I'm in generics in this video we're going to talk about specifically the phototransduction cascade so we're going to dig into the layers of the retina a little bit more talk about those layers and then we're going to talk about how we can turn light rays into electrical signals that basically produce vision all right so let's go ahead and start here so first thing we're going to talk about is the different layers of the retina so what am i doing here if you see here you guys are probably like wait where the heck did this come from don't worry all we did is if you guys watch the video on the structure of the eye we all we did is we're taking a little section right here we're taking a little section right here from the retina and zooming in on it okay so now we're actually zooming in on all the different components of the retina isel's go ahead and get started on this thing so first things first the retina is actually made up of three main cell layers like I said its outer layer out here this outer layer out here which is made up of these maroonish like cells this is the outer pigmented layer of the retina so what is this right here this is the outer pigmented layer of the retina all right all right so the outer pigmented layer of the retina this is an important layer why because the outer pigmented layer consists of these little granules see all these dots in there all those dots are consisting of a specific type of molecule called melanin you know melanin is basically like a pigment it helps to be able to prevent what happens whenever light rays because you know whatever light is actually coming into the eye let me actually this sometimes can be a little confusing for people it's come up to this little eye here for a second imagine here I have the light rays okay so here's the light rays these photons of light are actually moving in through the cornea note that you know that goes through the actual lens and then they'll actually move through the layers of the retina so they're moving through the layers of the retina so if you think about the rays of light are actually coming this way this way this way in this way and then they're stimulating these photoreceptors which working talk about but what happens is these photoreceptors some of the end points here this little like reg of bumped little edges part of the actual photoreceptors they're kind of embedded the discs are embedded into these actual outer pigmented layer of the retina and why is that important because these melanin granules are important for absorbing some of those light rates and preventing the scattering of light and preventing the actual reflection of that light so that doesn't disturb the normal visual pathway so that's one of the important functions of this melanin that's within the outer pigment layer another thing if you remember we said that there's going to be blood vessels here's what's called the from the colloid you're going to have these actual choroidal blood vessels so this whole thing right here is supposed to represent a blood vessel so this is representing a blood vessel from the choroid and what happens is this blood vessel gives off some of the substances that we need for the actual retina so what happens is certain things are actually going to come out of this actual choroidal vascular supply and from this it'll go into these cells these outer pigmented epithelial cells these cells will process that substance and then push it out here into the retina to provide nutrition or oxygen or other different types of substances to the retina by diffusion okay so that's one function of it so one function is to prevent the reflection of light to absorb the light rays and prevent it from scattering the other function is to provide nutrients supply to the actual nutrient oxygen other different types of vascular supply to the retina and it controls it's acting like a barrier it's acting like a barrier to prevent it controls what substances leave the blood and come into this actual tissue where the redness so it's acting as a barrier it's also giving nutrition and oxygen and other different types of supply from the blood and it's actually absorbing any light rays to prevent reflection of the light and print the scattering of light rays alright that's that should be good enough for that bad boy all right if you want to know some of the discs from these actual photoreceptors some of them actually degrade well they actually get released some of these old discs here from these actual photoreceptors these outer pigmented layer that have a retina they can phagocytose that okay so if you want they have four functions absorbing a light rays and prevented from being reflected and scattered they provide a blood vessel supply to this area by diffusion nutrient diffusion here and they control what leaves the blood comes into the retina area and if phagocytosis any types of cellular debris from like the actual photoreceptors like their discs okay alright good and then we get into the photoreceptor layer okay so these guys here there's all these cells here are photoreceptors okay so photoreceptors there's actually two different types of photoreceptors we're going to take a little bit of time to discuss this so this is our actual photo receptor layer so the photoreceptors there's two types what are those two types of photoreceptors there's rods and then there's cones and we should take a little bit of time to talk about this because this is one of the big big things here so photoreceptors the two types one is rods and the other one is of cones we should be able to differentiate between these two now what are the rods good for okay the rods specific function is for a specific type of vision they caught scum they call this type of vision here schoo topic