closed it actually causes the rods to turn off so turn off so basically without light the rods are on because these sodium channels are open sodium is flowing through and the rods are turned on they can actually produce an action potential and activate the next cell and so on but as soon as they're turned off what happens is a look is very interesting because let's let's just look at this rod over here so what happens is really interesting because there's this other cell over here that is called the bipolar cell and we'll just give it kind of a neutral face because it's bipolar this cell there are actually two different variants so there are on center and they're off center bipolar cells off center so the on center bipolar cells normally are being turned off when this rod cell is turned on but as we mentioned due to the phototransduction cascade the rods turn off which actually turns on the bipolar cell so basically on Center bipolar cells get turned on with light and get turned off when there is no light so that's how they get their names so when the bipolar cell gets turned on it activates a retinal ganglion cell which then is sends an axon to the optic nerve and then into the brain and so that process is known as the phototransduction cascade and it basically allows your brain to recognize that there's light entering the eyeball