Intraocular muscles nerves anatomy pupils and lens

Author:

Sam Webster

Keywords:

eye,sympathetic,parasympathetic,CN II,CN V,nasociliary,sphincter pupillae,dilator pupillae,ciliary body,ciliary ganglion,short ciliary nerve,long ciliary nerve,pupil,iris,lens,cornea,anatomy,Sam Webster,Swansea,dontbeasalmon

Subtitles:
[Music] okay so in previous weeks we've been doing various things with the eye we stayed outside the eyeball this week we're going to get inside the eyeball and we're sticking with nerves this time we've got a workout how sympathetic nerves get into the eyeball how parasympathetic nerves get into the eyeball and how the intra ocular muscles are innervated so there's still quite a lot to do I think what we'll do is we'll start anteriorly so we'll start with the bit we can see we'll start with the pupil and the iris because you know the pupil gets larger and smaller and then we'll go a step back and look at the lens and the muscles that affect the lens and then we'll do the last bit of the retina which I'm not going to do in any detail at all I'm just gonna say retina optic nerve that's about it and then we should have there's also a general sensation as well remember the trigeminal nerve that sends a branch inside the eyeball to get to the cornea if we layer it up like that the complexity that you see on some diagrams should seem less confusing all right good luck okay so I've got yeah I got a lot more thus today let's have a look inside this eye beware this one this is just the eye balled without the muscles and stuff you can see you get an idea at the ballet look all the little nerves that are going in there if I take this apart so the see-through bit is the cornea and then of course we've got the iris here and the pupil in the middle and you know the pupil changes size in response to the amount of light available so if it's very bright the pupil gets smaller and if it's not very bright the people slowly dilate right if you flash a light if you shine light into somebody's eyes you should see that in fact if you shine light into one eye you should see the same reaction in both pupils so what we must have is in the iris then there are two muscles we have one circular muscle which is sphincter poopley and then we have a muscle like a radial muscle with the fibers running in this direction around the outside kind of in the in the direction of lines that you see the iris in and that's dilator pupil e1 is innovated by sympathetic nerve fibers and one is innovated by parasympathetic nerve fibers so both of those are motor autonomic nerves so obviously you don't have control over pupilary dilation and constriction it's an autonomic response now which one does which the way to remember is Robbie and headlights right so you know the sympathetic nervous system and adrenaline are responsible for the fight-or-flight response part of our fight-or-flight response if you think about the rabbit in the headlights who stopped still wide eyes dilated pupils as well maybe so dilated pupil II the muscle with the radial fibers like that that's innovated by the sympathetic nerves and that causes the pupil to to widen also remember we talked about the the muscle up here the the the superior tarsal muscle which also has sympathetic fibers which is causes the lifting of the oil it's and but the but see so that means that the opposite then the parasympathetic nerves innervate sinter poopley which then running radially around here cause the sphincter to close in the pupils to shrink in response to bright daylight those are all smooth muscles within the iris on the other side of the colored part which kind of aren't really shown on here the next bit thank you see there's the lens so the pupil is a hole and the lens is on the other side this is the anterior chamber under here so what about the lens then you're probably aware that the lens changes shape the job of the lens is to focus light on the retina at the back of the eye right and we're not going to go into the physics of all of that but there is a lot physics ER when we look at something far away when we focus on a distant object the lens is quite thin and it focuses that distant light onto the retina so that we see a sharp image whereas when we're reading when we're looking at something close up when we're focusing on a close object the lens fattens and by fattening it again focuses the light from that near object on the retina to give us a sharp image now that's what asked mammals do is we should change the shape of the lens I think birds of prey have got I think they can even change the shape of their cornea and they might even have two fovea on the on the retina they which accounts for their amazing eyesight but us mammals we do that by changing the shape of the lens if you change the shape of the lens how are we going to do that it's going to be muscular and again it's autonomic because we're not I mean you can kind of you you can kind of control the focus a bit can you but essentially it's under autonomic control that the way in which this works is a little bit backwards inside the eye there are three layers of tissues one of those layers is called the vascular layer and the vascular layer where it exists around the lens because obviously it's going to leave a hole for the lens there is smooth muscle arranged within that vascular layer the muscle in this vascular layer is called the ciliary body so the ciliary body when you see in an illustration of course you only ever see in 2d like cross-sectional things so it's like it looks like a flat thing but we've imagined the ciliary body as a sphincter as a circular sphincter like other sphincters in the body and it runs around the lens now the lens is suspended from the ciliary body by fibers by like ligamentous fibers by collagen fibers these fibers get called zoella fibers or zhonya nerve fibers this area gets called the zone new lives in which is a great name so these fibers of Zin if you want to the contemporary term would be to call this as suspensory ligament