vision okay so topic vision scotopic vision so it's more for dim light or dark light or like a fuzzy dim light right so dim light and dark light so they're more for actual dim light dark light or another way of describing it is like lights in different shades of gray so light that is found in different shades of gray so light in different shades of gray okay so that's the function of these rods they play a role in what's called skoettel big vision so controlling the you know being able to see and dim light dark light or being able to see in different shades of gray now one of the things is because they play a role in our vision at that type so basically night vision you can also think about scotopic vision is like the night vision so these are the photoreceptors that are functioning during the night time but here's the difference here when you think about these guys if you guys have ever been in a dark room can you really determine like if I have like a certain structure could you determine every single edge of that structure because you have very very precise like edge detection of that structuring no I might not be able to tell the color of that structure either so at night certain types of visual acuity is affected so they lose their visual acuity and they lose the color at night right so this is not going to be like a very very great vision for acuity so it's not good for visual acuity okay so they're gonna have a hard time being able to see different types of shapes and edge detection and also color so will have a decrease ability to see color decrease what's called edge detection and decrease in acuity of the vision right okay so a decrease also in the acuity of the vision okay so there's going to be a decrease in this color vision they're going to have a decrease in being able to determine the detect like detect edges on some type of structure and they might have a decrease in the acuity okay so that's for the rods now the reason why they have scotopic vision and they play a role within this dim light this dark light this fuzzy light the different shades of gray is because they consist of a specific type of pigment they consist of a specific type of pigment and this pigment is called rhodopsin so it's called rhodopsin and rhodopsin is actually made up of two things one other things as it's made up of what's called retinal okay so it's made up of a substance called right now okay the other component is a protein called opsin so option is a protein and right now is a pigment and we'll talk about why that's important so rhodopsin is the pigment consisting of retina which is basically a derivative it's a derivative from vitamin A so you know vitamin A actually can get converted as retinal if you exceed eyes it so if you remove off of it two protons so if you get two protons off of it you can convert vitamin A into right now all right so that's rhodopsin but rhodopsin is very specific to the dark light or the dim light or that fuzzy light right that is one of the big differences about rods is therefore scotopic vision dim light dark light different shades of gray they're not very good at being able to determine color or edge detection or acuity right and they consist of a pigment called rhodopsin which is made up of retinal which is a derivative from vitamin A and a protein called opsin okay that's good for right now cones cones are cool little buggers these guys consist of they're actually more very very important for it's called Soto peak vision photopic vision so they play a role in photopack vision these ones are very good for visual acuity and for edge detection and for color vision but they're not very good at being able to see things in dim light and dark light and different shades of gray that's why they're just basically kind of opposite in function so they're four foot open vision so these is good for color vision now what do I mean okay what'd that mean man all right don't worry I got you our cones we have three different types of what's called these pigments you know there's actually we have it down here let's write it right here there's a specific type of pigment in cones we're going to kind of bring it down same kind of thing here this pigment that's found in cones is actually called fo Thompson flow Topsham which is kind of like it's also referred to as I Oh Dobson but specifically we're going to call it here fo Thompson and the reason why is there's different types there's fo Thompson one fo Thompson two and fo Thompson three why am i mentioning this because info Thompson what it allows for is this fo top set pigment can detect certain wavelengths of light you're like holy Frick is bringing chemistry into this don't worry all I'm trying to say here is is that these photons can pick up different wavelengths of the visible spectrum to determine three main colors so these phoE Thompson's are very very sensitive to three specific wavelengths one is the actual blue blue visible part of a blue part of the visible spectrum okay so they can pick up the color blue another one is red okay so they can pick up red within the visible spectrum and they can pick up green within the visible spectrum if you guys know a little bit about your actual chemistry though you'll pryor remember the term rui-zhi Biv right ROYGBIV and all this was trying to say is is that as you're going this way right the energy is increasing so energy is increasing frequency of this light is increasing but the wavelength as it goes this way the wavelength which is sure like lambda this is decreasing so energy is increasing frequency is increasing but the actual wavelength is decreasing so in other words if we were to pick up these colors that we were talking about blue is higher in energy but it's actually lower in wavelength Green is