hi everyone welcome to Professor ohms lectures in anatomy and physiology I'm professor Pablo this series of videos was intended for use by students enrolled in my human anatomy and physiology courses at Del Mar College if anyone else out there in YouTube land finds these helpful great use them otherwise they're really designed for my students how I test how I teach you what I think they need to know and as you know we're in the coronavirus shutdown so I'm a face to face guy trying to teach online so this is my method of delivery so you know they're done crudely if you hear any barking or in the background for my dog or my kids because this shut down I'm having to do all this in my dining room so you know please bear with me alright this particular video is an INT is intended for my students in my human anatomy and physiology to course we're doing a series of visual physiology videos hopefully you just watch the video on the nature of light and what pigments are in the visible light spectrum at the end of the video I made reference to the fact that we're going to talk about the difference between what are called rods and cones so if you're following along in my class we're gonna be on the bottom of page 15 or near the bottom where it says drawn describe a rod and cone so the cells the neurons there's a specialized group of neurons in our eyeball in our retina of our eye called photoreceptors so we still haven't really started talking about the neural tunic we're going to but I'm setting up for that so the cells in our eyes the neurons in our eyes that detect photons of light are called photoreceptors because they receive photons of light photoreceptors contain the visual pigments and we just talked about the visual pigments in the last lecture those substances that absorb photons of light within the visible spectrum I know I'm squeezed over there forgive me okay now there are two major classes of photoreceptors and this is my abbreviation for receptor so I'm going to make photoreceptor that way just for short hair one of those types of cells called a rock and the other one is called a okay we're gonna draw on describe rods and cones and I'm gonna draw them here in this space before I do I want to give you some concept as to what some of the terminology I'm going to be using is so now if I drew an eyeball here this would be the front of the eye where the cornea is and then the back of the eye where the sclera would go back towards the optic nerve from the optic nerve comes in a little bit inferior to here as we know that would be our fibrous tunic and then we have a layer of blood vessels called the vascular tunic that comes into the eye and makes up a layer of the choroid it has the ciliary muscle and the suspensory ligaments and it also makes up the retina I'm just going to use already for our vascular tunic okay not the man that makes up the iris and pupil now just deep to all of that there's gonna be a dark layer of tissue that's going to cover just deep to the to the choroid layer it's gonna go all the way around here it's gonna run on the back of the iris and it actually is going to cover the back of the ciliary muscle as well that dark layer is going to be called a pigmented epithelium we're going to define what the pigmented epithelium is after we talked about the rods and cones okay now inside of that there's a layer called the retina and the rods and cones are going to be lying inside the retina the retina is going to cover the inside of the eyeball here like this but it stops short of the front of the eye and that would be our aura sirata lining the inside of the eye the retina would be like the paint on these walls here lining the inside now the reason the retina stopped short at the aura Sarada and doesn't cover all of the inside of the eye like the pigmented epithelium does is because because of the openings where the cornea and the pupil are no mite coming into the eye at any crazy angle it's going to land anywhere more forward than this spot I didn't draw that very well so why would I put photoreceptors where no photons can land so Mother Nature is efficient so we don't put photoreceptors where we don't need them and so the aura sirata is the end of the retina where there's no more photoreceptors normal retina but the pigmented epithelium continues on to the back of the ciliary muscle and the iris now these cells since this is outside the eye and this is inside the eye and my drawing this would be outside the eye and this would be inside so we're literally cutting sort of this and looking at what I look at a rod rods are funny shaped bipolar neurons when you study part one A&P you know that we had neurons in that many many dendrites called multipolar neurons and a single axon where they would synapse and release neurotransmitter we also had bipolar neurons that as the soma in the middle like this and we had unipolar neurons unipolar neurons are usually incoming sensory neurons in the spinal cord well since these would be the dendrites and this would be the axon it turns out that photoreceptors are really just kind of modified neurons they have dendrites and an axon but the dendrites don't look right and it turns out that the dendrites are going to be facing towards the outside of the eye facing the back walls if I draw this neuron this would be the axon then we would have a solo with the nucleus of the organelles and peri carry on and then I have this long rod shaped or cylindrical shaped segment since this is facing outside this is called the outer segment of the rod cell and about where the somata would be the cell bodies this would be called the inner segment of our rod cell and the reason they're called a rod is because they look like a rod up here they're cylindrical shape the outer segment of the rod that's facing