they're rather suspensory ligaments around the body there's another one outside the eye you've come across lots of suspensory ligaments at all it's a good general term but um the lens is suspended from the ciliary body by fibers running around in between them and that would be the suspensory ligament so again that's a circular suspensory ligament so now what happens right when the ciliary body contracts it's going to shrink cuz it's a sphincter right and when the ciliary body relaxes its gonna open up because it's a sphincter and that's what sphincters do what effect is that going to have on the lens but if the lens is in the middle and it's it's suspended from the ciliary body by those collagen fibers by the suspensory ligament when the ciliary body relaxes and gets bigger the lens is going to get stretched and flattened so when the ciliary body is relaxed you're going to be able to focus on distant objects when the ciliary body contracts then those that suspensory ligament is also going to relax and the lens is going to shrink and it's going to fatten which means that when the ciliary body contracts you're able to focus on near objects this is essentially controlled by parasympathetic innervation so these thus the muscles in the ciliary body are controlled by parasympathetic nerves so when there is parasympathetic innervation it contracts and you focus on near objects and when that parasympathetic innervation is removed the ciliary body relaxes the the the lens gets stretched and you focus on distant objects hopefully the way I've explained it seems really straightforward because the time you came to Italy it often seems a bit backwards if I describe to you why it's confusing you're gonna get confused if you've not be confused by it then you're not confuse nice that's good right next question where do those parasympathetic nerve fibers come form well we looked at the extra ocular muscles and we saw that most of the extraocular muscles moving the orbit were innovated by the oculomotor nerve very well named coriander 3 and also know the trochlear in abducens nerve well it turns out that the intra ocular muscles are also innovated by the oculomotor nerve good huh so the oculomotor nerve produces mostly somatic motor neurons for those extra ocular muscles but also parasympathetic neurons it produces a bunch of preganglionic parasympathetic neurons which go out and make their destination are the Paris of a smooth muscle fiber smooth muscle fibers of the of the ciliary body and the sphincter poopley muscles right do you remember where the oculomotor nerve comes from it's a bit bigger on this model than it was on the other model but the oculomotor nerve comes out of the midbrain here and some reason the parasympathetic nucleus involved in this is is quite famous and people seem to remember its name the heading of Westphal nucleus and do you remember last time we were talking about the cerebral aqueduct here and the midbrain here well the eddying Avesta foul nucleus is very close to the other nuclei of the oculomotor nerve it's in the midbrain and a pop site as we saw there dives through the superior orbital fissure at the back of the orbit because that's how pretty much everything gets from the cranial cavity into the orbit unless you're the optic nerve or similarly special and then those preganglionic parasympathetic nerves get to the ciliary ganglion and the ciliary ganglion is one of the four parasympathetic ganglia of the head say four there's four on each side so parasympathetic and sympathetic nerves we have pre ganglionic neurons coming out of the central nervous system they find their way to a ganglion a collection of cell bodies they synapse with another neuron and that postganglionic neuron goes off to his target organ now in the case of the the the parasympathetic nerves going to the the eye going inside the eye that off that's the ciliary ganglion and in the ciliary ganglion and that's a parasympathetic ganglion those preganglionic parasympathetic neurons parts of the oculomotor nerve cranial nerve three they've run in there they meet our postganglionic parasympathetic neurons synapse with it and that neuron shoots off and through these nerves here it's getting to the orbit these nerves get called the short ciliary nerves because stuff randy gets called ciliary but if they're a short ciliary nerves then there must be longer ciliary nerves these other nerves going up here these longer ones those are the long sinewy nerves so those postganglionic parasympathetic neurons run with the short ciliary nerves could get into the eyeball and then they whip off to those Targhee muscles that we were talking about earlier cool huh next question if that's how the parasympathetic neurons gain their how do these sympathetic neurons get into the eye well all sympathetic neurons come out of the spinal cord in the thoracic levels one right so that preganglionic sympathetic neuron comes out of the spinal cord and you know about the sympathetic trunk right the trunk of sympathetic ganglia running down the the posterior thoracic cage the posterior thoracic wall posterior pelvic posture you neck and what have you right well the main way the sympathetic neurons travel around the body or one of the main ways is by following arteries so of course all the orbit all right and then here's a huge red blood vessel here and you can see that it's covered in little white lines what's the blood vessel that's the internal carotid artery because that's gone inside the cranial cavity and makes this room s-shape and at the end of our earth shape it one's posterior to the superior orbital fissure so postganglionic sympathetic neurons work their way up the sympathetic trunk and then they follow the common carotid artery an internal carotid artery up into the cranial cavity and they stay with it until it runs posterior to the orbit and then they jump off those sympathetic nerves jump off run through the superior orbital fissure because everything goes through that right and then they run auntie