 Retina consists of 10 layers in which from outer to inner what we see is the outer is the pigmented layer and near this pigmented layer we have the photoreceptor layer which consists of the rods and cones. Then throughout the layers there are basically we see there are two pathways one is the vertical pathway where we see three types of cells that is the photoreceptors then there are bipolar cells and then there are ganglion cells and there is also a horizontal pathway where we see the horizontal cells and we see that these horizontal cells are connecting the different photoreceptors and then there are emercrine cells and these emercrine cells connect the bipolar cells with the ganglion cells. So just to see what are the 10 layers you see that the outer most is the pigmented layer then there is the photoreceptor layer then there is the outer limiting membrane which is basically separating the outer segments of the photoreceptors from their inner segments then there is outer nuclear layer outer nuclear layer and when we are talking about nuclear layer there is the nucleus present so there is nucleus of the receptors here then there is inner nuclear layer inner nuclear layer has the bipolar cells and basically the nucleus of the bipolar cells is there in between the outer nuclear layer and in a nuclear layer we have the plexiform layer so plexus is forming here synapses are forming here so we see photoreceptor synapsing with the bipolar cells and horizontal cells also synapsing with the photoreceptor cells and the bipolar cells then after in a nuclear layer there is inner plexiform layer so again there is a plexus forming synapses are forming so we see synapses of the bipolar cells with the ganglion cells bipolar cells with the emercrine cells and emercrine cells with the ganglion cells we will see later how they are all connected then finally we have the ganglion cell layer which are basically the output neurons of the retina and then there is the axons of these ganglion there which form the optic nerve and finally there is the inner limiting membrane so in between the three cells you see outer nuclear layer inner nuclear layer with the bipolar cells and the ganglion cell layer we have the plexiform layer where the synapses are occurring now you see the direction of the light direction of the light is from the inner to the outer side okay so the light has to cross basically all these layers and then it reaches to the photoreceptors where it stimulates the photoreceptors and it also reaches to the pigmented layer which basically absorbs all the extra light and it prevents scattering so because of this prevention of the scattering the photoreceptors are stimulated from where the light is coming and the light doesn't scatter everywhere and stimulate all the photoreceptors so with this basic premise let us move on to the connections which are present within the retina and how the processing of the information takes place within the retina now in the retina we have something known as center surround arrangement this center surround arrangement is actually a very famous difficult topic of the physiology so let us try to understand how this center surround arrangement works and what is its significance now this arrangement is present for both the bipolar cells and ganglion cells and because of these arrangement these cells respond best to a small circular stimulus which is coming in their receptive field so let us see one example here you see here three photoreceptor cells are shown and they all three are connected to a single bipolar cell and what we see here that there is a center photoreceptor cell okay this is a simplistic arrangement by the way it is not that only one photoreceptor is connected to one bipolar cell we see later about that but in this arrangement what we see is there is a central photoreceptor and there are surround photoreceptor so these two are surround photoreceptors and how they are connected to the bipolar cell the surround is connected to the bipolar cell by means of horizontal cells horizontal cells and these horizontal cells remember are always inhibitory okay now suppose there is a stimulus with light in the center so here this yellow portion is showing the light and this light is stimulating only the central photoreceptor now how do photoreceptors respond to light they depolarize or hyperpolarize they always hyperpolarize so there will be hyperpolarization in this photoreceptor and because of this hyperpolarization there will be decrease in the release of the glutamate okay and this decrease release what it is causing it is causing the depolarization of the bipolar cell okay in this example we are seeing a bipolar cell which undergoes depolarization when there is presence of light and which stimulates the photoreceptor so this undergoes depolarization now if there is light in the whole field you see this example if there is light in the whole field what is happening the central one is getting stimulated also the surround of photoreceptors are getting stimulated so what will happen the central one will hyperpolarize there will be decrease in the glutamate release and there will be depolarization of the bipolar cell but because surround is also getting stimulated and there is hyperpolarization decrease in the glutamate release and what it will cause it will cause the horizontal cells to become activated and this in turn is going to inhibit the bipolar cell right so if the stimulus is coming from the center and the surround is is inhibiting the bipolar cell what will happen to the bipolar cell response there will be no response understanding so the response is best when there is only a central light because there is no inhibition from the surround in this example first example if we see what is happening surround isn't dark and if surround isn't dark these photoreceptors are not able to inhibit the bipolar cell and the bipolar cells response by depolarizing so this is a simple center surround arrangement where the response of the bipolar cell is best when there is a small circular stimulus in the center so let us try to see a little bit in 3d here what we are seeing there is an on-center bipolar cell that means this bipolar cell will become on when light is in the center on center bipolar cell okay so here you see I have made one 3d arrangement where there is a