actually going to be a little bit higher in energy than the red but it's actually going to be a little bit more in wavelength and then the red is going to be the lowest in energy but highest in wavelength okay so if you were to kind of write it like this this one is actually going to the highest wavelength this one will have the lowest wavelength and this will kind of be right in the middle so I'm not even going to put an arrow there I'm going to say this is kind of within the middle all right that's the whole purpose is that these fofo tops ins can pick up different wavelengths of light that's it so they play a role in photo peak vision which is for color vision and because they're very important during our bright light during bright light they're very good for visual acuity so they're good for bright light vision bright light vision and they're very good for like visual acuity right so they're very good for visual acuity so they can have very good they're very good being able to determine these shapes of sudden certain things right so edge detection so they're very good at that edge being able to see very very precise things okay so they're very good at precise vision all right that that should be able enough to explain that right now same thing the pigment here this boat opsin it is made up of different types of things here it's going to have the it's going to have like iodine and a part of it too so there's actually going to be so a little bit of iodine in this float opsin but it's also going to be consisting of opsin now here is the only thing that's a little bit similar between these two the way that they're stimulated is the same okay so their transduction process which we're going to talk about here in a second is the same that's kind of the really the only thing that is the same between these two okay their transduction process the same they both respond to light the same way which is really odd okay all right let's go ahead and keep digging in here so now that we know exactly what rods are good for we know what cones are good for let's go ahead and keep going into the layers and then we're going to discuss how this light is actually getting converted into electrical potentials okay what's the next part here so we know that we have the photoreceptor layer you know you have these special little cells located in between the photoreceptors within a horizontal plane so because there'll actually located within a horizontal plane and they're orienting these cells in a horizontal plane these guys are called horizontal cells okay they're called horizontal cells and these cells are very interesting we'll talk about them in a little bit later whenever we get into the transduction process like the signaling process they can secrete certain chemicals called GABA gamma-aminobutyric acid and the gamma amino butyric acid can actually inhibit these photoreceptors to basically control their it's very very important for being able to our adjustments when we go from bright light to dark light or dark light to bright light so they help to modulate that activity the adaptation in it we'll talk about that okay so that's the horizontal cells then we have this brown cell layer and this brown cell layer is actually consisting of what's called bipolar cells bipolar cells and these cells are not bipolar than manic and depressive it's because they have one dendritic sensen and one axon extension okay that's it and these are very very important also now in between kind of in kind like a vertical arrangement because they control the vertical motion these cells here are interacting and they're modulating activities occurring between the bipolar cells and the these green cells are called the ganglion cells they modulate the dis activity there are certain types of neurotransmitters these suckers right here these maroon cells are called amacrine cells and Makran cells and they're located within this inner plexiform layer we're not going to talk about the flexible layers maybe in future videos right right now we're going to talk about these layers very simply okay so now a macron cells going to be kind of within that what's called the inner plexiform layer then you're going to have this last layer here this last layer is consisting of what's called the ganglion cells and these ganglion cells are very very cool they have a dendrite extension that's connecting with the bipolar cells and their axons are coming together and making one big old structure here called the optic nerve and form the optic nerve okay so they all come together and form what's called the optic nerve which is known as cranial nerve - okay so that's what we know so far we know the layers we know that there's the outer pigmented epithelial layer the photoreceptors which is made up of the rods and the cones we know what they do when if there's the bipolar cells and they're basically acting as the relay between the photoreceptors and the ganglion cells we know in between the photoreceptors or horizontal cells which are modulating the activity somehow and the current cells which are also modulating the activity between the bipolar and the ganglion and we know that the actions of the ganglion cells are helping to form the optic nerve okay so far we've done pretty good and we've just diced out the layers of the retina we've identified what the rods and the cones do now what I want to do is I want to talk about exactly how this light ray let's make this light ray a different color that would be more but make more sense let's do it in orange so let's say that this is our light right here so this orange here is the light ray okay here's the light ray and what happens is I want to see exactly how this light ray these photons are producing these chemical changes and then electrical