Structure and function of the eye Flashcards
What are the bones making up the orbit of the eye?
- Frontal
- Sphenoid
- Lacrimal
- Ethmoid
- Zygomatic
What is the average anterior-posterior diameter of the adult eye?
24mm
What are the two segments of the eye? What are the separated by? What fluid do their contain
Two segements?
-
Anterior segment
- Contains aqueous humour
-
Posterior segment
- Contains vitreous humour
Separated by LENS
What are the 3 layers of the coat of the eye?
Outer to inner:
- Sclera
- Choroid
- Retina
Describe the structure and function of the sclera.
Structure:
- Hard and opaque
- High water content - therefore opaque
- Continous with the cornea
Function:
- Protects eye and maintains its shape
Describe the characteristics of the cornea.
Structure:
- Transparent
- Dome-shaped
- Covers front of eye
- Most anterior part of the anterior segment
- Low water content - therefore transparent
NOTE: If you hydrate the cornea (i.e. increase the water content) then it will become opaque
Function:
- Powerful refracting surface due to its convex (dome) shape
- Provides 2/3rds of the eye’s refractive (focusing) power
- Physical barrier
- Infection barrier
What are the 5 layers of the cornea?
Outer to inner:
- Epithelium
- Bowman’s Membrane
- Stroma
- Descemet’s Membrane
- Endothelium
Describe the structure of the stroma.
- Layer 3 of cornea
- Thickest layer
- Highly regular arrangement of collagen fibrils for transparency
- The fibrils arrangement and water content determines transparency and opacity
- No blood vessels
- Has corneal nerve endings provide sensation and nutrients to cornea
- Nutrients provided via neurotrophins (growth factors)
Describe the characteristics of the endothelium.
Structure:
- Layer 5 of cornea
- Only one layer of endothelial cells
Function:
- Pumps fluid out of cornea, preventing corneal oedema
Characteristics:
- Endothelial cells have no regeneration power
- Endothelial cell density decreases with age
- Endothelial cell dysfunction may result in corneal oedema and corneal cloudiness
- Makes sense because hydrating the cornea makes it opaque
Describe the structure of the choroid.
- Pigmented and dark
- Composed of layers of blood vessels that nourish the back of the eye
What is the uvea?
Vascular coat of eye ball - refers to the entire network of vasculature of the eye
Lies between the sclera and retina
Composed of 3 parts - shown in red on diagram:
- Iris
- Cililary body
- Choroid
These three portions are intimately connected - therefore a disease of one part also affects the other portions though not necessarily to the same degree
What are the layers of the iris?
Anterior - stromal layer
Posterior - epithelial layer
Describe the structure and function of the lens.
Structure:
- Outer acellular capsule
- Basement membrane - composed of collagen
- Regular inner elongated cell fibres
- Regular arrangement gives transparency
- The lens may lose transparency with age → cataract
Function:
- Provides 1/3rd of the eye’s refractive power
- Higher refractive index than aqueous humour and vitreous humour
Its transparency and elasticity are also useful properties for its function
- Transparent - lets light in
- Elastic - for accomodation
What are lens zonules?
Lens is suspended by a fibrous ring known as lens zonules
- Lens zonules consist of passive connective tissue??
Describe the structure and function of the retina.
Structure:
- Very thin layer of neurosensory tissue
- Lines the inner part of the eye.
Function:
- It is responsible for capturing the light rays that enter the eye
- Much like the film’s role in photography
- These light impulses are then sent to the brain for processing, via the optic nerve
- SUMMARY: Converts light into action potentials sent down optic nerve via photoreceptors
What is the function of the optic nerve (CNII)?
Transmits electrical impulses (action potentials) from the retina to the brain
Photoreceptors of retina → optic nerve → brain
Describe the characteristics of the optic nerve in the eye.
