- [Instructor] All right, we are starting special senses. So that is our new system or your sensory system. Now, we're going to review five different senses overall. The first one will be touch. The second one will be olfaction or smell. The third will be... taste. The fourth will be hearing. And the fifth will be sight. Now let's start with the first one. The first one will be touch. Now I'm not gonna draw this out for you because we already learned about the integumentary system where we covered epidermis, dermis and hypodermis. Now these will be in the dermis that I'm gonna be talking to you about. And I'm just going to give you a list of them so we don't rehash the integumentary system. We're gonna pay attention to five different types of receptors that will be essentially mechanoreceptors that will enable us to experience touch in different ways. So let's just start with our list. Our first one for type of receptor is gonna be something called Merkel's discs. Now remember these are all in the dermis of the integumentary system, and each one is going to be specific for the type of touch it will sense. So this first one will sense light touch. The second one will be... Meissner's corpuscles. I always think that's a fun word to say. Those will do fine touch. Fine touch, so those are more like on your fingertips for really fine detailed touch for your body to pick up. The next one will be Ruffini endings and those will sense stretch. And you know I always try to come up with weird ways to remember things that will stick in my brain. So Ruffini, ruff makes me think of a dog. So I think about whenever you wake a dog up and they do a huge stretch to wake themselves up, that's how I remember this. But you know, as always, take it or leave it. The next type will be Pacinian corpuscles which will sense deep pressure. Another weird way of remembering things is Pacinian it always makes me think of like Pac-Man. So I think about Pac-Man sensing deep pressure, like shoving your hand into the side of the face of a Pac-Man. Again, you know I have weird things, weird ways to remember things. Now, the last one that I want you to know for types of the types of receptors in the skin are free nerve endings, which sense pain and temperature. All right, so let's see. Is that all I want to cover? Yeah, so that's the basics that I wanna cover for the first special sense, which is touch. Now, we're going to move on to our next one, which we said is smell. All right, so special sense number two, olfaction or smell. You should remember that word from the cranial nerves that we learned. Cranial nerve number one is olfaction or smell. And remember we talked about that cranial nerve is the one most easily injured in motor vehicle accidents or car accidents. All right, so let's kind of approach this how we have with other things. I'm going to start by drawing someone's face from a lateral view. We're gonna do like a midsagittal cut, or it might be just off from midsagittal but let's start by drawing someone's face. All right. Obviously that's a lateral view. So I'm not gonna label that for you. We're going to start with one big area right here that smell will travel through. And that's going to be called our nasal cavity. Now, of course we have here, I'll add little nostrils in for this guy; he's gonna have nostrils. The smells are gonna go through his nose into his nasal cavity. Now our next color we're going to have our cranial nerve one. I'm just gonna represent by three little lines there. But in reality, it's a lot of little tiny ones going through our cribriform plate if you remember that. So I'll draw them a little bit longer. Again, that is cranial nerve number one, olfactory. Now remember, I just said that that was our cribriform plate that it goes through. So I'm gonna draw that in blue. Now what that cranial nerve is going to travel through the cribriform plate, and then go to something called the olfactory bulb. There's actually... so this is what I was talking about in terms of me creating just off from midsagittal because that green structure is our olfactory bulb, but we actually have a left and a right one that's like just lateral to the crista galli and a little bit like superior to that ethmoid bone. Remember, like we talked about with the cribriform plate. So then it goes from that cranial nerve one into the olfactory bulb. Then it's gonna go through something called the olfactory tract, but I'm adding one more structure in here. I wanna make this old little bit longer. And then we'll give this guy a little ear for reference. Now, do you remember? We have this little area where a specific gland sits. And it just kind of pokes in right there. That gland is our... pituitary gland. So this is gonna kind of pass around the pituitary gland. It's gonna look like it's passing through it but that's just because of the view that I'm showing you. So let's... we're going to have it traveling from our olfactory bulb. And now it's gonna go through something called an olfactory tract which is it gonna be right there, olfactory tract. And the last thing I'm going to label here is going to be some pink lines here. Now that is going to represent our olfactory cortex. Now, the reason why I drew three pink little lines like that is because our olfactory cortex is not a single structure. So it's a bunch of different regions. They tend to usually be centered in the temporal brain or excuse me, the temporal lobe, but they could go other places. But just again, know that that's not a single structure. So I'll put in here different regions just as a reminder. Now, this is interesting because olfaction is actually the only special sense that doesn't go through the