we are starting up with our first lecture video in the last section of our nervous system unit in AP1 which is special Senses all right so in the previous lectures you would have heard about especially mechanoreceptors and so in this chapter we're going to kind of take a look at the rest of the other sorts of receptors we see in the human body so primarily in this chapter this unit of this course we are going to be covering covering Vision hearing and equilibrium all right so if you remember back to our discussion of cranial nerves we mentioned the vestibular cochlear nerve cranial nerve and so I mentioned then what a vestibule is and that is the organ that helps to maintain sort of equilibrium in interference with that organ for example leads to sort of vertigo-ish type symptoms right so it's a really important organ for maintaining your upright posture and being oriented to what's around you and it's it's a you know really pretty critical thing if you think about it we'll also spend a bit of time talking about neurotransmitters and how they're involved with these sensory organs of course the other sense I guess if you want to think of it that way that we could talk about is olfactory and also taste uh but those two are covered in anatomy of physiology 2. in the digestive system we talk quite a bit about how taste happens as a sensation as well as how olfaction works so we are going to maybe mention them briefly in this chapter of AP1 and then you'll certainly hear more about it in ap2 all right so humans um even evolutionarily are a very very visual species everything about the way we sense the world around us is bias toward Vision right you can think about how uncomfortable you are even in a familiar place when it's dark it's just not something we do very well right night vision is not a thing that we do very well we are a diurnal visual species that needs to be able to see what's around us to be comfortable with our surroundings um that is evidenced by the uh relative processing power in our brain devoted to uh visual images right so um the optic lobe is a relatively large one the optic nerve are relatively large we have these other cranial nerves to control the eyes we've got a as a species we have a face that has a somewhat you're going to kind of laugh when you hear this but a shortened muzzle right so our faces are relatively flat as a species if you look at the most closely related other species to humans they have a much longer face and the reason for that is they devote a lot more of their processing power to things like olfaction but with humans being a visual species the face could be shortened that actually also helps the the visual perspective of the world around us um and you know we see stuff that's kind of what we do as a species a lot of our sensory receptors that we have in our body in general are just in the eyes alone right we think about all the other sensory receptors the mechanoreceptors the heat the cold crude touch fine touch vibrational receptors all these other kinds of things even when the olfactory and taste and Hearing and Balance sense 70 percent of all of those receptors are just visual receptors photoreceptors in the eyes all right so each eye alone has a million plus nerve fibers uh which you know is is a huge number when you consider that each one of those nerve fibers contains um multiple axons of multiple neurons right so it's a really uh densely packed region that has these photoreceptors in it we also go to a fair amount of trouble to protect our eyes we have reflexes built in that if something is all of a sudden coming at you unexpectedly you will blink to help protect your eyes um we have uh bony case around our eyes like where they sit in our skull is relatively safe compared to other places it could be so obviously you know the the loss of vision the potential cost of not protecting the eyes has to be much greater I'm sorry it has to be less than the benefit of protecting those eyes so uh we do a fair amount of things to help protect our eyes um we also have these little fatty cushions around our eyes to help give them some some protection kind of like we saw with a spinal cord having having fat in the epidural space the eyes also have fat around them to give them some cushioning this image here just kind of shows us a little bit of a it's a cartoonish kind of a crude breakdown but I actually like this one a lot of of the pathway of visual information from the world around us until eventually the point in time where it reaches our brain and we perhaps perceive that image so we're going to start off with the actual um visual world around us these two circles represent eyes this is the left eye this is the right eye so think about a face looking at you from out of the screen this is their right eye this is their left eye this is their left visual world this is the right visual world but the thing is and this also kind of goes to the maybe not the importance of protecting the eyes but the importance of having at least one functional eye is there's a lot of uh Crossing of information to opposite sides of the brain from both eyes so if you notice here we have the right visual world coming in but we also have a little bit of the left visual world coming in and so this part over here is actually shown across over to the other side of the brain eventually where it can be perceived by the optic lobe this pathway over here leads to essentially nerves traveling to the same side of the body to the brain where they can be perceived by the optic lobe so we have the outside world in general a little bit right side bias but the outside world in general being passed on to both sides of the brain so that that information can be processed and similarly we also have all this information about The World Slightly left side of the biased but that information again all of it is traveling to both sides of the brain so we get a really kind of like redundant pathway here of information flow but that's actually a good thing because if an optic nerve is damaged or if an eye is damaged you're still going to get visual information from the world around you into both sides of your brain so that it can be processed and and so that's a you know a nice little safety feature built into our optic system um just to kind of like quickly run this run this down we have