Cornea, lens and virteous Flashcards
-what is the function of the eye
- whole structure of the eye is related to its primary function as an organ of photoreception
- the eye is designed to convert light energy into electrical energy (action potentials) which the brain then interprets as an image
brain can only interpret electrical energy in the form of action potentials
what structure is behind the cornea
- fluid filled structure called aqueous humour
- which lies in the anterior chamber
what structure lies behind the anterior chamber
- the iris - which is the coloured part of the eye
- where is the anterior segment located and what does it include
- anything that is in front of the lens is the anterior segment
- -
the cornea, lens the anterior chamber , the iris
where is the posterior segment located
- anything below the lens is the posterior segment
where is the posterior chamber located
- the posterior chamber is part of your anterior segment - it is between the iris and the lens
what is the differnece between the posterior and anterior segments and the posterior and anterior chambers
- the segments are the parts of the eyes in front of the lens and behind the lens - these are not specific structures and these segments are compirsed of several structures
- and the chambers are specific parts of the eye -
what is the average axial length of the eye
- the axial length is from the front of the cornea - - the length of the eye from front to back -
the average axial length is 24mm
- this affects your vision because of focusing - if your eye is of the appropriate length and it has the right power - it all comes to a single point - if the eye is too short - your eye would focus on two seperate pints and your image would be blurred - and if your eye was too long the light would focus on two seperate points - which means that you would have a blurred image because it is not focusing on a single point
if you have an eye that is too long what refractive error would you have -
- if your eye is too long you would be short sighted - ,myopic
- axial length - 26-29mm
- because if your eye is too long that means that images that are coming from very far away - they will not be focusing on a single point - if you are looking at a near image the light will be in a divergent pattern - which means that your eye will have to do a lot more work to refract it - so that by the time it gets to the back of the eye it gets to a single point.
- that is why if your eye is too long you become short sighted because near objects the light is not coming straight on it is already coming in a divergent pattern - - your eye has o do more work the more distance their is the better it is for the eye - having a long eye allows you to see close objects better - but then your not able to see far away objects because if your looking at anything that is far away and the light is coming straight on by the time it gets to the back of the eye it is not focusing on a single point
- light is focused in front of the retina
- increase in axial length of the eye increases the converging light rays to be focused in front of the retina
if your eye is too short what refractive error would you have ?
- you have axial hypermetropia which means that you are long sighted
- eye = too short - 20mm
- near objects appear blurry
- light rays entering the eye are focused behind the retina
- short axial length of the eye
in what segment is the virteous humour?
- the virteous humour is in the posterior segment of the eye
what is the refractive index
- how fast light travels through a material
- air has a refractive index of 1
what is the degree of refraction porportional to
- degree of refraction is proportional to the difference in refractive (n) between the two optical media
- refraction = bending light
when you are moving from one medium to another e.g. air to water - or air to aqueous humour - any sort of difference in medium changes the direction - of the light
- the bigger the difference between the two media - the greater the change in direction -
- the light will be refracted more if you have a greater difference in the refractive index between the two media - the light will be bend a lot more
- -
why is the cornea a important structure in terms of refraction
air has a refractive index of 1 and will hit the cornea which has a refractive index of 1.37- so their is a difference of 0.37
- at the corneal interface - at the cornea you will have a tear film - - we call this the air/tear film interface
- because their is a big difference in refrcative index their is a fairly big difference in the direction of the light -
when it comes to the lense - it is in the aqueous humour
- aqueous humour has a refractive index of 1.33 and the lense has a refractive index of 1.41 - the difference is only 0.08- which means that although the lens is important in refraction - it will not have a much of an impact as the cornea will that is because their is not much difference between the refractive indicies of the aqueous humour and the lens -
how much refractive power does the cornea account for
- the cornea accounts for 2/3 of refractive power of the eye (43 dipoters)
describe the structure of the cornea
- transparent structure - if it is not transparent then the light will not be able to get through - cornea needs to be transparent
- oval shaped - 10.6mm vertically and 11.7mm horizontally - shorter vertically than horizontally - when you are measuring the pressure of the eye the thickness of the eye affects it
- thinner centrally (- 540 um) and thicker peripherally ( - 670 um)
- presents a physical barrier against trauma and infection
why does the cornea account for more refractive power than the lens