thalamus. Most other things do. So again, just to review, you're gonna smell things through your nose. It's gonna go into your nasal cavity. Your cranial nerve number one is going to... transport that sensation of smell through the cribriform plate into the olfactory bulb. Then that's gonna go down through the olfactory tract and into the olfactory cortex, which is a bunch of different regions, mostly in the temporal lobe. Now, olfaction is really important. Remember we talked about car accidents but it can also be an important diagnostic feature for different diseases. For example, Parkinson's disease, oftentimes the first few things for patients that have Parkinson's disease will present with are resting tremors in their hand, so almost just like something like this, or like it's called like a pill rolling tremor, or impaired sense of smell or impaired olfaction. All right, so that is our second special sense. That is smell. Now, what we're going to do is we're going to move on to the third one, which is taste. So I'm just gonna move this paper up. Taste, this one's pretty simple. All we're going to do is we're going to first create a list. Now we have taste buds, and those are essentially chemoreceptors... or they're receptors that receive chemicals. And there are a few different types of taste buds that pick up different tastes. So there is salt, sweet, bitter, sour, and umami. Now I'm going to draw a little tongue here. I'm just gonna make it pink for fun because tongues are pink. And we're going to split it up into two different regions. Here is the... posterior 1/3 of the tongue. So I'll just draw a little dotted line across it. Now, if you remember we learned this when we learned the cranial nerves. I'm gonna leave this up to you as a little test question before I write it out. Think about what cranial nerve innervated the posterior 1/3 of the tongue. I believe I put it on your cranial nerve chart. If I didn't, that's on me. But if you do remember, it is cranial nerve... nine, glossopharyngeal. Now, we're going to take a look at the anterior 2/3. So I'll put this dotted line in our blue here too. This is our anterior 2/3 of our tongue. And the nerve that helps us sense taste in that anterior 2/3 is cranial nerve seven which is facial nerve. Now it's a specific branch of the facial nerve, actually, which I'll write down the name for you is chorda... tympani. And that's all you need to know about our third sense taste. I'm going to go into a little more detail for our fourth and fifth special senses just because it's a little bit more of an elaborate system. It's a little more complex in terms of when patients have issues with those senses and just a little more involved overall. So let's move on to our fourth sense, which is hearing. Alrighty. Now let's start off with a coronal cut of the brain. I'm gonna do my best to attempt to not mess this up. It may or may not work. All right, there's my weird, funky looking brain, as you know. Let's see, let's draw some ears on the sides too. Now, these two little lumps here will be our... let's see, where do I wanna write this? This will be our temporal lobes. Now if we're hearing from our right side of our ear, it's going to come in through our ear and we'll go through more details about how it does that but it'll come in through our right part of our ear. You're going to have some of it go to, oops, I forgot to label, right and left. You're gonna have some of it go to our left temporal lobe. And then you're also gonna have some maybe go to our right temporal lobe. And we're gonna do the same thing with hearing from the right-hand side. Some of it's gonna go through your ear, and it's gonna go into our right temporal lobe. And then the other part is gonna go into our left temporal lobe. Now, the reason why I'm drawing this out for you is because this can be really important for anyone with a brain injury or stroke. As you can see, hearing from your right and left ear, it goes to both sides. So that's what's really nice is if you have an injury, for example, in your left temporal lobe, you're not going to lose hearing because if you, let's say, even if we... I'm looking for something to block, it, ah, I'll use an eraser. So this eraser is going to be our stroke. Let's say it takes out part of our left temporal lobe. As you can see, we're still gonna have hearing coming in from the left ear and from the right ear. So someone will not lose hearing. However, if you have damage to the actual single nerves, so if you have damage where my eraser is now it will damage the nerve itself. And you'll lose hearing just from that ear. So hearing tests are actually really important for neurological diagnosis, and it can be just very useful. So keep in mind that you can have lesions along different parts of the nerve or the brain and that it'll present with different outcomes. All right, so what I wanna do next is I wanna go into some detail about the actual pathway of hearing and how that actually occurs. It's going to be a little bit complex or not quite complex but just it'll be long visually. There's a lot that we're gonna draw out together. So I'm going to actually use a piece of paper that is, that will be, is this hot dog style? I think it's considered hot dog style like this. So it's gonna be a little bit of an extension of everything. If you are using a notebook that's open, you think about using two different pages for this whole pathway but we're gonna split it up into three different parts. So actually what I'm gonna try to do is I'm gonna fold this into thirds. Or you can just cut it into thirds with your own visual capacity on attempting to fold it, just to make it easier on myself. That's