changes within these photoreceptors okay so we're going to treat this structure over here as it's a photoreceptor meaning it could be a rod or a cone we're going to specifically look at it as rods but remember the cone mechanism is the exact same all right so let's go ahead and egg in here all right first thing what do we say there's going to be these photons of lights right so these photons so here's these light rays how does this affect this rod here so we're looking at a rod right now okay we're looking at a rod but again the mechanism would be very very similar for the cones to just involve different types of pigments so these light rays they come in and they hit a special structure here so here's going to be the outer segment and there's going to be these little disks right here's a little disc and here's the space inside of the disc here's the outer membrane on that disk what happens is these light rays will move through this outer segment and it'll hit a specific structure here on this disk here on this disk let's do this in maroon there's going to be that rhodopsin molecule here is your rhodopsin so here's a rhodopsin molecule and what did I say happens when it hits the rhodopsin well first off what's rhodopsin made up of it's made up of retinol and it's made up of opsin so let me show you something here I'm going to show you the two components here first thing I'm going to show you is the retina there's going to be this retinal structure here and the important thing of red nails that exist in two different forms right one is it's actually existing what's called the transform and the cysts form so let me just number this out real quick four five five six seven eight and then nine here okay so one thing that happens here is here's our right now but there's a methyl group here on three now on that third carbon there and then there's a methyl group here on seven but here's what happens let's say that this methyl group is pointing down all right let's say that it's pointing down like this if it's pointing down like this this is going to exist in what's called the eleven sis Retta now if you hit this eleven sis right now that's what you're going to doing you're hitting with these light rays if you hit it with this light rays what it does is it converts the eleven sis retinal into a different structure so here's the light ray so that's the light it's hitting the eleventh this right now and converting the eleven sis right now it's just kind of flipping the structure around and converting it I know it's so weird that that's all it does but all it does is it kind of just flips the structure around a little bit here and converts it from this eleven sis right now into what's called all trans right now okay so that's going to convert into what's called all trans red now so now we went from eleven sis to what's called all trans right now okay so that's what you did you took the light rays the light rays hit the eleven sis right now and converts it into all trans retinal why is that important because then out of this you're going to separate from the all trans retinal the opsin protein and then the option is going to go off and perform a specific function okay so what happened here we converted eleven sis into all trans retinal and then what happened here from this broke away the protein what was this protein that broke off the option what happens is the option goes off and activates a specific protein here it activates a very very specific protein or there's a protein over here there's like a transduce in protein let's say here is actually going to be this transducin protein so here's a little transducin protein so it activates a protein called transducin this transducin protein will then go and activate a special enzyme over here there's an enzyme here and this enzyme is called phospho diester ace so it activates a special enzyme called phosphodiesterase why is this important let me draw another phosphodiester is over here too here's why this phosphodiester ace is important because there's actually something else happening at the same time this is occurring there's actually another protein on this surface here that's usually functioning very very well and this enzyme here is called adenylate cyclase I'm sorry dentally cyclase actually no I lied this is called gwon allow cyclists this is called gwon allow cyclase what guan allows cyclase is doing is it's taking a converting a molecule called GTP and it's converting into what's called cyclic GMP cyclic GMP why is this important because cyclic GMP is normally bound to these special types of channels on the membrane so it's bound to special types of channels here on the rod cell membrane let's draw these channels here let's say here I have one channel okay here I have another channel here and then here I have two more channels here here's another channel and here's one more channel okay why is this cyclic GMP important cyclic GMP has little pockets on these channels here has little pockets that it can bind to so cyclic GMP could actually bind on to this channel here it could bind onto this channel here and it combined onto this channel here and again this channel here when it binds onto these channels it activates these channels here and opens up the channels to allow for specific ions to flow these ions are going to be sodium and calcium if these ions are flowing into the cell what are they doing to the inside of the cell they're making the inside of the cell positive if you make the inside of the cell positive it tries to bring about a excitatory postsynaptic potential tries to bring the actual cell to threshold to generate what's called an epsp these cells these photoreceptors do not generate action potentials you have to remember that they do not generate action