Eye cranial nerves of the extraocular muscles anatomy


Sam Webster


eye,anatomy,human,abducens,abducent,trochlear,nerve,oculomotor nerve,CN III,CN IV,CN VI,extraocular,muscle,superior orbital fissure,clinical test,orbit,eyeball,lateral rectus,superior oblique,levator palpebrae superioris,Sam Webster,Swansea,dontbeasalmon

[Music] right this week we're gonna talk about the cranial nerves that innervate the muscles that move the eyeball last week we looked at this nice simple model this week we're progressing to this thing now for some of you who want the brief version we can do this cranial nerve three the oculomotor nerve innervates all of the muscles of the orbit except for lateral rectus which is innervated by the abducens nerve cranial nerve six and the superior oblique muscle which is innervated by the trochlear nerve cranial nerve four that's it that's what you need to know you haven't even made it to the intro title [Music] but for those of you that are here because you want a little bit more and to be honest that first beer you could've got from anywhere right don't need to play anyway we're going to we've already looked at the muscles of the orbit the extraocular muscles we're then going to follow each of those cranial nerves three four and six from the brainstem into the orbit and to the muscles we'll talk about um what happens if those nerves get injured and to be honest there's not that much to talk about it's fairly straightforward but the model isn't now the reason this model actually this models a little bit tired this model is very complex fairly well used and a little bit friable so it's a little bit tired but hmm if we gain here you can see there's a huge number of nerves in there and that's because there are nerves not just going to the muscles we're going to talk about today but there are cranial nerves going to the retina nerves are controlling pupillary dilation and pupillary constriction there are nerves that are affecting the focus of the lens there are nerves going to the lacrimal gland and there are nerves going to the the other general sensory bits that we talked about the other week with a lot with a trigeminal nerve so that's why that's so busy I'm not going to talk about all those things today we will talk about all of the other cranial nerves and all the other nerves that go into the I like the intraocular muscles and our sort of thing cuz that's cool but let's just chunk it right so today it's these muscles and the nerves that innervate them and we'll try to find those as best we can on on that model we'll tease it apart we should start with the brain and the brain stem so you know what happens when I try and turn this model upside down it tends to fall apart so we've learned every we will take the cerebellum out and then we'll balance it and squeeze it there we go okay so the cranial nerves are numbered superior to inferior there's a sequential thing clearly of one two three four five six seven eight so cranial nerve one is the most superior cranial nerve 12 is the most inferior or choral if you like because we're in the central nervous system so clean on nerve three then is the oculomotor nerve it's almost completely filled with somatic motor nerves it does carry a little bit of parasympathetic motor nerve fiber stuff we'll talk about that another time but most of it a motor neurons going to the somatic muscles of the orbit that we use to move our eye now the nuclei for cranial nerves three four and six are within the brainstem and actually here's a good point all right so here's the brainstem there's the midbrain the pons and medulla is the spinal cord this is the cerebral aqueduct there's the fourth ventricle when we talk about the periaqueductal gray that's the gray matter around the cerebral aqueduct and is here very close to the midline that we find the nuclei for these cranial nerves that are innovating the muscles of the orbit the muscles of the eyeball what that means for you is that a lesion near the cerebral aqueduct in the midline is likely to affect the nuclei for cranial nerves three four and six on both sides do you see what I mean the nuclei aren't aren't like far apart the left one and the right one on a long way apart from one another in the brainstem they're close to one another so if you get a midline lesion it's likely to affect both nuclei left and right so you like to see effects in both left and right eyes and cranial nerve three it's nuclei more rostral more superior and cranial nerve six we find it's nuclei closer to the fourth ventricle so that's an interesting note but essentially pop-pop-pop we have the somatic motor nuclei Queeny owners pop anthem because that's a well-known thing the crayon is do pop right so cranial nerve one is the olfactory nerve cranial nerve - is the optic nerve easy-peasy which means that the next nerve we want to see will be cranial nerve three and in fact here's the pons here we can see a little nerve it's not huge just popping out they're just superior to the pons that for your clear motor nerve that's cranial nerve three and then let me look at the pond where we see this big nerve on either side well that's the trigeminal nerve it's big because it's carrying a lot of neurons for this high-definition sensitivity that we have in the skin of our face and then cranial nerve for the trochlear nerve comes around from the posterior pons so I almost said it right see there that's the trochlear nerve coming around so we're going to have three is up here Craig another four comes around from the back cranial nerve five is the big one around here and then that means cranial nerve six actually it helped because this model there also