going to be helpful because we will be splitting things into three different parts. So again, we're still on that sense, special sense of hearing. We're going to split this into our, oops, I wanna make sure that you can see most of this page. There we go. Our external for outer ear, our middle ear, and our internal or... inner... ear. All right, so those are our three different components that we're splitting this up into. You in turn have, I don't know if anyone has ever heard of like otitis externa. That means external ear infection. Otitis media means middle ear infection. And otitis interna means inner ear infection. And so those are different diagnostic terms that people will use when a patient presents with an ear infection. They'll kind of label it based on what part of the ear someone has an infection with. Now I'm going to also, let's see, I'm gonna give myself one more fold here as a reference. You don't have to do that, but I'm going to. Alrighty. This horizontal line that I just created will be kind of my little guide in terms of the different aspects of the ear that I'm going to draw. Now, I'm gonna draw a few different things that will separate the different sections of the ear but let's start with the external or outer ear. Let's do... all right, so we're gonna draw a little ear. That pink is going to be our C, auricle or pinna. So that's that whole big floppy section over your ear that you have on the outside. Now you're going to have a portion of it that is the very outer part of that outer ear. And then that's going to be drawn in blue here. So someone's got a funky colored ear. But that part is called the helix. And you have another section that's called the antihelix. It's kind of like in Spanish where the arm is called (speaking Spanish) and the forearm is called anti- (speaking Spanish), so a little fun fact for you there. Now, remember, does anyone remember we put our fingers on a certain part of our ear when we were talking about it having innervation or branches coming from the vagus nerve which does our... excuse me, parasympathetic innervation. Do you remember what that little piece was that we talked about? I'm hoping you do, but even if you don't I'm going to draw it in. It's this little section right there and it's called a tragus. All right, so now we're gonna move to our next part here. We're moving through our external outer ear. That's a little, that's obviously way longer than our actual external auditory canal is, but that's our external auditory canal, which goes from this whole auricle or pinna part and it connects us to the piece that separates our ear from the external ear to the middle ear. So that again is called our external... auditory canal. Now we have a specific membrane that's gonna separate the external and middle ear. Let's see, that makes this a little bit thicker. All right, so that pink structure that I just drew is called the tympanic membrane. I'm gonna draw it under our list of terms here on the external or outer ear, although it is considered remember that separating piece that separates the external from the middle ear, so tympanic... membrane. All right, so what's interesting is that tympanic membrane is actually what we know as the eardrum. So I'll write that down, eardrum. And it's called an eardrum because it is vibrated with sound waves that come in through that external auditory canal. Now, after that, it's going to go, the sound waves are gonna go in and hit a series of different bones. All of these bones are actually gonna be housed in that petrous ridge. Remember we talked about that when we were talking about the skull, and I may have put it on one of your quiz questions. But so these are the bones that are gonna sit in those petrous ridge. They collectively as three are called ossicles. So I'm just going to write that name down for you so we have it, ossicles. Our first one, I'm just gonna draw in the approximate shape but it's gonna look a lot smaller. So this is all for reference. These little bones that I'm gonna draw are all gonna be super big here just for you to see it. But in real life, they're super, super tiny. So we can take a look at that later, but just know that these are blown up for size but they're way smaller in real life. The first ossicle is called malleus because it kind of looks like a mallet. The second ossicle is named incus, and the last one is called stapes. So MIS, so that abbreviation from lateral to medial on both sides, of course. Now these ossicles, the function of them really is to amplify sound. And the actual definition or the meaning or etymology of ossicles is little bones. So remember we have those three little bones. You're also going to have this little tube that's gonna come right down through here in blue. Actually that's just a little area that the ossicles are sitting in. I changed my mind. I'm going to use green here. Now that little green tube is your eustachian... tube... or auditory tube. That's actually gonna go down and kind of meet up with your nasopharynx which is like the back of your throat kind of behind your nose, to get air into that cavity. But what's interesting about it is if you're sick and you're making a bunch of snot and mucus and all of that, it can fill up too much. Remember, 'cause we talked about nasopharynx, naso means nose, and that's where this leads to. And if your snot is filling up all of your nose and then it ends up traveling up into the eustachian tube from the nasopharynx, it can actually kind of put pressure and keep the tympanic membrane from vibrating, which is why sometimes when you're really stuffed up you might feel like you have trouble hearing. Now the little separating structure, remember, between