Eye convergence and accommodation anatomy


Sam Webster


eye,human eye,anatomy,convergence,vergence,accommodation,accomodation,acommodation,cross eyes,vision,near sight,far sight,focus,lens,ciliary body,Sam Webster,Swansea,dontbeasalmon

listen to that who builders banging no seagulls kicking off it's quiet for once right today we are throwing back to the ally when we look to the I did a bunch of videos on the I is a lot of an afternoon me I we talked about accommodation but we didn't talk about vergence I think I hinted at it we'll do that now nice and briefly so the style is we'll talk about accomodation again we'll just go over that and the anatomy involved and then we'll talk about vergence what it is in convergence and then the the functional anatomy involved reason I was reminded about this is that when I start looking at things close up its gained slower and slower for my eyes to focus on the thing that's closed up like my smartphone is getting really annoying mostly because it says I'm getting older like today when I was trying to change the batteries in this microphone it's got a little tiny battery and now I want to see which one it was so accommodation is when you go from focusing on something in the distance so focusing to infinity to focusing on something close up if you're washing a computer now or you've got your smartphone or a tablet whatever you're quite close to it right your eyes have accommodated so let you do that now if I take the eye apart we've talked about all the bits of the eye now in something here and I'm going to pinch some of the labeling from the previous video that I use so I can do it again let's do all the thing again right that's why I'm wearing should see in that video - sure and this one I'm wearing a t-shirt you probably never would have noticed two more weeks of the summer and I have to start dressing like an academic again because students are back anyway what happens is we have the lens you know the purpose of a lens the purpose of a lens is to focus the light onto something else in the eye we have the retina that's the photosensitive bit and light is focused on the retina and then that information goes to the brain and we make sense of and we perceive the world around us visually now the lens is surrounded in a ring you see the ring go around here so the lens is kind of a rain thing it has another round thing going around it and this is a ciliary body and the ciliary body is a muscular thing surrounding the lens and the lens then is is suspended from the ciliary body by the suspensory ligaments what that means is that if the ciliary body becomes a bigger ring then the lens gets stretched if the lens gets stretched it gets thinner and that's for focusing on distant objects the lens is thin and if the ciliary body contracts and gets smaller then the lens is no longer pulled by those suspensory ligaments so the lens also shrinks and it becomes fatter and is that state that the fatter lens that focuses light from a near object onto our retina and the last accommodation the changing in shape of the lens to focus light onto the retina so we do that quite easily and then reflex there so the optic nerve cranial nerve 2 is carrying sensory information from the retina back to the brain so that's the afferent limit this reflex the brain then does a whole bunch of magic and recognize it really needs to focus on something close up and then sin's motor nerve impulses to the ciliary body causing it to contract and get smaller causing the lens to shrink and fatten to focus the light on the retina when we're looking at a nearby object that Ephraim's limb is carried by cranial nerve three the oculomotor nerve and those nerve fibers are actually parasympathetic so that's accommodation now what about vergence and convergence there is something else we need to do if we're going to focus on a nearer object now the retina is not equal all over the retina you know this you move your eyes to look at something and the reason you look at something is because there's a region of the retina that is higher definition there's a high def bit than the other part of the retina so that when you look at something the light from that something that you're looking at is focused on the macula so the macula has a higher density of sensory cells so a high a higher resolution of vision and within that you have the fovea which has an even higher resolution right so that's why you move your always around so you if you read a book you have to move your eyes around to focus on the thing because around here it's all a bit blurry isn't it but around here the thing you're looking at is is super high-res so when you're looking at this an object the sort of think about the angle of gaze right or the the angle that the light is passing to the rest and when you're looking at a distant object you your eyes are kind of parallel right so the the light coming in from the tours is kind of parallel you're looking at something at infinity but as you well know when you look at something close up if you classic one is following your finger to your nose you go across right why do you go across right well that's convergence and if you practice enough you can do this yeah without using a finger oh and what's happening there is that the the the eyes are moving in opposite directions that's the important thing so with convergence the eyes both adduct right and with divergence they both go the other way you don't really do that I could something I can do that I can do converges without here I could you can cross ideas converges but I don't think I can make my eyes point in opposite directions I could anyway so convergence when you look at something close up and then when you look at something in the distance divergence brings the the lot of the eyes kind of parallel to each other the angle that the light comes in through the pupil to the retina those lines are parallel right got it so in final question have you worked out why we have to do this yes the reason we move the eyes with convergence is so that we can keep the macula and the fovea focused on the thing that's close to us so if we're going to keep that by knock Euler vision if we keep both eyes focused on the thing that's near to our eyes we need to make the the pupils point in a little bit we needs cause adduction all right anatomically then yes this is a right eye this is the right eye so what muscles are we using to adduct both of these eyes yes we're using medial rectus so with convergence we use media or rectus to bring the angle in of the eye to adduct the eye which means that if one eye is stronger than the others even if the the the medial rectus muscle is stronger on one side than the other then you're gonna quite early on and quite close to the nose you can start to get a double vision because what I will move fine and the otherwise I won't quite move appropriately to get bit diplopia right so if that's converts so for divergence how do we move the angular vision laterally again how do we abduct the other vision the absolutely eye that's lateral rectus isn't it so both of those muscles well so the medial rectus then is innovated by the oculomotor nerve cranial nerve three but of course because you've seen all the eye videos and you know this stuff inside out lateral rectus which abducts the is innervated by abducens cranial nerve six so hopefully that was a nice short addendum to the eye videos explaining how accommodation and convergence work together all right and hopefully I've edited that together in less than three cups of tea and it's a nice short video good see you guys next week [Music]