sensory information right coming in that information is is perceived by sensory receptors photoreceptors in the eyes and they achieve action potential as a result so it's a series of potentials that lead up to action potential when that occurs the cells axons transmit via these nerves uh to the optic tract in the central nervous system through the thalamus remember the thalamus is our sensory relay Center for all this other sensory information coming from the body including visual the thalamus will then send the information about um image essentially image data to that particular part of the brain so that is kind of the quick breakdown we also have a series of other structures around the eyes to help protect them from various things we have what are called tarsal glands that produce well the essentially the lubricating layer around the eyes when your eyes dry out that layer is not being produced and it's actually very uncomfortable right so it keeps your eyes moist um so eyelids we talked about a little bit as being protective and they certainly are that um there are uh with the eyelashes in particular there is a bit of a filtration function that occurs so that dust particles for example can be kept away from the Eyes by the eyelashes there are also ciliary glands ciliary glands between the eyelashes um that also provides some some secretion as well the conjunctiva is the membrane that lines the eyes and connects to the eye's surface so it's like the outermost layer of the eyes that is in contact with the outside world think about the outermost layer of the eye that is the conjunctiva so it's uh if you've ever heard the term conjunctivitis that is an inflammation of the conjunctiva which is typically or is caused primarily by a species of bacterium that infects that particular layer um normally the conjunctiva is the one that actually produces the mucus to lubricate the eye but like I said it can become infected inflamed and if you've ever had pink eye or have had a child that had pink eye it's it's relatively unpleasant those glands begin to secrete a thicker more mucousy secretion as opposed to the clear secretion that normally is secreted uh and so one of the symptoms of pink eyes when you wake up in the morning after having your eyes closed and having these glands secreting mucus onto your eyelids all night long is that in the middle of a pink eye infection you'll wake up and not be able to open your eyes because they're kind of like glued together by this mucousy gross sort of pus that is being secreted anyway that was fun um very appetizing I know um the question that sometimes comes up is about eyebrows and what functions eyebrowserve aside from like you know facial expression kinds of things looking surprised for example um but the eyebrows actually are somewhat important again especially for reducing glare on like far away objects the eyebrows can actually help produce some anti-glare properties as well and a little bit of filtration although not nearly as much as what the eyelashes would provide on the lacrimal apparatus is the gland that produces the lacrimal fluid um so this is uh you can see here the um the gland above the eye there are ducts that lead onto the surface of the eye and those ducts eventually drain into the lacrimal Sac which then drains into the nasal cavity right so we have that pathway there which is uh pretty I think interesting little series of channels and and Pathways to get to where it needs to go those tiers that are being secreted contain lots of other properties aside from just lubricating the eye there is a watery sort of nature to it there is some mucus production as well so it is a good lubricant there are antibodies there there is lysozyme there which I don't think we've talked about much in this course we talk about it a little bit when we talk about skin we talk about it more when we talk about the digestive system in ap2 but lysozymes are like antibiotic substances they are chemicals that will at least maybe not kill but prevent or maybe even limit I mean in a worst case scenario limit the growth of bacteria on the surface of the skin it does the same thing in the eyes it's an antibiotic substance that helps to Stave off infection anywhere in the body where you've got this this sort of interface between the the inside of your body and the outside of your body makes a really really good place for things getting in that you don't necessarily want to get in so pathogens parasites those kinds of things it's really important to have immune cells and antibiotic substances and those kinds of things in places where bacteria would tend to invade and that's one of the functions of the tears secreted by the lacrimal apparatus we've talked before also about cranial nerves and I mentioned a little bit about muscles that control them and that kind of thing I am sort of showing you guys the muscles that are moving the eye on this slide but that being said I'm still not going to hold you to a specific muscle paired up with a specific cranial nerve I mentioned in my previous lecture either online or in person depending on which one you had that as long as you know that cranial nerves three four and six are motor motor nerves to control the muscles that control the eyes I don't necessarily expect you guys to remember that well the superior oblique muscle is controlled by the trochlear nerve as long as you know that or nerve four as long as you know the nerve four is the trochlear nerve and it controls a muscle that moves the eye I'm happy all right you don't have to know which is which as far as I'm concerned uh the lateral rectus is the one controlled by the abducens and then all of the rest of them the other four are controlled by the ocular motor nerve or cranial nerve number three but again as long as you know that three four and six are the ones that control the movement of the eye in its socket uh I'm happy with that it's a motor or they're all motor nerves um they all attach to a muscle that muscle allows the eye to either turn one way or another depress the eye turn the eye to the sides turn it back toward the middle Elevate the eye whatever as long as you know which cranial nerves are involved I'm thrilled all right we talked about the conjunctive already it's the outer layer these are epithelial cells I don't think we mentioned that