Structure and function of the eye

Question 1

What are the bones making up the orbit of the eye?

Answer 1

• Frontal
• Sphenoid
• Lacrimal
• Ethmoid
• Zygomatic

Question 2

What is the average anterior-posterior diameter of the adult eye?

Answer 2

24mm

Question 3

What are the two segments of the eye? What are the separated by? What fluid do their contain

Answer 3

Two segements?

• Anterior segment
  
  • Contains aqueous humour
• Posterior segment
  
  • Contains vitreous humour

Separated by LENS

Question 4

What are the 3 layers of the coat of the eye?

Answer 4

Outer to inner:

1. Sclera
2. Choroid
3. Retina

Question 5

Describe the structure and function of the sclera.

Answer 5

Structure:

• Hard and opaque
• High water content - therefore opaque
• Continous with the cornea

Function:

• Protects eye and maintains its shape

Question 6

Describe the characteristics of the cornea.

Answer 6

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

Question 7

What are the 5 layers of the cornea?

Answer 7

Outer to inner:

1. Epithelium
2. Bowman’s Membrane
3. Stroma
4. Descemet’s Membrane
5. Endothelium

Question 8

Describe the structure of the stroma.

Answer 8

• 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)

Question 9

Describe the characteristics of the endothelium.

Answer 9

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

Question 10

Describe the structure of the choroid.

Answer 10

• Pigmented and dark
• Composed of layers of blood vessels that nourish the back of the eye

Question 11

What is the uvea?

Answer 11

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

Question 12

What are the layers of the iris?

Answer 12

Anterior - stromal layer

Posterior - epithelial layer

Question 13

Describe the structure and function of the lens.

Answer 13

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

Question 14

What are lens zonules?

Answer 14

Lens is suspended by a fibrous ring known as lens zonules

• Lens zonules consist of passive connective tissue??

Question 15

Describe the structure and function of the retina.

Answer 15

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

Question 16

What is the function of the optic nerve (CNII)?

Answer 16

Transmits electrical impulses (action potentials) from the retina to the brain

Photoreceptors of retina → optic nerve → brain

Question 17

Describe the characteristics of the optic nerve in the eye.

Answer 17

• 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

Question 18

Describe the location and characteristics of the macula

Answer 18

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​

Question 19

Describe the characteristics of the fovea.

Answer 19

• 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)

Question 20

COME BACK TO THIS What is retinal detachment?

Answer 20

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

Question 21

What are the different types of tears?

Answer 21

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

Question 22

Describe the lacrimal system

Answer 22

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

Question 23

Describe the functions of tear film.

Answer 23

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
• Bactericide

Question 24

Describe the structure of tear film.

Answer 24

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

Question 25

What is the conjunctiva?

Answer 25

* 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

Question 26

What are the two chambers of the eye?

Answer 26

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

Question 27

What does the ciliary body do?

Answer 27

Secretes aqueous humour

Question 28

Describe the flow and drainage of aqueous humour.

Answer 28

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

Question 29

What is glaucoma?

Answer 29

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

Question 30

What are the two types of glaucoma?

Answer 30

Primary open angle glaucoma - **commonest** Closed angle glaucoma

Question 31

What causes primary open angle glaucoma?

Answer 31

Trabecular meshwork dysfunction *- therefore aqueous humour not being drained properly*

Question 32

What causes closed angle glaucoma?

Answer 32

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*

Question 33

State the risk factors, presentation and treatment for closed angle glaucoma.

Answer 33

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

Question 34

What is the difference between central and peripheral vision?

Answer 34

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

Question 35

Describe the structure of the retina.

Answer 35

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

Question 36

Describe the structure of the macula and fovea in terms of retinal cells.

Answer 36

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*

Question 37

What are the 2 main classes of photoreceptors in the retina?

Answer 37

* Rods * Cones

Question 38

How do the photoreceptors work?

Answer 38

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

Question 39

Describe the structure and function of the rods.

Answer 39

* 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

Question 40

Describe the structure and function of the cones.

Answer 40

* 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

Question 41

Where is the highest concentration of cones and rods found?

Answer 41

Cones - on the fovea Rods - 20-40 degrees from fovea