knit numbered sequentially cranial nerve six is this guy in the midline here there's abducens the abducens nerve all right so that's where they come out of the brainstem you can see here's the pond so use that as a visual Joppy I haven't got a skull new for school right I got a skull I think I could do a head and neck Anatomy that skill crazy man you remember that within the orbit there are two fishes so there's a superior orbital fissure and an inferior orbital fissure and the superior orbital fissure is the fissure that you know it's a big slit in the posterior part of the orbit and that connects the orbit with the cranial cavity so all of those cranial nerves three four and six all pass through the superior orbital fissure because that you get to the orbit if you're not the optic nerve because that has its own canal right cranial nerve three actually splits into two before it goes through the superior orbital fissure just splits into superior and inferior branches nothing too funky about it and then the superior the superior branch runs superiorly so it runs up to innervate these two muscles here now last week we talked about superior rectus but I avoided squishy that's a double layer there I avoided talking about the muscle that sat on top of it I mentioned it briefly at the beginning but the muscle on the top there is Lolita palpebrae superioris and what levator palpebrae superioris does is it inserts into the skin of the upper lip of the upper library upper library it doesn't come from the common tenderness ring back here it actually I think it comes from the sphenoid bone and the posterior orbit and it runs anterior leaning see how it runs in line with superior with on top off superior rectus but this muscle levator palpebrae superioris is going to run to the skin of the upper eyelid and to the the Tarsus now the Tarsus is like it's a sheet of connective tissue within the eyelid and what levator palpebrae superioris does is it elevates the eyelid usually against gravity so it keeps your eyes opening on you relaxing levator palpebrae superioris your eyelids fall and that's usually because of gravity levator palpebrae superioris is innervated by the oculomotor nerve cranial nerve three but also interestingly it has kind of another muscle or other muscle fibers in there the superior tarsal muscle now the superior tarsal muscle that's actually innovated by sympathetic neurons which means that yeah you know in response to fright you the wide-eyed effect so the the fight-or-flight response is a sympathetic response and part of our sympathetic response is to cause the superior tarsal muscle to contract to your eyes and more widely and also the the pupils dilate and more have you willing that into future weeks it seems that the superior tarsal muscle has tone when your eyes are open because when sympathetic innervation to the orbit is lost you tend to get ptosis Potosi drooping of the eyelid on that side Horner's syndrome is a very interesting example of this with Horner's syndrome an injury or a lesion in the shoulder actually affects the sympathetic innervation to the orbit and the result of that is is it's an effect on the pupil saving it but also Potosi of the eyelid on that site if you lose sympathetic innovation the eyelid tends to droop a little bit going to look up Horner's syndrome I'm not gonna go into it now we need to do more groundwork it's very very interesting so that has levator palpebrae superioris and back to the oculomotor nerve in a superior branch of the oculomotor nerve innervates these two muscles up here make two palpebrae superioris and superior rectus or is then the inferior branch of the oculomotor nerve runs inferiorly and it's going to innervate the medial rectus muscle the inferior rectus muscle and the inferior oblique muscle around here so the oculomotor nerve innervates all of the muscles the movie.i that aren't innovated by abducens and the trochlear nerve now if we have a look on this model here's the lacrimal gland I'm taking so there's that would be the Tarsus muscle let's take that nerve away there is levator palpebrae superioris and superior rectus the nerve on the other side is a branch of the superior part of the oculomotor nerve the take off lateral rectus so these are the branches of the trigeminal nerve here if I take this off now what we're really doing here is we're getting into this is the superior orbital fissure here so this is so this is this branch here and if we take this off this branch running through here here these are the this is let's see how this is going to inferior rectus so this is the inferior branch of the oculomotor nerve and then this up here this is the superior branch of the oculomotor nerve what I'm running up to there and you can see in here that there is an awful lot going on so here then this is the trochlear nerve cranial nerve 4 so the trochlear nerve running around there to the superior oblique muscle and you can see that here is that's it passing through the superior orbital fissure there in this gap in the bone and then so look there's lateral rectus so this is innervated by the abducens nerve so if we if we pivot this around there's the abducens nerve innovating lateral rectus and you can see again running through the superior orbital fissure here so this is one of those models that it's really useful to study from because it's so incredibly detailed but not terribly useful to use in a video so let's go back to my simpler model so you remember up here the muscle they're running kind of in the bone there that's the superior oblique muscle it's called the superior oblique muscle because the last part of the muscle then runs a bleakly across