before the other thing that can happen or does happen here is the conjunctiva is it's even though very thin and pretty delicate it's actually a nice protective covering over the um excuse me over the cornea the cornea is the next layer in and if you've ever experienced a scratch cornea or anything like that damage to the cornea you know how unpleasant that is so protecting it is is kind of a good idea and the conjunctive it does a decent job of that like I said though it is a thin layer so it's not going to protect against um you know really fast moving objects or really large objects or sharp objects damage will still happen but in you know normal day-to-day situations the conjunctive is going to help protect the cornea from friction and from damage and those kinds of things and this is also where the tears come into play the cornea is the uh the transparent uh really curvy front part of the eye so you notice in this picture the cornea kind of has a greater degree of curvature to it than the rest of the layers underneath do so that's you know an important feature of the cornea this is a really important feature for focusing light you can think about uh well this is the idea of um taking a magnifying glass and with sunlight causing things to catch on fire it's kind of the same idea but in a much less extreme way you're taking this curved surface like a magnifying glass lens is curved and you're focusing light onto a point now obviously in the cornea we're not going to focus it to the degree where anything catches on fire but it is generally sort of loosely the same idea okay so it's it's converging light focusing light on the rest of the eye the sclera is the this is the part that is opaque uh it's relatively fibrous it's it's um actually relatively tough uh this is the um the white part of the eye and square is the white part of the eye and this is um also where muscles attach this is the place where um essentially everything within the sclera is protected by it right it's it's a relatively as far as your eyes go a relatively tough outer covering of connective tissue that does a pretty good job of protecting most everything else within um the you get this picture here you see the vitreous humor this this fluid is what actually maintains the shape of the eye so having the right amount of fluid in this space is actually very important if you don't have enough fluid or if you have too much fluid that can actually cause a distortion to the shape of the eye which can then affect as you might expect Vision as a result of that there's also a thickening of the vitreous humor that occurs as we get older so our eyes are less able to change shape when they need to change shape because the vitreous humor is is a thicker more viscous kind of a substance as we age uh the choroid is this blue layer in the picture up here this is essentially your blood supply for getting nutrients to the cells of the eye as well as getting getting rid of waste the choroid is also pigmented right so this is um what makes uh the part of your retina that looks black is the is the choroid right so obviously um in the back of your eye where the retina lives the black appearance is because of of that chord substance this is another way to manipulate light to our advantage you can think of it that way because if light coming into the eye through the pupil were to reflect off the retina onto some other surface and reflect back perhaps or even just reflect back in general that would very greatly distort the image that you were seeing if that were the case ciliary bodies are held by held open essentially by suspensory ligaments this is also um what holds the lens in place there is an aqueous humor within that as well and and like we said um in terms of causing the lens to change shape this is actually what does that so you might know for farcasing Ferguson for focusing on very far away objects the shape of the lens will be different than for focusing on very very close objects okay so that's kind of the the thing is when you're focusing on you know your hand in front of your face uh the lens really needs to be a different shape than if you're focusing on a bird far off in the distance right so these muscles in the ciliary body are the ones that allow for that changing and flexing of the lens to give a more clear image or present a more clear image to the retina which can then be interpreted eventually by the brain um so they are actually very critical as well in terms of maintaining the right shape for the lens the iris is a pigmented structure this is what gives their eyes their color pigmented in in some individuals not in others the inner ring of the iris is a circular muscle uh it is accompanied by an outer ring that's made up of radial muscle all right so we have a circular muscle and a radial muscle and so the the combination of those two rings of muscle or muscles is to be able to dilate or constrict the pupil right the the hole essentially that allows light into the eye so that it can be bounced off the retina and perceived by sensory cells that is the uh the pupil that does that so the iris is what actually controls how much light enters the eye and so this can be affected by a lot of other environmental factors this can be affected by um you know going from a dark room out into a bright sunny day or going from bright sunny day into a dark room obviously pupil needs to either constrict or dilate to adjust for the new amount of light coming in so you can actually see what you're trying to see these are also affected by the sympathetic and parasympathetic divisions of the peripheral nervous system um because if you're in a fight-or-fly situation if you're in a sympathetic nervous system activation kind of mode you're actually going to have that dilation of the pupils to allow more light to come in if you're in more of a parasympathetic mode more of a rest and digest mode these muscles can uh essentially relax RSI in that case they would actually contract to make the people get smaller so that less light comes in and you have less probability or less chance of damaging the retina of the eye by too much light right so you don't want to have too little light because it's doing that means it's going to be difficult to perceive the image that you're of the object that you're trying to see but if you have too much