the back wall of the I sort of pacing the pigmented epithelium and it is a long cylindrical structure that is filled with little disks of membrane that are stacked in here now I don't know how many of you all know this unless you may be working a bank or in the business but when I was a kid and we would get money we would put little rolls of coins together like a little stack of nickels rolled up in a paper roll so when I look at a rod the outer segment of rod is filled with little disks that are made up of membrane similar to the cell memory if I took one of these disks out there are little disk shaped structures almost like a little hockey pucks and if that disc and the membrane of the disk are what we called the visual pigments and that would be the visual pigment that we described in the last video called rhodopsin now I know I used the red color but rhodopsin does not absorb red photons and so all of these visual pigments are literally staggered in here thousands of them in these discs so that if a photon of light tries to penetrate past the cell the likelihood it's going to get through this cell if it's within the visible spectrum and miss all the pigments is pretty low okay now just in case it does we don't let the photon of light to bounce off the back of the eyeball then bounce back out imagine if I shot a tennis ball at a really high rate of speed it would be bouncing off the walls in this room and every time it hits something it loses a little bit of energy because the walls would absorb some of it it would slow down and then eventually it would stop rolling around the room or start rolling around the room well I don't want a photon of light that is outside the visible spectrum to bounce off the back wall bounce off the back of the iris and then slow down and then trigger vision and there's a reason for that I'm going to use a different color okay if I shot on photon of light in here let's say it's ultraviolet light purple but if I shot some UV light in here it's at such a high velocity but as it comes into the eye if it bounces and bounces and bounces and bounces and then gets absorbed over here as it loses energy it then triggers visual physiology I wouldn't see the photon of light as coming from over here my eyeball would think that it's coming from somewhere over here a few seconds after I art actually could have seen it so when a photon of light hits a photoreceptor we want it to trigger that photoreceptor or get absorbed well that's what the pigmented epithelium does this layer here there's going to be a layer that we call the pigmented epithelium I'm gonna put PE there for pigmented epithelium if the photon of light is not absorbed by it further the photoreceptor it will get absorbed by the pigmented epithelium another thing the pigmented epithelium does is this as we trigger these visual pigments when they get hit and there's a part of them then that's made out of protein when they get hit with the photon of light they change shape and as the photon of light is absorbed eventually they will reset so the next photon can cause it and every time they get hit they trigger and then reset well just like anything if you folded and unfolded and folded and unfolded it will start to break apart even a piece of metal so the visual pigments can wear out over time the newer ones that are manufactured or closer to the soma the older ones get pushed out as they wear out we can actually shed those discs and the visual pigments and they get absorbed by the pigmented epithelium and so the pigmented epithelium is filled with visual pigments as well but it does not trigger a photo transduction since it's not a neuron so now when we describe a rod this is how we describe them verbally rods have an elongated or cylindrical outer segment filled with numerous membranous discs that contain the visual pigment rhodopsin again rods are photoreceptors that have elongated or cylindrical outer segments filled with numerous membranous discs and those discs contain the visual pigment rhodopsin now one of the things we know about rods I described in the last video that rhodopsin is active in a dark room or in dim light and they they do not allow us to see any color they are black and white vision or shades of gray now when we describe a cone a common is also going to have an inner segment which is an axon a soma with an abyss and their outer segments are actually tapered or conical in shape like this hence the name a cone they are also filled with these little membranous discs although some books will say that they have folds in the cell membrane but there's all these little membranous discs and the disk ingots shed and they get absorbed by the pigmented epithelium as well and on these discs are the visual pigments if I pull one of these discs out what I would see is on this disk instead of rhodopsin I might have one of the other visual pigments if this one makes the visual pigment to absorb those photons of the blue part of spectrum then we would say that these this would be IO Dopson 3 and this cone is sensitive to blue light or we sometimes refer to it as a blue cone because they're sensitive to photons in the blue part of the spectrum if I had another cone that made I adopted one then it would only make the visual pigment IO dobson one and only be absorbing photons in the red party spectrum so it would be called in red cones if they made IO Dobson two it would absorb a green light so we have three distinct populations of cones so before I go there let me just say this when we describe a a rod we say to have an elongated or cylindrical outer segment filled with membranous discs that contain the visual pigment rhodopsin cones have a conical or tapered outer segment filled with membranous discs that contain the visual pigments I adopt someone