external and middle ear was the tympanic membrane. We're going to have something that separates the two here as well. So the separating factor between middle ear and internal ear is going to be our oval window. Now, that covers that for the middle ear. We're now moving into that internal or inner ear. You're going to have a little structure. Actually, it's a little bit bigger, but I'm gonna call it little. And it's gonna look, you may have seen something in pictures that looks like a little bit like a seashell. So that's what this is gonna look like. And that is going to be our cochlea. Now we're going to have another section over here that's kind of gonna branch off from the cochlea. That is called the vestibule. And within the vestibule, you have your utricle and saccule. Now coming off from that, you have three canals that we call semicircular canals. I'm just gonna draw them as three little canals here but they're actually oriented in a much more complex way. And you can see that in images online or in your book. Those are your semicircular... canals. And this whole area is very important for balance in general. And the semicircular canals basically tell you where you are in space. Now, after it either it comes to the cochlea and the vestibule and semicircular canals, you're gonna have different nerves that transport the certain information that originally came from the outside of the ear. I have a nerve that comes out of the vestibule. I wonder what that's gonna be called. Welp, it's called vestibular nerve. Then you're going to have a nerve coming out of the cochlea. I'm using lots of colors here. What's a color that I haven't used yet? I need the green again. This is going to be called cochlear nerve which makes sense, right, coming out of the cochlea, cochlear nerve. Now both of those are going to come together, and it's going to farm one of our fantastic cranial nerves which is just gonna be boring gray because that's the only color I have left. That is going to be cranial nerve number eight which is... vestibulocochlear nerve. Remember nn is my little abbreviation for nerve. Now we have this beautiful, if I don't say so myself, beautiful picturing of the different aspects of the ear. Now, as we're going over this I want to explain to you that we've different types of hearing in different sections. So in this external or outer ear, we're gonna have something called conductive hearing. Now conductive hearing means that... it's a type of hearing that is produced because of sound waves moving through. Now, that's gonna, remember, go through and make the tympanic membrane or eardrum vibrate because of those sound waves. We're also gonna have conductive hearing in the middle ear because those sound waves are gonna take these three little ossicles or bones and they're gonna make them actually vibrate to amplify that sound as it makes its way through the oval window. And then in our internal or inner ear, we're going to have... neural hearing because the cochlea is essentially going to change the sound that's being transmitted from sound waves into an actual neural impulse. That's gonna go out through our vestibular and cochlear nerves to create the vestibulocochlear nerve to give us all of those different impulses for both and balance and hearing. So again, your vestibular nerve will do more balance just like we talked about with this vestibule right here, and then your cochlear nerve will transmit hearing. And then together, they'll go through that cranial nerve number eight and give that message to the brain. Now, this is very important because especially, you know we talked about the vestibule as being very important for balance. It can also, if you have an issue with your vestibular system, you can have something if you've ever heard of vertigo, which is basically a sensation feeling like you're spinning. And there are so many different causes of vertigo like there can be issues in your brain that impact the vestibular nerve or the vestibular aspect of vestibulocochlear nerve. You can have issues going on with your semicircular canals and your vestibule with your utricle and saccule. What's important to know though as a future clinician as many of you will be is that there are some fairly easy fixes for simple types of vertigo. So there is a vertigo called BPPV. And with that, a common complaint that patients will present with is, for example, feeling that when they turn their head, like if you're rolling in bed and your head is turning from one side to the other, that they get dizzy, and then that subsides after a while, like 30 seconds to a couple of minutes, that they're actually really simple maneuvers to diagnose and treat that type of vertigo or dizziness. Now I've had patients, for example, who had brain tumors, and she had damage to the part of the brain where the vestibulocochlear nerve is going to travel to. And it had some issues in that area in the brainstem as well. For that, there's not necessarily anything we can do to treat actual structures because the structures have already been damaged. But at that point we can do vestibular rehabilitation which is usually done by physical therapists. And we can essentially train someone's vestibular system to be less sensitive. So for this patient, for example, she would get really dizzy with different visual stimuli. She would, especially when she was sitting and waiting at a stoplight, the cars that were passing by in front of her would really throw her off visually and make her feel dizzy and really impacted her vestibular system. So what I did was I did some retraining, like I took videos of cars passing by and I had her watch them and basically trained her