-
Optic disc = visible portion of optic nerve
- Visible when you look through a fundoscope
- Vision blind spot = where the optic nerve attaches to the eye and hence there are no photoreceptors
-
Macula is found temporal to the optic nerve
- i.e. Macula is near and lateral to optic nerve
NOTE: With both eyes open, we don’t notice the blind spots becuase the other eye compensates for that part of the visual field
Describe the location and characteristics of the macula
Location:
- Roughly in the centre of the retina
Charactersitics:
- Small + highly sensitive
- Therefore, it is the part of the retina responsible for detailed central vision
- Allows us to appreciate detail and perform tasks that require central vision such reading
- Centre of macula is fovea
Describe the characteristics of the fovea.
- Your fovea is the most sensitive part of the retina.
- It has the highest concentration of cones, but a low concentration of rods
- This is why stars out of the corner of your eye are brighter than when you look at the directly
- But only your fovea has the concentration of cones to perceive in detail
- Macula allows detailed central vision
- Fovea gives the most detailed vision (highest resolution)
COME BACK TO THIS What is retinal detachment?
Vitreous Humour liquifies and detaches from retina with age
If there is a tear in the retina, the vitreous humour floods behind the retina and detaches it from the other coatings, leading to blindness
What are the different types of tears?
3 Types of Tears:
- Basal – constantly produced
-
Reflex – in response to irritation
- Afferent: CN V1 (ophthalmic branch of trigeminal)
- Efferent: Parasympathetic nerve
- ACh neurotransmitter
- Emotional – crying
Describe the lacrimal system
Lacrimation = tear production
- Tears secreted by lacrimal gland
- Drains through the two puncta
- Puncta = openings on medial lid margin
- Drainage route:
- Puncta → canaliculi → tear sac → tear duct → nasal cavity
Describe the functions of tear film.
Functions:
- Tear film maintains smooth cornea-air surface
- Oxygen Supply to Cornea
- Normal cornea has no blood vessels
- Gaseous exchange takes place across the tear interface
- Removal of Debris
- This is due to both the tear film AND blinking
- Debris can be captured in the tear film
- Blinking creates a fresh new tear film
- This is due to both the tear film AND blinking
- Bactericide
Describe the structure of tear film.
Three Layers:
- Superficial oily (lipid) layer:
- Outermost
- To reduce tear film evaporation
- Produced by a row of Meibomian Glands along the lid margins
- Aqueous tear film:
- Produced by lacrimal gland
- Contains bactericides
- Mucinous Layer:
- Innermost
- On the corneal surface to maintain surface wetting
- This facilitates diffusion
What is the conjunctiva?
- Thin, transparent tissue that covers the outer surface of the eye
- It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids
- Reflects back upon itself to line the inside of the eyelids
- It is nourished by tiny blood vessels that are nearly invisible to the naked eye
What are the two chambers of the eye?
Anterior chamber
- Between cornea and lens
- More specifically between cornea and iris
- Filled with Clear aqueous fluid (i.e. aqueous humour)
- Aqueous humour supplies nutrients - to the avascular ocular tissues
Posterior chamber
- Between iris and lens
- Also contains aqueous humour
What does the ciliary body do?
Secretes aqueous humour
Describe the flow and drainage of aqueous humour.
Flow:
- Intraocular aqueous humour flows anteriorly into the anterior chamber (along the green arrow)
Drainage:
- Drains out of eye via trabecular meshwork and into canal of Schlemm
- 80-90%
- Uveal-scleral outflow
- Uvula → sclera - drained into vessels which are different from the other pathway
- Proportion - the remainder of fluid not drained into the trabecular meshwork
What is glaucoma?
Caused by sustained raised intraocular pressure compressing back of eye
↓
Retinal ganglion cell death & enlarged optic disc cupping
↓
Visual field loss & blindness
NOTE:
Optic disc cupping:
- The fibres which go into the optic nerve essentially all go round the outside
- This is the proper ‘disc’ part of the optic disc
- This means there is a space in the middle which is the ‘cup’
- As the nerve fibres within the optic nerve begin to die as a consequence of glaucoma, the ‘cup’ becomes larger
What are the two types of glaucoma?