I own opsin - or IO Dopson three if a cone makes IO Dobson 1 it can only make I out up someone and absorb in the red part of the spectrum if a cone makes I own a person - it can only absorb in the green part of the spectrum and it only makes I own up sin - and if it makes a opposite 3 then it's a Bluecoat it can only absorb in the blue part of the spectrum so now on the bottom of page 15 we've drawn and described the rods and cones elongated or cylindrical outer segments visual pigments in the disks tapered or conical outer segments with membranous discs that contained the visual pigments I adapt some one two or three again Cohen can only make one type of visual pigment either I don't option one or two I own up to two or out of three they can't make more than one so the information I'm covering here is on the bottom of page 15 and it is all explained on page 16 in this little paragraph so make sure you read that and you understand it table so now we have this idea of what rods and cones look like we have this idea that they're facing the back of the eyeball now we're going to go to the very beginning of the neural tunic so we're going to jump back to page 14 for a moment and we're going to go and the order of the note set finally all right so we're going to be doing page 14 where it says the neural tunic and my notes set now I hope you got this I hope you've drawn it or have a good vision of it or an idea of it in your head I'm gonna erase it so that we can cover all of this information alright so the first thing about the neural tunic that I want you guys to understand is that the neural tunic is the third of the three coats of the eyeball okay we said that the eye has three tunics that's where we started our visual physiology the first one we said was the fibrous tunic we said the fibrous tunic is dense connective tissue mostly and it includes mostly the splitter and the cornea the second tunic we said was the vascular tunic the vascular tunic allows blood flow into the eye and has a lot of blood vessels but it has a number of structures it includes the choroid layer that includes the ciliary body it includes the iris and pupil we've talked all about that now we're doing the third tunic of the eye called the neural tunic because the neural tunic contains the neurons the neuro tunic is made up of two structures one of the structures it's made out of is called the pigmented epithelium I talked about this in the last video but I want to repeat it sometimes I repeat things on purpose because I want you to hear it over and over and over and over again till it sticks into your brain okay so the pigmented epithelium there's a layer of cells in the retina we should say containing pigments that can absorb photons of light that are not absorbed by the retina okay we can say that the pigmented epithelium is a layer of cells that contains pigments that can absorb the photons of light that are not absorbed by the retina or by the photoreceptors and sometimes we say that it absorbs those photons of light that are outside the visible spectrum because it can't absorb some of those all right so now the second layer of the neural tunic is the retina the retina contains three layers of neurons that send action potentials out of the optic nerve okay that's not really the best way to define it but the retina is three layers of neurons one of those layers of neurons is going to be the photoreceptors that receive the photons of light absorb their energy it will change their resting membrane potential and alter their physiology and then they're going to signal to other cells the layers of cells and will eventually send action potentials out the optic nerve okay I know my penmanship is not the best but I'm trying my best all right so now let's talk about retinal organization okay so on page 14 and asks you to define diabetic retinopathy we might talk about that but ultimately I think you should look that up as a matter of fact I'm gonna write this down for a second and I'm gonna move on to the next page okay so now when it comes to the vascular tunic its pigmented epithelium that absorbs photons of light not absorbed by the photoreceptors or those outside the visible spectrum the retina contains the photoreceptors that absorb the photons of light in the visible spectrum and two other layers of neurons that transmit the signal to our brain so now when it comes to what we call the red wall organizing actually I'm you talking about diabetic retinopathy real quick diabetic retinopathy you can look this up I want you to look it up but it's a pathology of the retina that is associated with diabetes now in diabetes we don't have the ability to process sugar properly to turn it into energy in our mitochondria we can't get the sugar into the cells neurons are the biggest wimps of the human body and if I cut off their sugar supply or their oxygen supply they start dying off rather rapidly so in diabetic retinopathy the photoreceptors in the eyeball are dying because they can't get their sugar it's a pathology of the retina due to diabetes now our blood vessels our blood cells have smooth muscle and then they utilize calcium to contract if you remember muscle contractions we may peel those little calcium channels allow calcium to flow in and calcium pumps pump the calcium out we need energy from sugar to pump calcium out of our smooth muscle in our blood vessels in the walls of a blood vessel without the sugar the calcium stays trapped in the muscle cells and can start to crystallize or calcify and as they calcified the blood vessel will eventually get clogged off almost like calcium salts in our modern lines clogging off our water decreasing the diameter and eventually it can clog up a water pipe so in diabetic retinopathy the calcium channels can function and the