vestibular system to not be as sensitive to that visual input. So just know that there are oftentimes treatment for vertigo, whether it's treating the structures by having a physical impact on them, or just retraining the system as a whole. I think that does it for hearing. Oh, something else I wanted to discuss is that, you know what? Nevermind, that's good. I think I'm gonna leave it at that; that's our hearing. Now we're going to move on to sight. Sight is probably the longest one that we're going to cover, so hang in there with me. Just 'cause site is so complex, there's a lot of stuff that we're gonna review together. Alrighty, number, oops, I'm using red. Number five, sight. Let's start with a sagittal cut of the eye. All right, so first let's draw... this is going to be, remember, a sagittal cut. So here is anterior. Here is posterior. I'm going to draw a couple different structures just to give us a little base here. All right, that first green structure, the anterior-most structure is the cornea. That's actually avascular meaning that there are no blood vessels in it. No, what else, oh, I drew the brown structure which would be good to know. That is called the sclera, which is actually white in eyes. But obviously I can't draw white on this white paper, so it's brown. And the sclera is actually continuous with the cornea. Now let's see, let's draw another layer a little bit like just deep to the sclera. Let's make it light green. That light green layer that's just deep to the sclera is our choroid. Now, that is our vascular layer of the eye. And then we're gonna draw one more layer that is deep to that in light blue. Alrighty, that light blue layer that we just drew is called the retina. More on that retina later, but just so you know that retina is really a neural layer, so that's where receptors are basically living. So you can see these receptors called photoreceptors that help us get to our optic disc to transport information to our brain. But we're gonna do one whole different picture of that. But something that we're gonna review later is something that's pretty important, and it is called the... it's like right on the retina. And it's a specific part of it called the fovea centralis, And I'll go into more detail when we do cover that, the retina specifically. Now there is a couple of little... or one body made up of a couple of different things. So I'm gonna draw this one body on the top and one body on the bottom. Those are called our... ciliary bodies. And within that you have ciliary muscle. Remember mm is our abbreviation for muscle. And then we're also going to have... ciliary epithelium. Remember that epithelium term, that outer layer? It's rearing its ugly head again. This is really important because the ciliary epithelium is what actually makes something called aqueous humor. And aqueous humor is what fills this whole front part of the eye that we're going to call... our anterior cavity. And if we have an anterior cavity, what do you think we're gonna have here? A posterior cavity. I'll just make a little note to the side. Aqueous humor is what fills up the anterior cavity. Oops, now you can see that. Vitreous... humor... is what fills up the posterior cavity. And just like the name sounds, aqueous humor is a liquid, and the vitreous humor is a very thick liquid or almost like a gel. Now what's gonna help us separate this anterior and posterior cavity is going to be something called a lens. And so our ciliary body is, kind of it's that muscle that helps control the lens but you're also going to have ligaments that connect that muscle to the lens. And those ligaments are going to be... here we go, little ligaments. That's our suspensory... ligaments. That's gonna collect to our big important structure here which I'm gonna make in a nice blue, and that is our lens. More on the lens later as well. We're gonna talk about how images kind of go through there. Now, a few more things to note anteriorly, you're going to have something called your iris and you're gonna have two different parts of your iris. We're gonna draw them both. You're gonna have some that are kind of lined up like this and some that are lined up like that. And I'm gonna draw you a separate picture of that in just a moment. But that as a whole is our iris which is a muscle. Now in between that is our pupil. This is important because this is going to separate us into two different structures within this anterior cavity. So this orange structure is going to be our anterior... chamber. And this here is going to be called our posterior... chamber. Oh, one thing I forgot to say is that remember that choroid, that thing that we drew in light green? That's actually going to be continuous with something that I want to draw in red if I can actually find my red pen, which I don't think I can. So next best thing is pink. So remember it's that green choroid that is consistent with this structure that I'm gonna draw right there. And that's your conjunctiva which basically lines the inside of the eyelids. Now, the retina we're also going to have, kind of traveling right through here we're gonna have a central artery and vein that's gonna help supply blood supply. But I think that's all I need you to know about this view of the eye right here. Now we're going to break this down into a couple more specific views of the eye. So you know, what I'm gonna do actually is the iris, in this darker turquoise right here, remember it's this area right here, we have two different parts of it. You saw a sagittal cut here, but now I'm gonna show you like a coronal cut of the eye so we can take a closer look at those, at the iris. Coronal cut... of the eye. I'm gonna draw in the