light same thing plus you can also have damage all right so you want to really make sure you have the right amount of light coming in through the pupil for the retina to be able to perceive and pass that information along but you also don't want to have too much that's just as bad as not enough if not worse so that's the pupil um kind of got ahead of myself a little bit but in bright light constriction in dim light dilation in sympathetic conditions dilation and parasympathetic you're seeing constriction right the lens we've talked about already when we talked about the muscles that control it the ciliary muscles uh it's transparent it's flexible it's curvy it adjusts based on on the distance to what you're trying to focus on and then the retina itself is kind of the more or less last one we're talking about until we get to the the optic nerves this is the part that actually is perceiving the image there is a layer of sensory neurons that is made up of photorescent this is specifically rods and cones which are stimulated by light we'll talk a little bit more about rods and cones but essentially the rods in your eye the cells that are more Rod shaped are important in perceiving in low light conditions but they're not very good at showing color cones are much better at showing color but they don't work real well in low light situations so we have a combination of rods and cones as photoreceptors to be able to perceive um the images of the world around us we then have relay and other Sensory neurons that carry information to the optic nerve and the optic nerve is that one carries information to remember both sides of the brain where it can be processed and um perceived as an image at that point right so what's really important to remember with all of these sensory things is it the processing is what occurs in the brain the sensory cells are just responding in a very very fundamental cellular level to a sensory stimulus they are being excited by something around them and they are then sending that excitement that's that that information onto the brain and if a Thalamus says okay we need to pass this on to the appropriate place it's passed on and then it's processed and you actually recognize it consciously as something a squirrel or a tree or whatever it is the sensory cells in the retina are not perceiving a squirrel they're seeing a pattern of color and shape and light and dark all of that's sent to the brain and the Brain says oh look squirrel all right so that's it's really important to have that processing step in the appropriate segment of the brain uh the focus is a is a yellow spot fovea um actually it's typophobia um is back here this is actually uh in essence the part of your eye that has the best capacity for high resolution high resolution image if you notice the retina so the retinas kind of shown up here um kind of shown down here and then it goes all the way back here all right the fovea is a little indent right there and that little indent because it has a higher surface area than if it was just a straight line this end end indent actually has more sensory cells packed into it and more sensory cells means better capacity for high resolution that's actually where if if possible you want to focus the light coming into your eye on the fovea to give you the best possible resolution for the image that you're trying to see so that's um yeah cones there because that's going to give you a better indication of color um especially rods tend to be more peripheral um and and actually you can you can sort of experience this if you go outside on a dark night and if you're trying to focus your light on something in like in your yard right um if you look directly at it and it's dark that light is being focused onto your fovea and remember your fovea has primarily cones so focusing light onto cones in the dark does absolutely nothing because the cones need light in order to be able to perceive an image so if you instead of looking directly at the thing that you're trying to see if you sort of like look off to the side just a little bit and use your peripheral vision now you're using some rods now you're focusing the image away from the phobia where all those cones are to an area of the eye that has more rods and rods are actually better at perceiving in low light conditions okay so that's if you've ever experienced that that's why that is I know if like for me um two of my dogs are black and so in a really really like dark moonless night it's kind of hard to see them and my only chance really is is when I go out to let them out for the last time at night is to kind of like look off to the side of where they think where I think they are and then I can usually see them or they'll move and I'll see them that way but otherwise it could be really tricky to uh to see them when it's when it's dark out so this is um like I said the fovea is the um the part of the eye with the best resolution okay so the iris of the eye is the Color part um this is the part between it's between the cornea and the lens so it's um sort of sandwiched between those two layers it's also attached to the ciliary body um we've talked about the pupil um adjusting the amount of light that comes into the eye to the appropriate level just enough light to see but not so much that's that it's possibly going to damage the eye um the back part of the iris is always a brownish or blackish color and so for people that have brown eyes or really dark eyes they actually have pigments all the way through the iris for people that have blue eyes the rest of the iris is clear it's unpigmented alright so when whenever you see like genetics problems and they talk about non-pigmented phenotypes they're talking about people blue eyes right blue eye people don't have um that pigment layer in it like brown eyed people do so it's easier to see essentially the back parts of the eye um that you wouldn't otherwise see if there was a pigment there the downside to that is having less pigments overall in the eyes makes it really difficult to see on uh really really bright sunny days especially if you're getting reflection off of water or frozen water like snow um it that glare is is really uncomfortable if you have really light eyes the the other pigment there does a nice job of shading out some of that light filtering out some of that