potentials they generate what's called receptor potentials or graded potentials excitatory postsynaptic potentials but here's the problem when light rays hits the rhodopsin it converts 11 cysts in to all trans which frees up the option the option activates a transducin the transducin goes and activates these phosphodiesterase enzyme so again what what to do over here we come over here and activate this phosphodiesterase enzyme guess what phosphodiesterase does it breaks down this actual cyclic GMP it breaks down the cyclic GMP if you break down this cyclic GMP then geum cyclic GMP is no longer going to be active if this cyclic GMP is not active then what's it not going to be able to do then it's not going to be able to bind onto these channels if it can't bind onto these channels these channels close these channels close and then what happens sodium ions and calcium ions will not move into this photoreceptor this rod in this case they'll actually be prevented from coming out I'm sorry coming into this actual cell right so let's review this real quick just to make sure it's all clear here first thing light rays hit the road opsin second thing it converts eleven Sisson to all trans third thing is it takes option and then it frees up option to bind onto transducin transducin activates phosphodiesterase now normally in this rod it's having GTP being converted into cyclic GMP through guano al cyclase and cyclic GMP is binding onto these sodium and calcium channels on the membrane keeping them open so that sodium and calcium can come in but whenever the light rays are there it debates this phosphodiesterase enzyme and what does the phosphodiesterase do it breaks down the cyclic GMP if cyclic GMP is broken down his concentration decreases if it decreases then he can't keep these channels open if these channels aren't going to be open they'll stay closed and then the sodium ions and calcium ions can't come in if the sodium and the calcium ions can't come in what happens to the cell it becomes less positive so the cell becomes less positive so if it becomes less positive or we can even think about it like this if it becomes less positive it's actually becoming a little bit negative so if it's less positive it can actually become a little bit more negative what is this called then the cell is becoming increasingly more negative or less positive this is called hyper polarization so it'll cause what's called hyper polarization if you guys can think about it like this let's imagine I draw here a graph for a second say I put here a graph and here on the x-axis is going to be time here on the y axis is going to be millivolts let's say here is the normal resting membrane potential then here's what you need to get to threshold right so there's your threshold potential all right generally these cyclic GMP channels are going to be keeping it how it's kind of depolarized right if they're if it's binding onto those channels bring in the sodium and the calcium in if it blocks it then then what happens it actually is not going to rise to threshold potential it'll actually drop below resting membrane potential so once once this happened then this is actually going to hyperpolarize it goes below resting membrane potential okay so that's called an IPS P IPS P is an inhibitory postsynaptic potential so whenever hyperpolarized this produces what's called a I P SP an inhibitory postsynaptic potential if you pretend for a second that in red let's say that these channels were open so cyclic GMP was bound then this would actually cause sodium and calcium ions to come in making the cell more positive it would start approaching what it would start approaching threshold potential if it starts approaching threshold potential is trying to excite the cell this is called a excitatory postsynaptic potential so it's trying to cause a slight depolarization in this case though because light hits this it causes it to hyperpolarized which is called a ipsp if that happens then what's going to happen to the action potentials that are moving down this axon it's going to be very very low very little if any action potential is going to be traveling down this axon then if that's the case then if there's very little action potentials or almost no action potentials you know there's specialized channels down here specialized channels down here called these voltage-gated calcium channels there's these voltage-gated calcium channels down here at the synaptic terminals these calcium channels these calcium channels want to open so that calcium can come in but if this cell is hyperpolarized this is not going to activate these channels these channels will be inhibited if these channels are inhibited the calcium can't really come in and if that's the case let's say that a tiny tiny bit of calcium does come in very very very little calcium comes in or almost no calcium comes in right you're going to release very very little if any neurotransmitters and what is this neurotransmitter that's actually getting released here it's called glutamate okay so to pick up where we were we said if the cyclic GMP is not bound the channels closed if the channels closed sodium and calcium can't come in the cell becomes hyper polarized that was the ninth thing if your type of polarized there's pretty much no action potentials that are moving down the axon if there's pretty much no action potentials moving down the axon it's not going to activate the voltage-gated calcium channels if pretty much no calcium is coming in there's going to be almost very little or no release of glutamate and if there's very very little release of glutamate how is that going to affect this let's come over here and see okay let's say that this guy releases very little glutamate so I'm going to put here glutamate