same color just for consistency. I'm actually going to draw the other aspect. I'm gonna make it a little more pink just to separate things for us, but I'm still gonna put those lines in there. Remember, these are those two-directional turquoise lines that I had in the other picture. And I just broke them down. I'm gonna make this brown just so we can separate the two different kinds of turquoise things right there. So the brown is going to be the outside part, these vertical lines right here. And then the pink right here is going to be these horizontal lines right here, right, since this is a sagittal cut and this is a coronal cut. So that brown outer part, that part of the iris is going to be called the pupil dilator, which will basically open up the pupil which is the sensor part right here, pupil. And remember we have our autonomic nervous system which is split up into sympathetic and parasympathetic. With our sympathetic system, it's gonna be our fight or flight system. So with that, we have to be able to get more light into our eyes, which we do by dilating our pupil to allow for that light to come in. So this pupil dilator is going to be controlled by the sympathetic... nervous system. And then our pink inner part is going to be our pupil constrictor, which is going to be controlled by our parasympathetic... system. All right, so that's the first thing that I wanted to break down from that big picture. Now I want to break something else down, also in a coronal cut. Let's do our lens. So this was our iris. Now we're gonna do our lens. Again, I'm gonna try to do it in the same color. So we have our lens, which is a circle here. And then we had those gray suspensory ligament. Oops, I'll write lens out, lens. We had the gray suspensory ligaments, and then our ciliary body which was pink. So this is another view of it. Remember here's our lens in the sagittal cut. And then here is our lens in the coronal cut. Here's our gray suspensory ligaments. And then here's our gray suspensory ligaments. And then here's our ciliary body. Here's our ciliary body, just to clarify. Now, I want to talk to you a little bit more about this lens and kind of show you how things move through the eye itself. So let's draw another eye. That's a lateral view of a midsagittal cut of the eye. Let's draw that or write that again, sagittal cut. This is anterior. This is posterior. Now let's draw a little book. All right, that's my little book. With that book, we're going to have letters A... B... and C that we're gonna read in that book. Now let's put our lens in there. I'm gonna make it the same color. I'm not gonna draw all the rest of the things just because, you know, time reasons. You guys got it. So letter A is going to... come up to that cornea that we have. It's gonna come through and hit a specific part on the lens. And then what's going to happen is it's going to go back to our retina. Essentially what happens is a concept called refraction occurs, which means we bend our light rays. Now keep in mind, our cornea right here towards the front is fixed while the lens is moveable. Now we're going to go from this letter A through our cornea, to our lens, and it's going to refract or bend the light rays. So they're going to be upside down. They're gonna go from the lens to go back and hit the retina towards the back of the eye. But this letter A is going to then be inverted to the bottom, and it's going to look like that, like it's upside down when it's on our retina. Now our letter B going to go into the cornea, same spot on the lens, and it's going to be inverted on our retina at the back of our eye. And then the same thing with C, it's going to come in through our cornea, to our lens and then travel here at which point it will be upside down there. Now, remember this is called refraction, meaning that it bends the light rays and it sends them upside down. Then from that area right there which I will label for you in green is the retina. Then you're gonna have fibers that travel in through all the way up to the brain. And then when it gets to the occipital lobe, then it's gonna turn that image right side up. So of course everything that we're reading is right side up. It's gonna get turned upside down and then it's gonna go out to the brain and be turned right side up again. That's why the lens is important. Now, one more thing I want to zoom in. Remember we said, we'd talk more about the retina. Well, now's the time. Here's a coronal cut of the retina. So let's just we're gonna make a big circle. And I'm going to... draw a couple of things. First thing here will be our optic disc, also known as our blind spot. That's where we have no photoreceptors or things that see images. So it's essentially our blind spot because we have no receptors there. Now I'm going to draw something. It's gonna be a little bit of a blob here. And that's going to be called our... macula lutea. And then inside of that, we're gonna have a specific spot that is our fovea centralis. Remember we talked about that little spot right here, fovea centralis? Our fovea centralis is where you see the clearest. And that's gonna make sense in one moment when I explain that we have a few photoreceptors, remember those receptors for vision. The first one is going to be cones. And those are gonna be red and green and blue. So the macula lutea is really important because those cones are really concentrated in there. And that's why the fovea centralis specifically is a spot where you see the clearest, because you have many receptors in that area. You're also gonna have some cones splattered around here. And then more towards the outside, you're going to have your second type of photoreceptor which is