Applied anatomy and Physiology of the eye Flashcards
describe the histological features of the cornea?
Epithelium – stratified squamous non-keratinised.
Bowman’s membrane (basement membrane of corneal epithelium)
Stroma – regularly arranged collagen, no blood vessels
Descemet’s layer
Endothelium – single layer (normal - 2500 cells/mm2)
what is a commonly used instrument in ophthalmology?
slit lamp
what can you see in this slit lamp image?
This art like structure you can see is the slit of rays that are passing through the cornea.
The outer white line that you see that is nothing but your Bowman’s membrane,
which was the basement membrane of the epithelium.
All of this thickness in here is the stroma made up of regularly placed collagen fibres.
And at the other end, the little white line that you can see there is the Descemet’s membrane.
through a slit lamp
you cannot appreciate the corneal endothelium. So that’s what a normal cornea looks like
using a slit lamp.
how would you describe the histology of the cornea?
Histologically – regular arrangement of collagen in stroma
No blood vessels
Endothelium cell layer has a pump that actively keeps the aqueous humor out.
If you close one eyelid and place an index finger on top of that eyelid and gently press down, what is that firm resistance?
So if you close one eyelid and place an index finger on top of that eyelid and gently press down, that firm resistance you are feeling is your intraocular pressure, which is caused by the pressure at which aqueous humor is maintained inside the anterior chamber.
So this fluid is under pressure, a pressure of 21 millimetres of mercury,
and that is therefore constantly trying to push against the cornea and trying to get into the stroma of the cornea.
The endothelium has a very active pump which helps to keep this fluid
So the endothelial health and therefore, the endothelial pump is very important in maintaining transparency.
So why might you lose endothelial cells?
When you end up losing endothelial cells, initially, the existing cells enlarge in size to cover that area and still keep the pump going.
But when you’ve lost enough endothelial cells, they cannot pump anymore, you’ll find that fluid starts to accumulate in the cornea.
And that leads in turn to opacification, because the fluid moves the collagen fibres in the stroma apart.
Well, there are some degenerative disorders that lead to that, but also in simple procedures such as surgery to remove the cataract.
You’re putting instruments into the space here to get to the lens, to remove the cataracts or opaque lens.
Endothelial damage can be a common side effect or complication rather, a common complication of cataract surgery.
how does the nature of healing the cornea differ from the rest of the body?
Unlike elsewhere in the body the very nature of healing of wounds of the cornea leads to loss of transparency!
how does Avascularity plays an important role in corneal transplant
The avascularity of the cornea is of benefit to surgeons when performing a graft surgery as it means there is a lesser chance of foreign antigens from a corneal graft being recognised by the recipient, so lesser chance of a graft rejection.
This has lead researchers to believe that the cornea is an “immune-privileged” site.
what does this image show?
corneal graft
describe the posterior and anterior part of the crystalline lens?
The parts of the eye in front of the crystilline lens is called as the anterior segment, the parts of the eyeball
behind the crystalline lens is called as the posterior segment.
You further divide the anterior segment into two chambers.
The bit in front of the iris is called as the anterior chamber.
And this tiny little space behind the iris, but in front of the lens is your posterior chamber.
And this tiny little space behind the iris, but in front of the lens is your posterior chamber.
We talked about the aqueous humor, which is a watery fluid produced by the ciliary body and occupies all of the anterior segment.
Therefore, both the anterior and the posterior chambers are filled with aqueous humor
that controls the intraocular pressure.
We also talked briefly about the aqueous humor: how it’s produced, how it circulates and how it gets reabsorbed.
And you use such a lens called as a gonioscope.
So there’s a person having the gonioscopy being done. You got the patient sitting on the slit lamp.
You have anaesthetised the front of the cornea and you’re putting in this lens,
which is going to allow light rays from the angle of the anterior chamber to come onto one of the three mirrors just here and reflect out from that,
which is what you are capturing from the slit lamp.
when you place a gonioscope, you’re standing in front of the cornea just there looking at the angle of the anterior chamber
just here. You see the iris just here. You see the cornea just here, the dome of the cornea.
And you can see the angle just here. If you push in and the angle is what we call as open,
you should be able to see a tiny bit of the end of the ciliary body and the scleral spur just after that.
Just beyond that, you should notice a light brown circumferential ring that goes all the way round.
And that is your trabecular meshwork.Remember that the Schlemm’s canal is deeper to the trabecular meshwork, and therefore you cannot see it using a gonioscope.
Why would you want to look at this angle of the anterior chamber?
As I said, the drainage of the aqueous is controlled by structures at this angle.
And sometimes you might have things, for instance, adhesion of the iris to the periphery of the cornea.
Then the iris, which is vascular, gets very sticky and sticks to the periphery of the cornea.
The aqueous now is unable to go into the angle
and drain through the trabecular meshwork. You call such angle as closed.
Sometimes there might be degenerative changes happening to the trabecular meshwork which means that they are no longer able to,
it’s no longer able to drain the aqueous humor. And all these conditions which impede the draining of the aqueous humor
leads to an increase in intraocular pressure. An increase in intraocular pressure is called as glaucoma.
And if left untreated, this pressure will ultimately end up affecting the optic nerve cells on the retina,
describe histology of the retina and choroid?