smooth muscle will start to harden and crystallize and the blood vessel continue can get constricted so if you look inside the retina there's a structure called the fovea and we have the optic disk if you remember from the models in lab and the blood vessels enter the optic disk as the blood vessels enter the optic disk they will fan out in a specific pattern and that pattern is unique to each individual but there's a certain pattern that should be there if these blood vessels get clogged with calcium then they won't show up they die off and so you get what's called when you get a lack of blood flow that kills the neurons our body in a last-ditch effort to try to sort of save those cells will start growing all these little branches and blood vessels around that blockage and so you get what's called a neovascularization neo meaning new vascular ization meaning forming blood vessels so sometimes your optometrists wrap the moments can look in your eye and go hey have you been diagnosed with diabetes they see some diabetic retinopathy anyway and neovascularization you need to know this definition you need to know that definition look them both up okay now in the notes that we're going to switch over to page 15 now we're going to draw on label the organization of the retina okay and this is important for our understanding of visual physiology so if I were looking in the eyeballs I'm gonna redraw this again here's the cornea here's the sclera the front of the eye and the back of the eye as you know my photoreceptors I'm gonna make a little blue cone-shaped cell here my kamon is gonna be facing in this direction obviously they're not this thick but I'm over magnifying the drawing is not to scale all of these cells would be squeezed into a very very thin layer but I'm gonna have my photoreceptor facing outward that way okay so if I were drawing this here would be my cone so I'm just gonna drop and large this drawing like this okay now the cone cells are going to be facing your pigmented epithelium which is right here so I'm gonna have to my pigmented epithelium again I'm just going to put PD here I'm gonna put the photoreceptor here so that we can see the layers in order now right beyond the pigmented epithelium there's going to be my choroid layer of my blood vessels and then outside of that is going to be sclera since the square is filled with collagen fibers I'll make it pink because collagen stains pink on your slides and it's going to be a very thick layer this almost is going to look like the slide of the retina that we do in lab so this would be the sclera the red would be our choroid and then I have pigmented epithelium and then I have my photoreceptors and the nucleus of the photoreceptor would be here in the soma and again there would be disks out here with pigments now the next cell in in row is going to be a cell that looks like a classical bipolar neuron here with the dendrite and the axon so this layer is called the bipolar cells the third layer of cells are going to be these short funny-looking cells that send axons out the optic nerve and so these cells are referred to as a ganglion remember a game Leon is a collection of neuron cell bodies outside the CNS so I would have a bipolar cell here and then I would have a ganglion cell send their axons out and if I had another series of three cells let's say I have a rod here then a bipolar cell then a ganglion cell all the ganglion cells no matter where they're at in the retina send their axons out the optic nerve to our eyeball now this is kind of counterintuitive to what you would think you would did since a photon of light that's going to come in the cornea it's going to pass through the retina I'm not the retina through the lens and then be focused on the retina you would think that the photoreceptors would be the cells that are facing where the photons of light are coming in because the photon of light is going to come in this way this is our retinal organization the photoreceptors are up against the wall I'm sorry the pigmented epithelium the photoreceptors are facing outside then we have bipolar cells and ganglion cells okay so backwards if I asked three of my students to line up and I'm going to throw a tennis ball if you catch a hundred of the tennis balls that I throw you get a point you would think that the first person who is has to be up against the wall the next person standing in front of them and the next person standing in front of them if I if they catch it one person kicks the other that person kicks the next person and the third person can stump on something on the floor that lights up on the wall so I can see where the tennis ball was caught you would think that it would be the person facing me would be the phone and receptor but imagine if the person facing me is up against the wall and then there's someone standing next to them and are in front of them and someone else standing in front of them and then they step on a button to tell me if they caught it when I throw a tennis ball the person who is the farthest away from me catches it then signals the next person which signals the next person and they tell my brain so a photon of light is going to end and if that photon of light lies outside the visible spectrum so if I do a photon that is outside the visible spectrum one that cannot be absorbed by our photoreceptors that photon of light will come in it will pass the ganglion cell it will pass the bipolar cell it will pass the photoreceptor and then will be absorbed by the pigments and epithelium that's the order of structures that a photon of light would pass into as its absorbed okay I like to ask that question sometimes please list the structures in order that a photon of light that lies outside the visible spectrum will pass into the eyeball if that photon