the eyeball to attach passes through this pulley here and that's innovated by the trochlea nerve cranial on the four trochlea means pulley it's the Latin word for pulley which is why gets called the trochlea nerve and then the lateral rectus here is innovated by the abducens nerve cranial nerve six and it's called the abducens nerve because this muscle abducts VII abducts the vision right and that's it so the trochlear nerve is a very small nerve and it crosses sides as it passes from the brainstem to get out into the orbit and because it innervates the superior oblique muscle we talked about the superior oblique muscle have no role in in rotating the eyeball but because it's affected quite small if you have an injury to the trochlear nerve you might not notice much effect because we talk about the superior oblique muscle helping with that looking down and out looking down looking you know depressing the gaze and abducting somebody with damage the trochlear nerve might notice like a bit of diplopia bit of double vision when reading use your highs going down the page or when you're going down some stairs and you're looking down and what I can't move quite as it should so you get a little bit double vision bit at the appropiate that's all so cranial nerve six the abducens nerve is another small nerve because it's only innovating one muscle this is huge compared to how big the muscle is within the orbit the muscles like it's like that long it's you know in that it's a tiny little tiny little muscle so it's a small nerve again this one comes out of the pons as we saw and the nerve crosses sides so it causes from one side of the brain to the other side of the body and if you have an injury affecting the abducens nerve the effect is a little bit more obvious because the lateral rectus and the medial rectus muscles working position right so if the lateral rectus muscle has no innovation then there's nothing to oppose medial rectus so when the I would be in the neutral position the affected I would tend to be pulled a little bit medially because me that the resting tone of medial rectus pulls immediately rather than being balanced out by the pull of lateral rectus right so that means you'll have a little bit of a medial strabismus so strap is must be a bit of cross ties right so again we get a bit diplopia a bit of medial strabismus it will be possible to move the eye into the neutral position mostly probably by relaxing medial rectus you know but it will probably be impossible very difficult to abduct the eye past the neutral position if the abducens nerve has an injury now if there's a lesion affecting the the nuclei of the abducens nerve it gets a bit more complicated too complicated for right now in fact but that's the most common that's the most common sign yeah bit of media strabismus so we know what about the muscles and we know all about the nerves and something I think I touched on the other week was the fact that when somebody does a cranial nerve exam and they're testing the muscles of the orbit the muscles that move the eyeball one of the tests is to get the patient to follow the finger as it moves in an aged shape and ask if they get any double vision to make sure they can follow the finger make sure that both eyes move together and what-have-you but the movements we asked them to do don't match up with the movements that we described the muscles as doing and what we're doing is well if we consider just one eye we can test the lateral rectus so we can test cranial nerve six by asking the patient to abduct their eye you you wouldn't do one eye closed you do with both eyes to make sure they're both moving together right I'm just making sure you understand I'm just talking about this eye so we we test lateral rectus the abducens nerve and we can test medial rectus which is part the oculomotor nerve her you know three by adopting the eyeball so if we get somebody to look laterally the the gaze is in line with superior and inferior rectus muscles so only superior rectus will let them I'm turning my head on I'm cheating Bad's though look laterally superior rectus then is the only muscle that can elevate the gauge and inferior rectus is the only muscle that can depress the gaze because the pupil comes in line with the muscles because the muscles are running out and ankle gonna have a look at that video if you don't know what I'm talking about likewise then if we move the vision medially the only muscle that can elevate the gaze is inferior oblique because the way it pulls on the back of the eyeball almost so inferior oblique elevates the gauge and then superior oblique depresses the gay so by doing that you're testing cranial nerve six growing your nerve 3 when you're near 3 cranial nerve three cranial nerve three cranial nerve three cranial nerve for that makes sense the abbreviation some people use is can't even remember lr6 so4 ao3 I can't usually remember these things so LRC its lateral rectus quite it is six pregnant no six ASO for superior oblique craig'll endure for AO all the other muscles in the orbit and renovated by cranial nerve three that's helpful for you learn that but does that make sense with the so that's it those are the three cranial nerves that innervate the muscles of the orbit it's not terribly complicated because you have one to one muscle one no to the other nostril then one than all the other muscles and what is complicated is when we look at how those nerves get into the eye and innervate the intra ocular muscles and other bits and bobs and we'll do that another time after many more cups of tea all right hope that was helpful see you next time let's hide well they certainly [Music] you