this is really weird because glutamate is generally an excitatory neurotransmitter so you may think though okay it's not going to release very little glutamate very little glutamate is being released as probably isn't going to activate it actually it's the exact opposite it's a very very weird example very little glutamate is actually going to emulate this bipolar neuron okay it's going to stimulate this by pulling on so what's happening here is if you're releasing very little glutamate and if you release very little glutamate that's going to stimulate this actual bipolar neuron so glutamate must be causing some type of mechanism where it's actually getting rid of cations and the inside of the cell is becoming negative but if there's very little of that glutamate coming over here very little cations will be leaving the cell and very little cations will be leaving the cell the cell will remain positive and will become depolarized and in this case it produces what's call if it stimulates it it produces with holiday EP SP so this will produce ATP SP what does that mean it's going to generate these nice receptor potentials remember it generates receptor potentials no action potentials the only one that generates the action potentials is really just these actual bipolar neurons not these bye I'm sorry not the bipolar the ganglion cells so the receptor potentials are produced by the photoreceptors in the bipolar the action potentials are generated by the ganglion cells okay so it releases very little glutamate which produces these epsps because excitatory postsynaptic potentials which actually trigger a little bit of movement down the axon which is then release more this is this is where it gets really funny it releases a lot of glutamate if it releases a lot of glutamate you'd be like oh it's probably to inhibit it no it stimulates this one it stimulates this one and this generates action potentials on this guy and again it will release a lot of glutamate here and if you release a lot of glutamate here this will help to generate action potentials moving down this axon again themself and then if the actual action potentials are moving down this ganglion cells axons what does that mean them that means that the action potentials are going to be moving down the optic nerve is going to be increased and if the action potentials that are moving down the optic nerve are going to be increased that will take it into the actual occipital lobe specifically where the primary visual cortex is and help us to perceive the actual image that we are seeing at that point in time all right so to recap that here you're going to have light rays hitting this rhodopsin converts eleven fists right now to all trans releases the option obstinate activates transducin transducing activates phosphodiesterase phosphodiesterase breaks down the cyclic GMP and to the GMP broke the break down product then after that if the cyclic GMP is not present in these you need stick a GMP to keep these channels open if these channels do not have the cyclic GMP they stay closed sodium and calcium can't come in the inside of the cell becomes increasingly more negative or less positive and it hyperpolarizes the cell which causes an eye PSP very little receptor potentials are going to be moving down this axon very little receptor potentials if there's very little receptor potentials it's going to cause very little calcium to come into the axon terminals and very little glutamate is going to get released if very little glutamate is released onto the bipolar cells it stimulates the bipolar cells and produces epsps and then they'll generate increased receptor potentials down their axons which would release a lot of glutamate onto the ganglion cells if you release a lot of glutamate onto the ganglion cells that stimulates the ganglion cells and triggers action potentials down their axons which eventually become a part of the optic nerve holy crap we did that part now okay now I want to talk about how the horizontal cells and the amacrine cells are affecting this process real quick the horizontal cells are actually receiving so you know whenever if these this glutamate is being released here this actual guy here let's say here it can release glutamate not just onto the bipolar cells but onto the horizontal cells and when it releases this glutamate onto the horizontal cells it actually stimulates the horizontal cells and then as a result the horizontal cells release this chemical called GABA gamma-aminobutyric acid and then gamma amino butyric acid guess what it does it goes in and hits this photoreceptor you guys are probably like wait what Frick I'm so confused now let me explain it one more time here this guy's releasing glutamate come up with glut there right that stimulates this guy this guy and responsible release gaba gaba will then inhibit this photoreceptor the whole purpose of this is is that whenever this is getting hit with light rays you want this guy to become less sensitive to those light rays right you don't want them to become very very sensitive you want to become a little bit more desensitized to these light rays so what this guy is doing is how these horizontal cells are functioning is whenever we're going from light to darkness or from dark to light he's helping us to adapt to that he's adapting to the changes in light going from dark to bright light or bright light to dark light okay that's the function of these horizontal cells so they're basically maintaining or modulating this actual movement right or these actual photoreceptors sensitization all right sweet the amacrine cells are doing very similar of that okay the whole function they're still trying to sift out the function of the amarak results what they do know is because that the immigrant