of light triggers vision let's say it's a blue photon then when that photon of light comes in the eyeball and if this is one that's in the visible spectrum or what we'll say is a visible photon a photon of light that lies within the visible spectrum it will come into the eye and get absorbed by the photoreceptor and trigger it it turns out that when we're in a dark room photoreceptors are tonic receptors they're always releasing neurotransmitter sewing this bipolar cell we're blind when they absorb a photon of light there's a change in resting membrane potential there's a series of physiological steps similar to olfaction when we amplify the signal where the dentate cyclase and cyclic amp key it's a different series of steps we have phosphodiester agents transduce it and i'm not going to get into the steps but let's say that in a dark room with no photons this photoreceptor is releasing neurotransmitter telling this so we can't see anything when the photon of light is absorbed by the photoreceptor it stops releasing neurotransmitter then the bipolar cell fires an action potential telling the ganglion cell that it's gonna fire an action potential and then we send the signal out of our optic to our brain so let me repeat this when I have a photon of light that lies outside the visible spectrum what would be the order of structures that it passes as it's absorbed it would pass by the ganglion cell the bipolar cell the photoreceptor and is absorbed by the pigmented epithelium if a photon of light lies within the visible spectrum what would be the order of structures that fire action potentials or a fire a signal telling our brain we're seeing first the photoreceptor fires or actually it stops releasing neurotransmitter but it fires the second cell is the bipolar cell then the ganglion cell is triggered and then we send an action potential out the optic nerve to our brain so this is our retinal organization that you should know and that would be the order of cells that things are photons of light will need to pass by if they're not inside the visible spectrum or the order of cells that are triggered with fire if it lies within the visible spectrum now I'm going to cut the video here because we got to go to some other details of vision but we're we're just about done as a matter of fact before I do that let me make sure I want to stop okay I'm actually making up these videos as we go along so I don't really have them planned out in advance just like when I walk into class so let me do one last thing with you guys and then we'll actually be done with vision all right so there's two questions that are on page 15 of the note set that I got want you guys to understand right after this retinal organization so one of them is what is the fovea centralis and was the optic disc so I'm gonna erase all these notes and I'm gonna talk to you about what the fovea is and so a little bit more about the organization of the retina okay now I wish I didn't erase that eyeball there but I'm gonna redraw it anyway so it'll be night neither is finger okay so here's your cornea here's the sclera and as you know the optic nerve comes off a little bit low and downward actually a point immediately also we know we have the lens here with the ciliary muscle and the suspensory ligaments and the iris and the pupil I'm not interested in that I'm interested in the retina itself now we know that the optic disc is a spot on the retina I'm going to define the optic disc first because it's easier the optic disc is a spot on the retina it's a disc shaped spot on the retina where all axons of ganglion cells exit the eye via the optic nerve and there's actually no photoreceptors on the optic disc so fo photon of light lands on the optic disc we're actually blind to it we have a blind spot in each eye and by the way just so you know both of our eyes are not focused exactly on the same point they're off just a little bit which gives us a three-dimensional awareness of the world we don't see a flat world we see it 3d world but the optic disk doesn't have any photoreceptors on it so each eyeball has a blind spot as a matter of fact there's a really cool thing in your textbook somewhere I don't know where it is but there's a picture and you're not menuing your textbook it has a cross and a dot if you stare at the cross close your left eye and stare at one object and move the book eventually the other object disappears because the photons of light are landing on your optic disk so you can see the blind spot if you can you can experience it in each eyeball by following that little experiment now the fovea when I look at the retina if I can cut the eyeball in half this way and look straight into the eye so I'm looking at the redness of big disk then the optic disk is a little bit inferior and medial but there's a spot right in the center of the visual field called the macula you may have heard of macular degeneration that's where the neurons in the macula are dying usually in diabetic retinopathy within the macula there's a smaller spot inside of here called the fovea also called the fovea centralis because it's in the middle of the macula the central part of the macula now one of the things I want to show you is I'm going to redraw this next to it for a reason if I'm looking at the retina when we're staring at an object most of the photons are coming straight in the eye and they're going to hit this area and the ones that are spreading out might land into other parts of the eye but some of them are blocked but an overwhelming majority of the photons of light if I'm staring at a particular object are going to land right in a specific spot which should be right in the center of our visual field it turns out that if I start at that spot I'm going to use red as my color for cones turn