Primary open angle glaucoma - commonest
Closed angle glaucoma
What causes primary open angle glaucoma?
Trabecular meshwork dysfunction - therefore aqueous humour not being drained properly
What causes closed angle glaucoma?
Increased pressure pushing the iris/lens complex forwards, blocking the trabecular meshwork
- Can be acute or chronic
- Vicious cycle
- As the aqueous humour is not being drained, the pressure increases even more
- This pushes the iris/lens even more forward causing even more blockage of the trabecular meshwork, reducing drainage
State the risk factors, presentation and treatment for closed angle glaucoma.
Risk factors:
- Small eye (hypermetropia)
- i.e. Shorter eyeball than ususal
- Narrow angle at trabecular meshwork
Presentation:
- May present with sudden painful red eye with acute drop in vision
Treatment:
- Peripheral laser iridotomy to create a drainage hole on the iris
What is the difference between central and peripheral vision?
Central vision:
- Functions:
- Detail day vision
- Colour vision
- Fovea has the highest concentration of cone photoreceptors
- Reading, facial recognition
- Assessed by visual acuity assessment
- Loss of foveal vision → poor visual acuity
Peripheral vision:
- Functions:
- Shape, movement, night vision
- Navigation vision
- Assessed by visual field assessment
- Extensive loss of visual field → unable to navigate in environment
- Patient may need white stick even with perfect visual acuity
Describe the structure of the retina.
There are 2 Parts to the retina:
- Retinal pigment epithelium
- Transorts metabolies between choroid and photoreceptors
- Neuroretina
3 layers of the neuroretina:
- Outer Layer/ 1st Order Neurones:
- Photoreceptors - detect light
- Middle Layer/ 2nd Order Neurones:
- Bipolar cells – local signal processing to improve contrast sensitivity, regulate sensitivity
- Inner Layer/ 3rd Order Neurones:
- Retinal ganglion cells – Transmit AP (signal) from eye to brain via optic nerve
Describe the structure of the macula and fovea in terms of retinal cells.
Macula:
- Also known as the macula lutea (yellow patch)
- Pigmented region at the centre of the retina of about 6 mm in diameter
Fovea:
- Forms the pit at the centre of the macula due to absence of the overlying ganglion cell layer
- Fovea has the highest concentration of photoreceptors for fine vision
Clinically can be assessed with an OCT scan (Optical Coherence Tomography) - allows you to see each of the retina’s distinctive layers
What are the 2 main classes of photoreceptors in the retina?
- Rods
- Cones
How do the photoreceptors work?
They synthesise their photopigment in inner photoreceptor segment then transport to outer segment discs
Photopigment in rods:
- Photopigment: rhodopsin
- With cofactor: 11 cis retinal cofactor
- Photons induce change in cofactor, inducing change in rhodopsin
- Activates G-protein pathway, generating AP
- Rhodopsin reacts most efficiently to 498nm light
Photopigment in cones:
- Photopigment: photopsin - 3 types
- S-cone: BLUE spectrum
- M-cone: GREEN spectrum
- L-cone: RED spectrum
These photopigments are technically GPCRs which when activated by light (photons) results in a signal transduction pathway which ultimately leads to AP generation
Describe the structure and function of the rods.
- 120 million rods
- Longer outer segment with photo-sensitive pigment
- 100 times more sensitive to light than cones
- i.e. requires less light to function
- Slow response to light
- Responsible for:
- Night vision
- Peripheral vision
- Scotopic vision
- This is the vision of the eye under low light conditions
- Happens completely through rod cells
- Recognising motion
Describe the structure and function of the cones.
- 6 million
- Less sensitive to light, but faster response
- i.e. require more light to function
- Responsible for:
- Day Vision
- Central vision
- Photopic vision
- Vision under well lit conditions
- Mediated by cone cells
- Recognising colour and details
Where is the highest concentration of cones and rods found?
Cones - on the fovea
Rods - 20-40 degrees from fovea
