hello if you have come first time to our channel i dr amrit welcome you to insight ophthalmology and if you are a returning subscriber thank you for following us today i'm going to take a lecture on anatomy of optic nerve as we all know that the optic nerve starts from the optic disc to the optic chiasm that is if we look at this diagram of a fundus of an eye over here is the optic disc so our optic nerve starts right from the optic disc and goes up to the optic chiasm let me first tell you briefly about the uh about the visual pathway although i will be taking a detailed lecture on visual pathway later so the visual pathway starts right from the retina so this is our retina and then the next what we have is the optic nerve optic nerve of both the sides will coils together and form the optic chiasma from the optic asthma we have the optic tracts then we have the lateral geniculate body finally we have the optic radiations and these optic radiations are ending in the visual cortex right so this is a very basic components of the visual pathway okay but over here in this lecture i will be telling you about the anatomy of the optic nerve that means it is starting from here that is the optic disc and going up to the optic chiasms so let us see some saline features about the optic nerve so our optic nerve first starts from the optic disc to the optic chiasm and it is actually nothing but the backward continuation of the nerve fiber layers of the retina so over here if you see the fundus if you look at the fundus of the eye there is a layer the innermost layer of the retina which is called the nerve fiber layer which is very shiny okay it is the backward continuation of all these fibers all these nerve fiber layers they go up to the optic disc and it is nothing but the backward continuation of these nerve fiber layers which forms the threat which forms the optic nerve one more thing that i want to know is that optic nerve is basically a sensory nerve and it carries the sensation of vision there are basically five main components of vision which we must all know the first is the light sense that is the optic nerve is taking the fibers of light reflex okay we are able to see light because of the objective second is the color sense the sensation of colors are also appreciated because of optic nerve third is the contrast sensitivity that means the ability to distinguish two objects based on their illumination that is contrast sensitivity third is the form sense the ability to differentiate two images especially based on their shapes that is visual equity okay fourth fifth one is the visual fields how much field of vision can you see so all these are the five components of vision and all of these five components are basically taking place because of our optic nerve okay yeah so optic nerve is uh is a sensory nerve but there are a lot of ways in which it differentiates it is different from the other sensory nerves or the other peripheral nerves of the human body so let me tell you what are those differences the first important difference is that unlike the peripheral nerves the optic nerve is not covered by the newly lemma that means that whenever the optic nerve gets cut it will not regenerate it will not regenerate so this is the first point second thing is there are about millions of nerve fibers which are present in the optic nerve but these fibers are very very fine they're about 2 to 10 microns however in the other peripheral nerves the diameter of these fibers will be about 20 microns so this is another difference and one more difference from the peripheral nerves is that it is covered by meninges optic nerve is basically considered to be an out pouching of the brain so if something is coming out from the brain it also takes along the covering of the brains that is the meninges okay so optic nerve is the only sensory nerve which is actually covered by the meninges okay so that is also one the optic nerve is about 50 millimeters in length and it can be actually divided into four parts starting from the optic disc to the optic chiasm okay so these four parts of the optic nerve are number one the intraocular part of the optic nerve that is the part which is present inside the eye it is about one millimeters the intraorbital part of the optic nerve that means the part which is present inside the orbit it is about 25 millimeters the intracanalicular part of the optic nerve that means the part which is present inside the canal the optic canal it is about nine millimeters and then finally the intracranial part of the optic nerve just before the optic chiasm the part which is present inside the upper inside the cranial cavity now uh i'll give you a very easy way to remember these dimensions so the intraocular part you can remember as 1 mm it is nothing but it is 1 square okay square of 1 is 1 the inter orbital part is actually 5 square okay what is the square of 5 it is 25 the intra canalicular part you can remember it as square of three okay it is nine and the intracranial is square of four that is four fours are sixteen all right yeah let us talk about the intraocular part of the optic nerve that is the part of the optic nerve which is present inside the eye that is why it is called intraocular part of the optic nerve and i already told you that it is about 1 mm in length ok so the intra ocular part uh how does it form actually it is a part which is present inside the optic inside the eye so our sclera actually has some amount of fenestrations inside it and these fenestrations or holes are actually called lamina cribrosa so the optic nerve nerve has millions of fibers i already told you right so those millions of fibers will pass through these laminar cribrosa through these fenestration and then they will enter into the choroid and finally they are going to enter inside the eye and inside the eye they are going to form what is this optic disc okay so this is very important to know that the intraocular part of the optic nerve is present inside the eye and how do they go inside the eye they pass through the lamina tribrosa okay so coming to the average diameter of the optic nerve this average diameter of the optic nerve inside the eye is about 1.5 millimeters okay so how much it is it is about 1.5 millimeters but if you see outside this clearer over here this part outside this clearer it will become almost double that is three millimeters just behind the sclera and why does this happen this happens because beyond the level of square that means inside the eye the optic nerve is not myelinated okay however when we go outside the level of sclera okay that means the retro laminar region i will tell you what is retro laminar region it will acquire myelination and because of that myelin the diameter of the optic nerve will get doubled from 1.5 to almost about 3 millimeters now the