- because their is a big difference in the refractive index between air and the cornea and the tear film interface the cornea is responsible for 2/3 of the total refractive power of the eye
- the eye itself has a refractive power of 60 diopters and the cornea is responsible for 43 and your lens is responsible for 17- 20 because it can change its shape
what is refractive power a function of and how does this affect the cornea and the lens
- refractive power is a function of the difference in the refractive indicies between two media
- air/ tear film interface at the cornea - large difference - high refraction
- aqueous/ lens interface - smaller difference - less refraction
how does the cornea maintain its transparency
- avascular structure (i.e. no blood supply)- no blood vessels in the cornea
- smooth epithelium
- regular stromal arrangement
- relatively dehydrated - compared to the aqueous humour is fairly dehydrated - their is not a lot of water in the cornea - the more water their is the more opaque something becomes
what are the 5 layers of the cornea
- corneal epithelium - top of the cornea - around 5 to 6 layers thick
- bowmans layer - this layer has no cells - acts as a support to the epithelium
- stroma- usually 200- 250 layers thick
- decesments membrane - basement membrane for the corneal endothelium which is at the bottom
- endothelium
describe the structure of the corneal epithelium
- stratified (layered), squamous (cell type thats in the corneal epithelium) , non keratinised epithelium
- 5- 6 layers thick
- 50- 60 micrometers thick
stem cells present at the limbus- limbus is the edge of the cornea - where the cornea ends and the sclera starts (palisades of vogt)
- these stem cells repilcate by mitosis allowing replacement of the epitheliel cells - if you lose corneal epitheliel cells then your cornea is unable to repair itself
- that can lead to epitheliel defects
- corneal epithelium responds to disruptions by amboeid sliding movements - - esentially covering the defect -
- corneal epi
corneal epithelium responds to disruptions by ammeboid sliding movements
describe the structure of bowmans layer
- acellular
- composed of collagen
- 8-12 um thick - very thin
- cannot repair itself - compared to corneal epithelium the stem cells cannot replacde those cells- because it is an acellular structure - corneal epithelium heals by scarring -
describe the structure of the cornea
- 90% corneal thickness
- 200- 250 layers of collagenous lamellae
- modified star shaped fibroblasts called keratocytes lie between each lamellae
- the lamellae are regualy arranged - which is essentail for corneal transparency
- if the cornea wasnt regualry arranged it would be white and you wouldnt be able to see anything at all -
- important because the sclera- which is the white of the eye- has the same structure as the cornea - because the collagen instead of being layer over layer is just a random arrangement you end up with a white structure.
how is the stroma involved in the maintence of corneal transparency
- regular arrangement of the collagenos lamellae - keatcoytes help maintain the regular arrangement
- regular diametre and spacing of collagen fibrils- allows them to maintain the regular arrangement - regulated by glycosamnioglycans (GAGS)
- No blood or lymphatic vessels present in the stroma - would interfere with light entering the cornea
- state of relative dehydration - not a lot of water in the stroma
why is the sclera white
- because of the irregular arrangement of the collagenous lamellae
describe the structure of descements membrane
- descements membrane is composed of a fine lattice of collagen fibrils (different from the collagen in the stroma)
- 8-12 micrometres thick
- acts as a basement membrane for the corneal endothelium
-
what is corneal oedema
- when the cornea becomes filled with water
- the descements membrane which is very thin begins to fold on itself
describe the structure of the corneal endothelium
- at the bottom of the cornea
- simple sqaumous epithelium - it dosnt have several layers -
- an epithelium is a tissue whose cells lie on a basement membrane
- the corneal endothelium is of the epitheleil cell type - epithelium = any tissue that lies on a basement membrane
- csince the conreal endothelium lies on descements membrane - which acts as a basement membrane - the tissue type is epitheliel - however the tissue itself is distinct from the corneal epithelium
critical role in maintaining level of corneal hydration
- has lots of mitchondria - lots of energy needed to actively pump water out of the stroma
because it lies on a basememt membrane the tissue type is epitheliel cells - the corneal endothelium which is the layer in the bottom has a epitheliel cell type because of descements membrane
corneal endothelium is esponsible for maintaining the corneas relative state of dehydration - corneal endothelium uses energy to push water out of the stroma
describe the corneal endthelium pump
- to maintain corneal clarity , endothelium has sodium pottasium atpase pump (active transport) to pump sodium ions ( low water potential) back out of the aqueous from the cornea against a conc grad
- water moves down the conc grad and the stroma is kept realtively dehydrated
-
why does damage to the corneal endothelium cause corneal odema
- corneal endotheliel cells have a very poor regenerative capacity - damage to the corneal endothelium can rapidly lead to corneal oodema and opacity since water accumulates in the stroma
- if it gets damaged their is nothing to push the water out of the cornea which means water keeps accumulating in the stroma - as water accumulates you end up with a opaque cornea - whitish apperance due to lots of water in cornea = corneal odema
what nerves innervate the cornea
- nerves that supply the cornea - you need nerves as a protexctive mechanism - because if you have something touching your eye you have a reflex to blink and that is called the blinking reflex -