rods, which help you see black and white. I'll actually make them little circles just so we can differentiate them 'cause that green and blue looks a little bit simpler, I mean similar, not simpler. All right, so here's the coronal cut of the retina. We have rods more towards the outside, cones toward the inside, our optic disc, which is our blind spot, and then our macula lutea with a bunch of cones in that area so we can see better. And then our best spot of vision is our fovea centralis which is where we can see the clearest. A few more things to cover in the eye, and then we'll be done with this sensory system. All right, so let's talk about visual fields. Now let's see, I'm going to draw a couple eyeballs out. So this is going to be a superior view. Here are two eyeballs. And these are connections to the eyeballs. So this is, here's our little nose right here. So this is anterior and this is posterior. Alrighty, so we're going to label a few different things. Oh, let me label one more thing, right and left. That pink arrow is our right... visual field. That green arrow is our left visual field. Now from our right visual field we're going to have arrows coming in here. And we're gonna have information traveling along here. We're also gonna have information traveling along here. And then from our left visual field, we'll have it hitting our eye from here and traveling along. And same thing from over here, it's gonna come from this right eye and travel right along here. All right, so this is important, and we're gonna label a few different areas. And I'll explain this mechanism in a moment. So right here, this area here and this area there, that is our optic nerve. If we have information from our left eye traveling through this optic nerve, if we have a lesion or a cut we're gonna lose vision of our left eye. Okay? We have another area, which is our... does that make sense? So I wanna make sure that that's clear. We have vision coming in from our left eye and it travels through here. So if we cut that, it's going to impair or it'll basically have us lose vision of this left eye which covers both the left visual field and the right visual field. Okay, now our next aspect here I'm going to draw in purple, and that's where things cross. That will be called our optic chiasm. Now, this is important because this optic chiasm is directly superior to sella turcica. Now, do you remember what sits in? Actually, you know what, that's so important that I'm gonna write it down, superior to... sella turcica. That is very important because do you remember what structure sits in the sella turcica? That's right, you got it, pituitary gland. So what's really interesting is if someone has a pituitary gland tumor, your pituitary gland is basically sitting right here. Now, if we follow these lines, if you follow this line it goes into this lateral area of our left visual field. And if you follow this line, it comes from this lateral area of our right visual field. If someone has a pituitary tumor, it's impacting this cross right where this happens right there at the optic chiasm. So oftentimes if someone has a pituitary tumor, their first symptom or their first sign will be loss of temporal or peripheral vision. So that's so important if someone just comes in and they say, "Oh, you know, I have trouble seeing off to the sides," if you don't know your anatomy, you don't know why that would be. So that's why this is so, so important. Now our last part that we're gonna label there is our optic tracts which are these guys right here. Now, those optic tracts are gonna go towards the occipital lobe. So I'll just label that for you. This is gonna go to the... left occipital lobe. And this is gonna go to the right occipital lobe. And remember those occipital lobes are the ones that they take that image that the lens had flipped upside down and they're gonna flip it right side up again. So remember what we talked about if you have a lesion of, for example, this right optic nerve, you're going to lose complete vision from your right eye. If you have a pituitary tumor in the center right here just over where this cross is in the very center, you're gonna lose your temporal or peripheral vision. Now, if you have a lesion or a cut or damage at the optic tract, what's gonna happen is you're gonna lose your whole right visual field. Now what's interesting is if you have a stroke in your right occipital lobe, remember we're gonna lose our whole left visual field again, and vice versa. If you have a stroke in your left occipital lobe, if we see we have these yellow colors coming from our right visual field, so we're gonna lose vision in our whole right visual field. Alrighty, so that covers that. One more thing we're gonna learn about the eye. All right, let's draw... an eyeball and a little nose. So we're looking at someone's face. We'll give them nostrils just for fun. Alrighty, so you have something that's technically part of the exocrine system. And that's gonna be a gland right up there called your lacrimal gland. And that's actually where tears are produced. Now, those tears are gonna go across your eye, boom, into two little things called puncta which are little tubes, little like openings leading to little tubes actually. Puncta, those are going to make way to our... little sac right here called a lacrimal sac. And that's going to make a way to our... nasolacrimal duct. So tears are gonna travel from lacrimal gland down across the eye into the puncta, through the lacrimal sac and then into the nasolacrimal duct, which is why our nose runs when we cry. And that's it. We covered our special senses; we covered... touch, olfaction or smell, taste, hearing and sight.