cells are involved in the connection between the bipolar neurons and the ganglion cells and what they know is is that this a Makran cell can actually release various different types of chemicals it can release chemicals such as dopamine it can release chemicals like acetylcholine anak release chemicals like GABA and glycine many many different types of chemicals and basically what they believe that it's doing is is it's inhibiting these ganglion cells why so that there is very very precise movement or very very precise action potentials that are leaving through the ganglia selves so again it's modulating the actual action potentials moving through the visual pathway the whole purpose of these horizontal cells and these amacrine cells is to modulate the actual visual pathway to make sure that it's very precise to make sure that we're adapting to it to make sure that the visual field is very very perfect okay so that's the function of these amacrine cells okay so now that we talked about that we got to talk about okay let's say that now you're no longer getting hit with light rays let's say that you actually are in dark so now there's no light race if there's no light rays than what's going to happen then well then it's just gonna be the exact opposite then if there's no light rays then won't this all-trans go back to 11 sis if there's no light rays so in the absence of light so no light over here right so you get rid of now you're in dark light or dim light right then what's going to happen here if there's this no light right if there's no light here this is actually going to cause the all-trans to go back to eleven fifths if that happens then the option will go back so now let's say it's like this let's say it's right here this will go from what all trans it'll go back to eleven sis all right then what will happen obstinate will come back and rebind here then it'll no longer be able to activate that's transducing if it can't go over here and activate this transduce and then what happens it's no longer going to activate the phosphodiesterase so now it's not going to activate this transducin here so this transducing is going to be inhibited it can't in actually activate the phosphodiesterase so then the phosphodiester is going to be inhibited if the phosphodiesterase is inhibited he can't break down the cyclic GMP into GMP so this process is inhibited so then what happens to the levels of the cyclic GMP it goes up if cyclic GMP levels go up then what does that mean oh that means I have a lot of this here so what would happen to all of these channels all of these channels would open up because cyclic GMP would be bound to them if all of these channels open up who's going to start flowing in sodium and calcium and as sodium and calcium start flowing in to this actual photoreceptors what's going to happen then the cell would become increasingly more positive and as the cell becomes increasingly more positive it's going to start depolarizing right but again it doesn't generate action potentials it generates graded potentials or receptor potentials so again what's coming in here sodium ions and calcium ions all right so sodium ions here and calcium ions here coming in coming in and coming in all right then it's going to convert it from hyperpolarization it's been going to become depolarized and when it becomes depolarized what's that called it's going to generate what's called a epsp now it's going to become a epsp if it becomes an epsp then what happens to the action potential I'm sorry not action potentials receptor potentials moving down the axon it's going to increase you're going to have a lot of receptor potentials moving down if you have a lot of receptor potentials moving down this axon what does that mean for these calcium channels these calcium channel is going to be super active and calcium is going to start flowing in very very excessively that's going to stimulate the release of more glutamate huh if I release more glutamate what does that mean then if I release more glutamate I'm going to inhibit these actual bipolar neurons if I inhibit the bipolar neurons I'm going to cause them to generate what's called ipsps if they generate ipsps they'll have less receptor potentials moving down if there's less for sector potentials moving down they'll release less glutamate if there's less glutamate there's going to be very little action potentials moving down the axons of the ganglion cells and down the optic nerve okay and it's because it's in the absence of light that should make sense so and then there's the absence of light there's very little action potentials moving down the optic nerve when there's a lot of light there's going to be very many action potentials moving down the optic nerve and again the horizontal cells and the immigrant cells are modulating these actual this movement up through the visual pathway horizontal cells by releasing gaba immigrant cells by releasing chemicals like dopamine or acetylcholine or dope gaba or glycine and basically modulating the vertical transmission and the photoreceptor transmission between these guys okay holy crap I think we pretty much covered all of that part for the photo transduction all right now what we're going to do in the next video guys is we're going to go ahead and talk about what's called the pupillary light reflex so we're going to have to talk about what happens during dark adaptation and this light adaptation and how our pupils respond to that and then after that we'll talk about the visual pathway itself engineers I really hope all of this made sense I really really do hope that you guys liked it and hope it helped if it did please hit the like button comment down the comment section and please subscribe re-engineers as always until next time