merely through here and they'll take whatever route they can get to get into the eye which means that they can on what some of them will run through the ciliary ganglion they won't sign up they're just running through it's just cabling it's just a convenient route for them to follow and they they're within also then the short ciliary nerves and they run into the eyeball and they do they're sympathetic jobs within the on label which is the motor bit we just we just talked about there is some discussion about sympathetic nerves also affecting the the ciliary body and the the lens you know opposing the parasympathetic innervation so that when you're startled you also focus on distant objects nine times how true that is but there you go so they'll do those sympathetic motor jobs we've been talking about but also of course sympathetic nerves one of the reasons they're following the arteries is because they're going to affect the smooth muscle in the artery walls and and be involved in regulation of where blood goes in blood flow so they're also gonna go into the retina and into the blood vessels within the retina and control blood flow through there some of them are going to run through the long ciliary nerves because why not you just trying to find any way you can get from here to the eyeballs you go through the long ciliary nerves and that's how the sympathetic nerves and the parasympathetic nerves get into the eyeball but there is a nother nerve going into the eyeball and it's kind of the real reason the long ciliary nerves exist but I've got to pick up this big model again I could do with the big do it like a tall thing he couldn't write the short ciliary nerves run between the ciliary ganglion and the eyeball those are the short ciliary nerves now really the long ciliary nerves are running across here and this nerve here these are branches of the trigeminal nerve that we talked about some weeks ago we talked about the trigeminal nerve and how it innovates you know carriage general sensory innervation from the skin of the face and the eyelid in the conjunctive or an ass or thing right now the mezzo ciliary nerve is a branch that's going to send some general sensory fibers through these long ciliary nerves to get into the eyeball and the ultimate destination of those neurons is is the cornea right so the the cornea has general sensation and you know if you try and touch her you really can you really don't want to and that's the the blink reflex so the I mean you know even if you were to touch the cornea likely with the bud of Cornwall you should trigger the blink blink reflex so general sensation is going to travel through those general sensory neurons through the long ciliary nerves through the nay's of ciliary nerves and then back through the ophthalmic branch of the trigeminal nerve and back to the brain and then trigger the facial nerve to close the oil herbs that that's the blink reflex they should happen but both eyes should happen on both sides so that was the real reason that the long ciliary nerves are there they're actually a branch of the nazo ciliary nerves carrying general sensation from the cornea the sympathetic nerves just run through there just for fun I think as most of the stuff that goes through the eyeball there's obviously one big one that we haven't talked about and that's the optic nerve by the way those that the general sensory nerves of the nazo ciliary know if they'll actually run through some of those will run through the ciliary ganglion as well once I napped with it they'll just run through and through the short silly wee nerves getting the eyeball these nerves they just seem to follow whatever path they can it's probably because in the embryo they're following similar chemotactic cues anyway the the optic nerve cranial nerve to the optic nerve is as I'm sure you're aware carrying special sensory information back from the retina carrying vision and I'm not going to talk about it too much because that's another very minute talk all in its own suffice to say that cranial nerve two is considered a direct extension of the central nervous system so it's not like a peripheral nerve in normal sense the retina and the optic nerve are are an outgrowth and extension of the forebrain so they're quite special and because of that if you follow the optic nerve through the optic canal and into the orbit it's covered by the three layers of connective tissue of PMA - arachnoid mater and dura mater with a bit of CSF and what have you and it yeah that's all I'm going to say is it runs to the eye carries vision one thing is worth mentioning is that within the nerve there is a little artery and a little vein and those are the central retinal artery and central retinal vein carrying blood vessels to the retina but that's it those are the nerves that enter the eye so now we've talked about the intra ocular muscles the sympathetic and parasympathetic nerves that innervate those muscles and how they get there we've added on a bit of general sensation and we've briefly mentioned the optic nerve if ever we talk about the optic nerve what we really should do is talk about the retina the optic nerve the optic tract tract the lateral geniculate nucleus and the thalamus and the yeah we should follow the whole tractor back and how all crosses over and what happens when you damage different bits because that's a whole topic in its own right but we'll save that for another day right so if you see complicated diagrams of nerves going to and from the Ani ball and passing through the ciliary ganglion don't get to befuddle just think about the nerves are going from one place and trying to get to another place and they're just taking whatever route they can and the only parasympathetic neurons synapse in the ciliary ganglion other nerves do pass through the single ciliary ganglion but they're just passing through it's just cabling at that point all right I've made a right mess now like you guys next week [Music]

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