The retina extends anteriorly just anterior to the equator of the eye, ending in a serrated margin called as ora serrata.
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In other words, it does not extend all the way to the front of the eye.
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At this point, it’s also worth going back and looking at that recording on embryology.
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And you might remember that the eye itself was developing from the diencephalon - one of the vesicles of the developing brain.
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And as it develops and reaches the surface ectoderm, that bit of the surface ectoderm thickened and invaginated,
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causing the optic vesicle to become a two layered optic cup.
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The stalk was going to go and form the optic nerve later on.
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But the two layered cup was going to go and form the different parts of the retina.
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Histologically, you will see that there are 10 different layers that form the retina.
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Before you ask me that question, let me tell you. No, you do not need to remember each and every layer.But just a few. These two here are quite important.
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The outermost part of the retina is the pigment epithelial layer.
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So called because its cells have a lot of pigment and it sits just on top of the choroid.
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The next layer up from the retinal pigment epithelial layer is the layer of rods and cones.
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Rods and cones are modified neurones and they are the light sensitive cells which have the photoreceptors.
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If you look at the rods and cones in detail they look like so.
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And it is their outer segment that contains the photo-active receptors.
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And it is because of their outer segment shape that you call them either
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the rods or the cones. So that is the light sensitive layer, the layer of rods and cones.
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And then the next few layers are nothing but the nuclei, the axons, dendrites of the next neuron, the next neuronal cell body, their axons.
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Until you come to this layer, which is the ganglion cell layer,
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that is where the cell bodies of the optic nerve resides and their axons come out at this layer here,
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layer nine, which is called as the nerve fibre layer.
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All these layers on the surface of the retina then congregate together and leave the eyeball as the optic nerve just here.
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So a couple of things to think about. The retina is sitting on the choroid.
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And the choroid, which is here, is highly vascular. You can see the different sizes of blood vessels in the choroid.
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So the outer layers of the retina are going to be supplied by choroid.
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We also studied that as the optic nerve forms, the central retinal artery, which is a branch of the ophthalmic artery,
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enters into the eye through the optic nerve. And you see it coming in to the surface of the optic nerve, which is where it’s going to
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divide into four branches and the blood from the central retinal artery is going to supply the outer layers of the retina,
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including your nerve fibre layer. So that is the first point you want
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to understand. The second point is about this optic cup.
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The two layers of the optic cup have a space between them which gets obliterated when the retina is formed.
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This outer layer gives rise only to the pigment epithelial layer of the retina.
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While the inner layer of the optic cup is what gives rise to all of these nine layers of the retina.
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And that potential space between the outer and the inner layers of the optic cup can form once again in an adult retina.
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And when that happens, the retina is usually said to be detached.
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So when you come across the term retinal detachment, it is worth understanding that it is not all 10 layers of the retina that are detached.
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But usually the inner nine layers detach from this potential space, from this tenth layer,
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because fluid has filled in the potential space between the two layers of the optic cup.
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The third thing to remember and understand is how light rays come in. You know
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that the light rays have to pass through from the front of the eyeball to the back
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and passes through all of these transparent structures - the cornea, the aqueous humor,
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then through the pupil, the lens and the vitreous to reach the retina.
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This is where the vitreous would have been.
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And so light rays actually pass through here before and through all of these eight layers before they reach the layer of the rods and cones.
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And surely that is not a very efficient method, is it?
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For the light to pass through eight different layers before it can act on the layer of rods and cones?
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So keep that thought at the back of your mind, because we will go and look at that again.
what is site of maximum visual acuity?
We also looked at the posterior aspect of the eye and how that could be seen using special instruments called as the ophthalmoscope.
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In fact, you will be doing this as one of your examination sessions in clinical skills.
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When you look at the retina,
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it looks like so. The bright orange colour is the blood supply of the choroid shining through the nearly transparent retina.
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This yellow spot in the middle is the physiological blind spot.
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And that is the head of the optic nerve. These blood vessels that come to the surface and radiate out are the central retinal artery
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and vein, their branches. But now I want you to focus on this bit here in the centre, and that is called as fovea centralis or simply the fovea.
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That is the point of maximum visual acuity.
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So when we are looking at something, for instance, a spot on the wall,
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the image of that spot is going to fall on your fovea. When you’re reading something on the slide,
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the image of that word you are reading is going to fall on that fovea.
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And as we read through the lines on a page, our eyes move so that the image of the word that we’re reading can fall on the fovea.
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So the fovea is that central part of the retina which has the maximum visual acuity.
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So what is it that gives it this maximum visual acuity?
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For one thing, it’s got more of the highly sensitive cones than the rods.
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There are almost no rods in the fovea centralis. Instead, all you have is a high proportion of cones.
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Secondly, remember how we said that it is not very efficient to make light pass through these eight layers before it reaches the rods and cones?
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Well, at the fovea you find that all of these other layers are pushed away to the side, which brings the layers of the cones almost to the surface.
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But because of that the fovea centralis is much thinner than the rest of the retina.
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So the fovea centralis is that central spot, which has the maximum visual acuity.
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It has the maximum visual acuity because it’s packed with cones
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and because the outer layers of the retina are pushed away peripherally making the fovea quite thin.