intraocular part of the optic nerve can again be divided into four parts okay and what are these four parts let me tell you the first is surface no fiber layer second the pre laminar region the laminar region and the retro laminar region and let me tell you that these four parts of the intraocular optic nerve are divided based on the presence of lamina cribrosa that's why we are hearing the word over your laminar laminar lamina okay because they are divided in with respect to the laminar region okay so let us see how how this works up so the first layer is the surface nerve fiber layer okay so this layer over here is the surface nerve fiber layer okay this is the layer what we see actually in our fundus examination in the optic in the fundus okay so the surface nerve fiber layer actually is made up of mainly mainly it is the nerve fibers about 94 are the nerve fibers and astrocytes are very less about only five percent just behind the surface nerve fiber layer and in front of the laminar cribrosa is the pre laminar region because it is present in front of the laminar cribrosa so what is the difference in this pre laminar region from the surface nerve fiber layer nothing that it is more dense okay more number of neurons and more number of astrocytes are present in this prelaminar region coming to the region where we are seeing this small small holes or you can say fenestrations this is a region of laminar cribrosa so this is the sieve like structures through which the optical fibers are passing and this is called the laminar region of the optic nerve because it is formed which is passing through the lamina cribrosa and finally we have one more which is called the retrolaminar region which is behind this laminar cribrosa region and in this retro laminar region the oligodendrocytes are an addition so what extra do we have here it is the oligodendrocyte and what is the function of oligodendrocytes they will secrete myelin and this is the reason as i already told you that the average diameter of the optic nerve head will also increase to about to about three millimeters behind the sclera i hope it is clear let us talk about the intraorbital part i already told you that the intraorbital part is a part of the optic nerve which is coming from the intraocular part to the optical foramina that is the point the okay so this highlighted portion is the intraorbital part which is a part present in the orbit and the length of this intraorbital part is about 25 millimeter i already told you that now this intraorbital part is usually very very sinus okay what i mean to say is it is like this and why is it sinus to allow for the ocular movements the eye has to look towards left right up and down and it should have a free movement if the optic nerve itself is very taut and tight and is not lengthy enough the movements will be difficult and that's the reason that the intra orbital part is also the lengthiest part it is about 25 millimeters in length and it is sinus so that the movements can easily happen okay and at the level of the optic foramina that is over here we have uh this optic foramina that is here let me show you here this is the optic parameter if you see this is the you know orbit diagram and this is the optic formula and this is the superior orbital fissure right so at the level of the superior orbital fissure we have a tendinous ring which is attached and through this common tenderness ring of zin it is called the ring of zin various extraocular muscles will be taking their origin that means the superior rectus the the medial rectus the electro rectus and the inferior actors of course takes a origin from down so all these mass inferior vectors also from here and free oblique will take it from down so these muscles will take origin from this analysis of zin okay and various structures will also pass through this analyst of zinn okay so that is one important thing that we should remember uh with respect to the intraorbital part of the optic nerve that at the end we have an optic foramina and at the optic foramina we have an annulus uh of zinn through which various extraocular muscles are taking their origin that is the superior rectus the medial rectus okay the inferior rectus and also the lateral rectus so what happens is whenever there is inflammation of this part of the optic nerve that is the intraorbital part so these muscles will also get inflamed okay so that is the reason that in retro-orbital neuritis what happens is inter-orbital neurite is whenever the patient tries to look towards left side or right side and usually with the horizontal movements and especially when they want to look up okay so there will be pain so in so that is the reason why there is painful ocular movements in retro bulbar neuritis and why is it so because of the involvement of the superior rectus and the medial rectus which are associated in very close relation to the optic nerve at the level of the optic foramina because of the origin from the analysis of zinc i hope it is clear next important point that you should know is regarding the central retinal artery now central retinal artery we know is an important artery and it takes its origin from the ophthalmic artery so this is the diagram which is showing the or various arteries in association to the optic nerve and as you can see the central retinal artery is taking its origin from the ophthalmic artery and it is actually passing inferiorly and slightly medially to the optic to the optic nerve okay so this this relationship we should know that the central retinal artery passes inferiorly and it is from the inferior medial aspect only that it enters the optic nerve so our central retinal artery actually this is a cross section which is showing that the central retinal artery is actually passing through it through the optic nerve it passes and how it is entering it is entering from the inferior i told you about the central retinal artery and also what i want you to know is about the ophthalmic artery so ophthalmic artery also has a very important relationship to the intraorbital part of the optic nerve it is present slightly inferiorly to start with and then it is lateral to the optic nerve and then gradually it is turning medially from above okay from lateral to medial side it turns and then finally it continues medial to the optic nerve so this important relationship is also very important with respect to the ophthalmic artery which is changing its direction from lateral side to the medial side in the intraorbital part of the optic nerve third relationship one more important relationship we should know with respect to the intraorbital part is with relation with respect to the ciliary ganglion and the various other nerves which are present inside the orbit that is the branches of the oculomotor nerve the nasoceliary nerve the sympathetic nerve