central photoreceptor obviously there are many central photoreceptors on which light will be falling okay and they basically converge on a single bipolar cell right so light on center causes a depolarization of the bipolar cell but if there is light on surround what will happen the surround is going to inhibit the bipolar cell and the same bipolar cell is going to hyperpolarize but there is another type of bipolar cell that is the off-center bipolar cell that means if the light is on center central photoreceptors are stimulated which are connected to the bipolar cell then these photoreceptors are going to release a decreased glutamate right and this in turn is going to hyperpolarize the bipolar cell so there are two types of bipolar cell one which depolarize when light is in center and the other which hyperpolarize when light is in center and we have seen that how this bipolar cell acts opposite when light is on surrounds so these bipolar cells have a different name as well so on-center bipolar cell or we can say off-surround bipolar cell because when surround has light then they are getting hyperpolarized similarly off-center bipolar cell or we can say on-surround bipolar cell okay so because when light is on surround then they are getting depolarized so you got it that how this surround is working basically there is inhibition via the horizontal cells from the surround so horizontal cells are mediating this center surround processing so we till now have been talking about on-center off-center bipolar cells separate but actually all connections are in two parallel pathways let us see how so you see same rods are connected to a on-center bipolar cell and an off-center bipolar cell so there are two parallel pathways going on so these are basically the central rods which I have shown there might be receptors here right surrounding these rods and they will exert the inhibitory effect on these on-center and off-center bipolar cells now what is the need of these two parallel pathways because it's like two information here one is like here depolarizing right with central right and one is hyper polarizing with central right and you see when it is hyperpolarizing this hyperpolarizing bipolar cell is also connected to another ganglion cell which also will hyperpolarize and this will connected to another ganglion cell which will also depolarize okay so these are on-center off-center ganglion cells the two pathways are going on and these ganglion cells actually generate action potential the bipolar cells horizontal cells amacrine cells and the photoreceptors they do not generate action potential there is graded potential in these cells but ganglion cells are the ones when the stimulus will cross the threshold there will be generation of action potential but if the ganglion cell hyperpolarizes will there be action potential no right so there will be action potential in one pathway and there will be no action potential in the other pathway so what is the use of such connection well this mechanism is useful for separating contrast borders in a visual image which is exactly between two photoreceptors okay now we will not go into details of all this that how this particular image is going to stimulate the bipolar cells and ganglion cells it will become too complicated but yes you should remember that there are two parallel pathways the rods being connected to both on-center bipolar cells and on-center ganglion cells and the off-center bipolar cells and off-center ganglion cells and this mechanism operates together with the horizontal cell mechanism which inhibits the bipolar cells from a stimulus which is coming from the surround now let's move on to ganglion cells and before directly moving on to ganglion cells I want to focus here on emicrine cells so let us see this diagram here what we are seeing you see this side there are cones which are connected to the bipolar cells right bipolar cells is there and these bipolar cells are then connected to the ganglion cells so here is the synapses now you see the bipolar cells are also sending information to the emicrine cells and emicrine cells are then sending information to the ganglion cells so there are two pathways by which the information is reaching to the ganglion cells one is direct pathway where it is from the bipolar cells to the ganglion cells and other is the indirect pathway that is the via the emicrine cells okay so that is very important why it is important you see I told you that how the bipolar cells are connected to the ganglion cells and the on-center bipolar cells on-center ganglion cells off-center bipolar cells off-center ganglion cells now here you see the first diagram where it is showing the action potentials which are coming off from the ganglion cells what is happening when the light is switched on here there is action potential so which type of ganglion cell it is it is an on-center ganglion cell when light falls on the center the ganglion cells are depolarizing obviously via the bipolar cell and there is generation of the action potential but you see when the light is kept on slowly what is happening the number of the action potentials is decreasing okay why is it so that is because of the inhibition which it is receiving from the emicrine cells so direct pathway it is causing yes more action potential and there is indirect pathway via the emicrine cells which is inhibiting the ganglion cells and the number of action potentials is decreasing that means we are maximally sensitive to a stimulus which suddenly comes to our visual field like for example an insect which is sitting in your visual field may not stimulate your visual field that much okay that information may not reach the cerebral cortex that much but if suddenly starts flying what will happen you will immediately detect it so our visual field is more sensitive to a stimulus which is suddenly come into our visual field and why we are seeing the retinal processing which is happening at the level of the emicrine cells and here it is showing what here it is showing the off-center ganglion cell off-center ganglion cell so when there is light in the center there is no action potentials which are coming and you see what is happening here when there is off the action potentials have started but later on again the frequency of the action potentials is decreasing so i hope you