- or if their is dust or wind
- opthalmic division (v1 ) of trigmeninal nerve
- splits into anterolateral portion of cavernous sinus into
v1 has 3 branches
lacrimal
frontal
nasocillary- gives off long cillary nerves
- long cillary nerves supply the cornea - provides sensation to the cornea -
terminal branches perice bowmans membrane
pass between epitheliel cells
how much refractive power is the cornea responsible for
- accounts for 2/3 of total refractive power of the eye (due to higher diifference in refractive at air/ tear film interface)
what are the 5 layers of the cornea
- corneal epithelium
- bowmans membrane
- stroma
- descements membrane
- corneal endothelium
how is the stroma involved in maintaining conreal transparency
- stromal collagen is regularly arranged and the stroma is kept relativley dehydrated for corneal trasnparency
how does the corenal endothelium keep the stroma dehydrated
- corenal endothelium activley keeps the stroma dehydrated through the corneal endothelium atp-ase pump
pumps sodium ions out into the aqueous -
what does damage to the cornea endothelium lead to
- conreal odema
- due to poor regernative capacity of the cells in the cornea
what is the cornea innervated by
- cornea is innervated via the long cillary nerve (which comes of the nasociallry nerve ) which itself is a branch of v1 (opthalmic division of the trigeminal nerve)
describe the lens
- located just behind the iris
- transparent structure accounting for 1/3 (17- 20) dipotres of total refrcative power of the eye
- dioptric power can change as the lens can change its shape
- lies behind the iris and in front of the virteous humour
- it has a biconvex shape
-
what is the process called when the lens changes its shape?
- accomodation
non accomodative lens become thinner
accomodative lens becomes fatter - refractive power of the lens increases
describe the structure of the lens
- enclosed in a capsule
- biconvex sructure
- continues to grow through life
- encircled by zonular fibres - zonules connect the lens to the cillary body which has a muscle which can contract - when this muscle contracts it is responsbile for accomodation - the lens is surrounded by zonular fibres
- lens gets thicker as you get older
what happens during accomodation
- your cillary muscles contract
your zonules become less tight - (tension in zoular fibres is released)
- which means that your lens is less stretched and becomes fatter - increased dioptric power
- and therefore more powerful
accomodation reflex kicks in when looking at a near object
what are the 3 components of the accomodation reflex
- convergene (eyes move in towards object of interest)
- miosis (increases focus) - pupils constrict - pupil constriction = miosis
- lens becomes more powerful to focus image (cillary muscles contract - zonular fibres become less tight - lens becomes fatter - increasing dioptric power)
what is a cataract
- loss of lens transparency - - when the lens loses its transparency
what are the causes of cataracts
- age - happens over years
- uv light exposure - e.g. exposed to lots of light
- trauma - e.g. hit on the eye - you will have an immeadite catarat
- radiation -patients who have radiotherapy
- steroids - patients who are on long term steroids
conditons such as downs , diabetes
congential - e.g. rubella
post operative
what does loss of lens transparency lead to
-loss of lens transparency leads to cataract treated surgically (phacoemulsification)
what is the vitreous
- structure at the back of the eye
- transparent gel like structure lying in the virteous cavity
- accounts for 2/3 of eye volume
- 2-4 x more viscous than water
- 98% water
- remainder is hylauronic acid, collagen, fibronectin . fibrillin and opticin
gel like consitiency depends on the collagen
central virteous is more liquid than the peripheral virteous
describe the structure of the virteous
- the virteous has certain points of attachement to the retina at
the ora serrata - structure at the anterior edge of the retina - virtoeus base - very strong - where the virteous is most strongly attached - hard to pull the virteous away from the eye at this place
optic disc margin - fairly strong
- perifoveal - fairly weak
peripheral blood vessels - usually weak
- the edges of the virteous are usually thicker in consitency
what is the main function of the virteous
- it gives the eye its shape and it maintains its shape
what is a posterior virteous dettachement
- thin potential space between the virteous and the retina may fill with fluid in virtoeus dettachment
- as you get older your virteous becomes more liquid - the virteous usually has a gel like structure when it becomes more liquid it pulls away from the retina
- usually occurs due to age related changes in the virteous
- may lead to a retinal dettachment if the virtoeus pulls on the retina
- a person with just a virteous dettachment wont lose vision - if it has created a tear in the retina this person will lose vision- in these cases they do need surgery
describe the structure of the virteous
- gel like structure
- transparent
what happens to the virteous with age and what are the consequences of this
becomes more liquid with age - a process called virteous syneresis
posterior virtoeus dettachment may occur with age
and may lead to a retinal dettachement (if the virteous pulls the retina along)
How does the cornea recieve its nutrition
what are some different types of cataracts and what symptons may a patient complain of
how is a posterior virteous dettachement differnet from a retinal dettachement and how are the two linked
what are signs and symptons in a PVD and in RD
why is vision blurry when opening the eyes under water, but clear with goggles on