light so not all that gets through um all right so let's talk about the muscles themselves uh we have the the um the center muscle which we talked about before circular fibers this is smooth muscle so this smooth muscle obviously then is going to be controlled unconsciously because smooth muscle is not something we have direct control over so this round layer of muscle around the eye has parasympathetic innervation this is the part that's going to cause constriction when someone is in a really high light conditions so by Contracting by Contracting this muscle under parasympathetic conditions rest and digest normal kinds of functioning those muscles contract and the pupil gets smaller it constricts right the dilator muscle is the opposite this is also smooth muscle but now we're talking about radio fibers so instead of wrapping around the eye these fibers are going to go like in a straight line away from the eye but all the way around so you'll have fibers coming off all the way around when those muscles contract usually due to some fight or flight sympathetic situation they are going to cause when those muscles pull all the way around the pupil the pupil is going to get bigger it's going to open up and it's going to a lot more light in and again that's the that's the response you'd expect to see in a sympathetic type situation all right so we have these two muscles we have the sphincter pupilly muscle and we have the dilator pupilar muscle as well that are going to cause a constriction and dilation of the pupil respectively in the in the um conditions that you'd expect to see them in um again with the retina um this remember is the part of the eye that actually forms the image the outer part of the retina is pigmented the inner layer is not it's more neural neurological in nature and it's going to look more like the cortex like you'd expect to see in the cortex of say an organ like a cortex of a kidney or cortex and adrenal gland it's it's shaped in a similar kind of a way and this is where the photoreceptors are these photoreceptors are equipped individually with various photo pigments uh otherwise known as rhodopsins and they are going to be stimulated by different wavelengths of light depending on on the concentration of these various proteins so some photoreceptors will detect one range of colors others will detect others and so forth again the rods especially are very good for low light conditions but you do not get a lot of differentiation of color so if you're retina was just made up of rods exclusively you would actually see the world in black white in Shades of Gray maybe a little more color than that but not not on a whole lot but you would be able to see much better at night that would be the trade-off um cones are the opposite so cones give you the detailed color vision but they don't work very well in low light so as long as you know you're in in relatively highlight conditions you get that nice color vision that we're accustomed to thinking about because of the cones on the retina of our eye we talked about the fovea as well the full name for the fovea is the fovea centralis this is that little depression if you remember in the image of the eye the retina came around in a curve like this but back here in the middle of the eye there was a little Notch cut out and then it continued back around on its curve that little notch in the center of the of the retina is the fovea centralis and so again remember we talked about this already the fovea contains only cones and this is where we will sort of instinctively try to focus an image because this is the essentially the part of your eye that sees the best the goal of your eye as an organ is to focus as much light as possible onto the fovea centralis to give you the best clearest view of whatever it is that you're looking at um so we have also the transmission of information right so we have these photoreceptors and they hand off the visual information again all that is is a is an action potential it's no different from really most other Action potentials but when a certain combination of cells are excited they're going to produce eventually an image process by the brain as whatever it is that you saw right so that's um kind of the overall very broad perspective about how the eyes are are going to work the ganglion cells are present where the retina ends and the pathway toward the brain actually begins through the optic nerve the optic disc is actually one of the really one of the things that a lot of evolutionary biologists have kind of struggled with and and this is actually really comes up frequently in people who are um uh anti-evolution in a way almost um is that the human eye is is not perfect um the human eye in a lot of you see this all the time will be kind of touted as this perfect structure that um senses the environment better than any other structure in any other organism anywhere and that's really not the case um our eyes are not perfect um everybody's eyes have a blind spot which is where the optic nerve crosses over the retina it essentially blocks or Shades the retina in in a sense from perceiving light from a certain angle so there actually is a blind spot and it's it's just off to the periphery from your your center field of view really kind of hard to demonstrate by yourself because you sort of know where your hand is even if you can't see it your cerebellum is telling you where your hand is if you move it off to the side of your head um but if you test this with somebody else you can actually show it reasonably easily um and and the other kind of interesting thing about this is that um other organisms that are not humans um actually have eyes that are even better at perceiving the world around us because they don't have a blind spot they have at least as many cones they have no blind spot and so the um the the image forming potential of something like an octopus or a squid especially is probably better than a human's which is pretty insane if you think about the fact that they are invertebrate organisms and we are in some ways much more complex than them right so they they actually have probably a better capacity to see than we do based on how their eyes are set up all right so we're gonna stop here for a second and I will cut this off get it posted and pick up with our second um segment of lecture