understood the role of the emicrine cells in the ganglionic cell responses and how they help us in detecting any stimulus which is coming in our field by the way remember one thing that neurotransmitters released by rods and cones are glutamate that we have been talking the neurotransmitters which are released from emicrine cells many have been detected and there is a inhibitory neurotransmitters GABA glycine which is going to inhibit the ganglion cells there are others as well dopamine is there acetylcholine is there and there are different types of emicrine cells also been detected so as we are getting to know about the retinal processing it is getting more and more complicated moving on to the last part that is the types of the ganglion cells there are two types of ganglion cells in humans that is the p-cells and m-cells and the properties arise basically by the type of the connections which are happening in the retina right so p-cells also known as the parvocellular cells and m-cells also known as a magnocellular cells p-cells carry the information to the parvocellular layer of the lateral geniculate nucleus okay so this information is getting segregated at the level of the retina itself and magnocellular layer carry the information to the magnocellular layer of the lateral geniculate nucleus now these p-cells have smaller receptive field smaller receptive field okay plus they respond to color vision okay so basically these p-cells are connected to which type of photoreceptors mainly cones correct and how they are having a smaller receptive field see i talked about convergence before the photoreceptor conversion from the cones to the ganglion cells is quite less especially in the fovea so it is like two cones are converging into one ganglion cell only however the convergence of the rods is too much high 60 rods converge to one ganglion cell so that is how these p-type of ganglion cells have a smaller receptive field that basically equivalent to the receptive field of two cones okay so these p-cells tell us about what we are telling that they have a smaller receptive field they respond to color vision so they tell us about fine details in the visual field any image if it is there then fine details of the image as carried by these p-cells and they also tell about the color however any black and white image these p-cells do not carry that information that information is carried by magnocellular cells and as a word suggests magno they have a large receptive field and how it will be because they are connecting mostly to the rods and that too large number of rods how many 60 rods is to one ganglion cell okay and this information tells us about the low contrast stimuli in our field of vision low contrast stimuli and black and white stimuli basically black and white stimuli but remember that their responses are transient transient responses okay so these magnocellular cells tell us about the rapidly moving signals rapidly moving signals meaning what you see we are telling that the rods as photoreceptors are more sensitive to light right and they are more in our peripheral field of vision so anything which comes suddenly in our field of vision that is detected by these rods and conveyed via the magnocellular ganglion cells through the visual pathway to the cortex so these magnocellular cells carry the information about low contrast black and white image but rapidly moving image and they have transient responses it is very important that when we have a rapidly moving sensations the cells should adapt fast their responses should stop very fast on the then the parbocellular these responses are actually sustained responses of the ganglion cells so let us see in summary what are the aspects of the visual image for which processing is occurring in the retina first is at the level of the rods and cones right so for that I have made another video there are videos on color vision also there is video on photo transaction where we talk about dark and light adaptation as well then what we discussed today is about the lateral inhibition okay which is very helpful in detecting the contrast in the image lateral inhibition so there is centers around the field and this contrast edges in the image are also being detected by the two parallel pathways which we saw that there is connection between the photoreceptors and the bipolar cells with both the on and off center types so there are parallel pathways then third we saw how the connection between the bipolar cells ganglion cells photoreceptors there are different types of the ganglion cells and how they help us in detecting the rapidly moving image right and the sustained image how it helps in differentiating the color the fine details of the image which is there so all this is happening at the level of the retina itself then there is color processing also happening that is how these ganglion cells are connected to the different types of cones and this is basically the color opponent theory which I have discussed in theories of color vision video so please have a look on that that how there is different types of connections between the sm and l types of cones with the ganglion cells so that is color opponent process theory okay then obviously the retina processing also talks about the location of the stimulus which is there in the visual field because that particular bipolar cell and ganglion cells in that particular connection it will get stimulated so that also we are getting the information from the retina and also the information of the specific direction of movement you see when it is moving from one side to the other then there will be a pattern of stimulation in the visual field of ganglion cells and also the connections in the retina are such maybe the rods here are connected to ganglion cell bipolar cell like this and again here they are connected to different bipolar cells and ganglion cells so there is overlapping it is not that it is segregated right so by this kind of connection we will get to know about the specific direction of motion which is happening in the visual field so I hope that I was able to make you learn about something about the retina processing of the image and how it is helpful in getting the details of the image which is present in the visual field thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel Physiology Open thank you