- because the refractive index is greater between cornea and air than the cornea and water
- higher difference in refractive index between two media = greater degree of refraction - which allows light to focus on your retina - difference in refractive index is important when your thinking about focusing images on the eyes
- by using goggles the swimmer is effectivley preserving the air/ tear film interface which provides a greater difference in refractive indicies and thus greater refraction to allow light to focus on the retina
- without goggles the difference in refractive index between water and tear film is so small that light isnt sufficently refracted to focus on the retina
why does the cornea have twice the refractive power compared to the lens
- the difference between the refractive index of air. - and the refractive index of the cornea is much higher than that between the aqueous humour and the lens which is why the cornea has twice the refractive power compared to the lens
- cornea is responsible for 2/3 of the refractive power whereas the lens is responsible for a 1/3
- air tear film interface at the cornea - large difference in refraction
- aqeuous/ lens interfafe - smaller difference - less refraction
what type of cells does the corneal endothelium contain
- the endothelium contains epitheliel cells
- because the corneal endothelium lies on a basement membrane - descements membrane which is above the corneal endothelium
- remember - anything that lies on a basement membrane is a of the epitheliel CELL type - this is different from the tissue itself
how many layers does the stroma have
- has 200- 250 layers of collagenous lamellae
how does the cornea recieve nutrients and oxygen
- the cornea is avascular - because it needs to be transparent
- instead the anteror cornea - the front surface of the cornea - recieves 02 mainly direclty from the air (which diffuses into the tear film)
the posterior cornea recieves its nutrients and oxygen via aqueous humour
- anterior cornea recieves its oxygen directly from air
- oxygen from the air diffuses from the tear film- and from the tear film it goes directly into the cornea
- some o2 and nutrients are also delivered indirectly by the anterior cillary arteries at the limbus
where is the limbus
- where the cornea ends and the sclera starts
what is corneal odema
- where the cornea stroma is being filled with water - this makes it lose its transparency
what nerves is the cornea innervated by
- the cornea is innervated by the nasocillary branch of v1 (opthalmic division of the trigeminal nerve)
what are the 3 branches of v1
- lacrimal
- frontal
- nasocillary
which of the branches of the opthamlmic division of the trigeminal nerve passes inside the ctr in the superior orbital fissure
- nasocillary branch
- the lacrimal and frontal pass outside the ctr
what happens during accomodation
-the cillary muscles contract ( they tend to contract inwards ) releasing the tension on the zonular fibres - because the zonular fibres become more relaxed and are not pulling the lens as much refractive and the lens becomes rounder which increases the refractive power which gives you extra power to see things nearby- it increases near vision
what symptons are most likely to be seen with a cataract
- gradual painless decline in visual acuity ( unless traumatic cataract)
- glare - often reported - as light scatters around the eye due to lens opacification catarcat = lens opacity - loses its clarity- so when the light is reaching it one point it is scattered in your retina- so that is why you have a glare
- as lens becomes denser - refractive power increases - this effectivley renders the eye more myopic and hence reduced the need for near reading glasses
- reduced need to wear glasses for reading - your lens becomes more dense- this increases refractive power - the eye is becoming more short sighted- if you have a more powerful refraction in the eye- that means light will focus more further in front of the eye- you become more short sighted which is why you dont need glasses for reading
what types of age related catarcacts
- nuclear cataracts - increase the refractive power - reduce the need for glasses
- cortical - - gives a spoke like pattern
- posterior subcapsule cataract - - lies in the middle of the pupil - because it is in the middle of your visual axis- it gives you lots of visual problems e.g. glare in the middle of the night - they have difficulty driving - because when headlights are shining in their eyes because it is in the middle of your visual field
what type of cataract is often seen in steroid induced catarcats
- posterior subcapsular is often seen in sterioid induced cataracts - and because they lie in the visual axis you often have glare symptons
what are some of the cataracts you can get from secondary causes
snowflake cataract - diabetes
- myotonic dystrophy - stellate cataract
- atopic dermatitis- eczema - - shield like cataract
what keeps the gel like consistency of the virteous
- the collagen
what is the most important differentiating feature between a virteous and a retinal dettachement
- visual loss- when a patient has a posterior virteous dettachement -the biggest concern = if they have visual loss- then you can be certain that they have also had a retinal dettachment - they may not yet have a retinal dettachment but sometimes they have a retinal tear in which case you may some blood vessels lying in the virteous
a pvd on its own does not cause visual loss
what is retinal dettachment
- refers to dettachment of the neurosensory retina - which contains the photorecpetos from the retinal pigmented epithelium
- leads to visual loss since the detached retina no longer detects light
can be caused by traction due to posterior virteous dettachment (pvd)
pvd on its own does not cause visual loss