and the abdasen nerve so all these nerves whether it is oculomotor nerve abducen nerve nasociliary sympathetic ancillary ganglion they are also related to our intraorbital part of the optic nerve let me show you hi how so this is our optic nerve over here and we can see that between the optic nerve and the lateral rectus so this is our lateral rectus if you extend this it will be like this so in between this lateral rectus and the optic nerve we have few structures which are passing and these few structures are the ciliary ganglion okay then we have the necessary nerves most of these nerves are passing lateral to the intraorbital part of the optic nerve okay so this was about the intraorbital part of the optic nerve now the third part of the optic nerve is the intra canalicular part of the optic nerve so in this intracanalicular part of the optic nerve i already told you that this is the part which is passing in between the optic canal okay and how much is the length of this intra canalicular part of the optic nerve the length is about nine millimeter this intra canalicular part also is very closely related to the ophthalmic artery and one more important relationship of this intracanalicular part of the optic nerve is with relation to the sphenoid sinus and the ethmoid sinus so we know if this is the orbit and these are our frontal sinuses which are present sorry and here we have the ethmoidal sinuses and behind that we have our sphenoid sinuses so over here will be our canal okay and the intracanalicular part of the optic nerve is very closely related to these two sinuses whether it is spinoid or ethmoid sinus and moreover the separation between them is very very thin bone and that thin bone is a very thin bony lamina okay so what happens is whenever there is a sinus infection also they can be retro bulbar neuritis because the infection can infection and inflammation can spread to this thin bony lamina and cause inflammation of the optic nerve so that is important with respect to the intracanalicular part another thing which you should remember is that this intra molecular part is a very narrow canal and there is an optic nerve also which is present so whenever there's injury to the uh bony fragments injury to the bone of the skull it's most uh commonly seen in road traffic accidents what we see is fracture of these bony segments and these bony segments can actually impinge on the optic nerve and cause what is called traumatic optic neuropathy or ton okay this is a very basic concept about dramatic optic neuropathy a lot of else a lot of other details are also there about it but remember that most of the time in row traffic accidents also it is the intracanalicular part of the optic nerve which will get damaged okay yeah finally coming to the fourth part of the optic nerve and that fourth part is the intracranial part of the optic nerve that is the part which is present inside the brain and this interactional part of the optic nerve is about 16 mm in length okay so this part of the intracranial nerve intracranial optic nerve is actually present above the level of the cavernous sinus and both these both the optic nerve of both the sides are going to meet at one location to form the optic chiasm chiasm is nothing but it is the crossing okay so as you can see over here this nasal fibers of this optic nerve and nasal fibers of this optic nerve are fusing together and crossing at a part which is called the optic chiasm okay and this happens over a region called die from a cell okay now what are the other important relations that we should know with respect to the intracranial part of the optic nerve is that we should know the relationship of the internal carotid artery the internal carotid artery as can be seen over here this part is mostly located below and lateral okay so it is present below and it is present laterally to this uh to the optic nerve okay and from this point only the ophthalmic artery will arise from the internal carotid artery below the level of the optic nerve so it is in the intracranial part the ophthalmic artery is actually below the level of optic nerve and then from the interconnellicular and the intraorbital part it will become lateral to the optic nerve and finally it will cross to the medial side all right yeah what are the other relationships to the intracranial part of the optic nerve the other relationships are the anterior perforated substance all these things will be present somewhere here the anterior perforated substance the medial root of the olfactory nerve and the anterior cerebral arteries all these are lying above the part of the optic above this part of the optic nerve on the meningeal sheaths of the optic nerve okay so we know that it is the only uh sensory nerve probably which is having the sheaths and the intracranial portion of the optic nerve that means the part which is present inside the inside the cranial cavity has only the fire matter okay just the innermost layer the innermost meninge only is present in the intracranial portion whereas if you talk about the other portion sorry this is not intercranial this is if we talk about the intracanalicular and the intraorbital portion they will have all the three layers of meninges that means the pyramidal the arytenoid matter and even the dura mater these three layers will be present in the intraorbital and in the intra canalicular portion of the optic nerve moreover the subarachnoid and the subdural spaces of the optic nerve are actually continuous with that of the brain and that is the reason that whenever there's a raised intracranial pressure there will be papilloedema why because all that pressure is being transferred to the optic nerve and you will see a raised disc okay yeah and all these layers will actually terminate at the level of sclera that means intraocularly there is no meninges okay meninges are present afterwards only they are not present up inside the eye so up to the level at this clearer all these meninges will get fused with this clearer right yeah and at and one more thing you should know is about the dura the splitting of the dura which happens at the apex of the orbit the apex of the orbit is nothing but the point where the optic phenomena is actually ending or the optic canal is ending so right at that point the dura mater will split into two layers okay and the outer layer will get continuous with the periosteum of the orbit what i mean to say is if the denominator is coming like this the outer layer will get fused with this bone okay the periosteum of the orbit and it is the inner layer which will form the sheath of the optic no it is only the inner layer which will form the sheath of the optic nerve okay so that's all about the anatomy of the optic nerve if you have any doubts kindly comment and ask in the comment section thank you and have a nice day