Structure and function of the eye Flashcards
1
Q
What are the bones making up the orbit of the eye?
A
- Frontal
- Sphenoid
- Lacrimal
- Ethmoid
- Zygomatic
2
Q
What is the average anterior-posterior diameter of the adult eye?
A
24mm
3
Q
What are the two segments of the eye? What are the separated by? What fluid do their contain
A
Two segements?
-
Anterior segment
- Contains aqueous humour
-
Posterior segment
- Contains vitreous humour
Separated by LENS
4
Q
What are the 3 layers of the coat of the eye?
A
Outer to inner:
- Sclera
- Choroid
- Retina
5
Q
Describe the structure and function of the sclera.
A
Structure:
- Hard and opaque
- High water content - therefore opaque
- Continous with the cornea
Function:
- Protects eye and maintains its shape
6
Q
Describe the characteristics of the cornea.
A
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
7
Q
What are the 5 layers of the cornea?
A
Outer to inner:
- Epithelium
- Bowman’s Membrane
- Stroma
- Descemet’s Membrane
- Endothelium
8
Q
Describe the structure of the stroma.
A
- 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)
9
Q
Describe the characteristics of the endothelium.
A
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
10
Q
Describe the structure of the choroid.
A
- Pigmented and dark
- Composed of layers of blood vessels that nourish the back of the eye
11
Q
What is the uvea?
A
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
12
Q
What are the layers of the iris?
A
Anterior - stromal layer
Posterior - epithelial layer
13
Q
Describe the structure and function of the lens.
A
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
14
Q
What are lens zonules?
A
Lens is suspended by a fibrous ring known as lens zonules
- Lens zonules consist of passive connective tissue??
15
Q
Describe the structure and function of the retina.
A
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
16
Q
What is the function of the optic nerve (CNII)?
A
Transmits electrical impulses (action potentials) from the retina to the brain
Photoreceptors of retina → optic nerve → brain
17
Q
Describe the characteristics of the optic nerve in the eye.
A
-
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
18
Q
Describe the location and characteristics of the macula
A
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
19
Q
Describe the characteristics of the fovea.
A
- 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)
20
Q
COME BACK TO THIS What is retinal detachment?
A
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
21
Q
What are the different types of tears?
A
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
22
Q
Describe the lacrimal system
A
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
23
Q
Describe the functions of tear film.
A
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
24
Q
Describe the structure of tear film.
A
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
25
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
26
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
27
What does the ciliary body do?
Secretes aqueous humour
28
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
29
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
30
What are the two types of glaucoma?
Primary open angle glaucoma - **commonest**
Closed angle glaucoma
31
What causes primary open angle glaucoma?
Trabecular meshwork dysfunction *- therefore aqueous humour not being drained properly*
32
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*
33
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
34
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
35
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
36
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*
37
What are the 2 main classes of photoreceptors in the retina?
* Rods
* Cones
38
How do the photoreceptors work?
They synthesise their photopigment in inner photoreceptor segment then transport to outer segment discs
Photopigment in rods:
* Photopigment: r**hodopsin**
* 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
39
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
40
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
41
Where is the highest concentration of cones and rods found?
Cones - on the fovea
Rods - 20-40 degrees from fovea
42
