Eye: Structure and Function

Question 1

What is the average anterior-posterior diameter of the orbit?

Answer 1

24 mm

Question 2

What are the three layers of the eye? Describe their properties and function.

Answer 2

Sclera - Hard and opaque, Maintains the shape of the eye (High water content)

Choroid - Pigmented and vascular, Shields out unwanted scattered light Retina

Neurosensory - Converts light into neurological impulses

Question 3

What are the two segments of the eye separated by?

Answer 3

Lens separates anterior and posterior segments

Question 4

Which humours are found in the two segments of the eye?

Answer 4

Anterior = aqueous humour 
Posterior = vitreous humour

Question 5

What name is given to the fibrous strands that suspend the lens from the ciliary bodies?

Answer 5

Zonules

Question 6

What are the layers of tear film?

Answer 6

Superficial oily - Reduce tear film evapouration (Produced in Meibomian glands along lid margins)

Aqueous tear film - From tear gland

Mucinous layer - Prevents wetting of cornea, produced by goblet cells

Question 7

Explain the lacrimal system.

Answer 7

Lacrimal gland lies superior to upper lid (Supertemporal?), provides watery component to tears

The two puncta (Sing. Punctym) stenose and collect the secretions, travel through canaliculi to tear sac and tear duct. (Tear duct ends in inferior meatus in pharynx - valve prevents reflux)

Question 8

What covers the outer surface of the eye?

Answer 8

Conjunctiva - thin, transparent tissue (also lines inside of eyelid)

Has many small blood vessels.

Question 9

Describe the production and drainage of aqueous humour.

Answer 9

Aqueous humour is produced by the ciliary body It is drained via the trabecular meshwork into the canals of Schlemm

Question 10

What is the role of aqueous humour?

Answer 10

Provides nutrients to the cornea and other tissues in the anterior chamber

Question 11

Describe vitreous humour.

Answer 11

It is 99% water trapped inside a jelly matrix, provides mechanical support for the eye.

In ageing it loses its jelly consistency, liquefies and can become detached from the retina Vitreous detachment in seen as floaters

Question 12

What are the potentially disastrous consequences of vitreous humour detachment?

Answer 12

Detaching from the retina could cause a small tear in the peripheral retina If there is a small tear, liquid vitreous could seep into the sub-retinal space and lead to retinal detachment If untreated, it can lead to blindness

Question 13

What are the two layers of the iris?

Answer 13

Anterior – stromal layer containing muscle fibres

Posterior – epithelium

Question 14

Describe how the retina and choroid contribute to the different parts of the iris and ciliary body.

Answer 14

Retina gives rise anteriorly to the ciliary body epithelium and the posterior (epithelial) layer of the iris
Choroid gives rise anteriorly to the ciliary body stroma and the anterior layer of the iris (stromal layer)

Question 15

What is the collective term for the choroid, iris and ciliary body?

Answer 15

Uvea

Question 16

What is the normal range for intraocular pressure?

Answer 16

11-12 mm Hg

Question 17

What changes can be seen in the retina in glaucoma?

Answer 17

Retinal ganglion cell death
Enlarged optic disc cupping

Leads to Progressive loss of peripheral vision, Blindness

Question 18

What are two causes of Glaucoma? What is the first line treatment?

Answer 18

Primary open angle glaucoma It is caused by a functional blockage of the trabecular meshwork

Closed angle glaucoma This can be acute or chronic It is caused by the forward displacement of the iris-lens complex –narrowing the trabecular meshwork (Small eyes (hypermetropic) Treatment: peripheral laser iridotomy)

Prostaglandin analogue - only 20% pressure reduction max

Question 19

Describe the structure of the lens.

Answer 19

It has an outer acellular capsule There are regular inner elongated fibres, which give the lens its transparency NOTE: may lose transparency with age

Question 20

Which two structures provide the majority of the refractive power of the eye?

Answer 20

Cornea = 2/3 
Lens = 1/3

Question 21

What layer of the eye is the cornea continuous with?

Answer 21

Sclera

Question 22

What are the consequences of prolonged contact lens wear?

Answer 22

Reduce the oxygen supply to the cornea

Question 23

What are the layers of the cornea?

Answer 23

1 - Epithelium - large turnover, tight barriers

2 - Bowman’s membrane

3 - Stroma - No bloodvessels but has nerve endings, transparent

4 - Descemet’s membrane

5 - Endothelium (Density of cells decreases with age, pumps water out of cornea)

Question 24

Where is the lacrimal gland located? What does it do?

Answer 24

Superio-laterally to the orbit, produces tears.

Types
Basal Tears – produced at a constant level in the absence of irritation
Reflex Tears – increased tear production in response to irritation
Emotional Tears – crying

Question 25

Describe the innervation of the cornea.

Answer 25

The cornea is very sensitive and it is innervated by the ophthalmic branch of the trigeminal nerve (CN V)

Question 26

What two things regulate how much light reaches the retina?

Answer 26

Pupil | Pigmented Uvea

Question 27

What are the types of vision?

Answer 27

Emmetropia - axial length match refractive power, rays fall on retina, no accomodation Amemtropic - Myopia (Near-sight), Hyperopia (Far-sight), Astigmatism, Presbyopia

Question 28

What causes Myopia and hyperopia?

Answer 28

Excessive globe length (axial myopia) common Excessive refractive power (refractive myopia) Treat with concave lens Insufficient globe length (axial hyperopia) common Insufficient refractive power (refractive hyperopia) Treat with convex lens

Question 29

What are symptoms of hyperopia?

Answer 29

Asthenopic - eyepain, frontal headache, buring sensation in eyes, blepharoconjunctivitis Early blurring in vision acuity (nature varies) Amyblyopia - uncorrected hyperopia, one eye takes dominance

Question 30

What causes astigmatism? 

Answer 30

Non-spherical cornea - light refracts differently along one perpendicular meridian leading to two focal points - unfocused vision

Question 31

What are the symptoms/treatment for astigmatism?

Answer 31

Symptoms - blurring, asthenopia, head tilting Treatment - Reg: Cylinder lens or other lens Irreg: Rigid CL, surgery

Question 32

What causes Presybyopia?

Answer 32

Natural associated with aging and cell death onset around 40, only affects near vision so treated with convex lenses

Question 33

What are some different types of lenses?

Answer 33

Spectacle Contact – less distortion, higher infection risk Interocular – Cataract lens replacement

Question 34

What are some types of occular surgery?

Answer 34

Keratorefractive surgery – laser?? Interocular surgery

Question 35

What is the difference between central and peripheral vision

Answer 35

Central - Day vision, colour, detail (CONE) Peripheral - Night vision, movement (ROD)

Question 36

What are the retinal neural layers?

Answer 36

Outer - Photoreceptors Middle - Bipolar cells (Improve contrast, regulate sensitivity) Inner - Retinal ganglion cells

Question 37

State some differences between cone and rods

Answer 37

Rod - longer outer segments, 100x more light sensitive, slower response 120 mil cells Cones - 6 mil cells, faster response, phototopic colour vision

Question 38

What are different forms of cone cells?

Answer 38

S cones - sensitive to blue light M cones - Sensitive to green light L cones - Sensitive to red light Rods are senstive at about 500 nm

Question 39

How is colour blindness tested?

Answer 39

Ishihara test (only for red-green deficinencies)

Question 40

How long do cells take to adapt to light/dark?

Answer 40

Dark: Cones 7 mins, Rods 30 mins (rhodopsin regeneration) Light: 5 mins, bleach photo-pigments, Neuroadapt to inhibit rod and cone function (Pupil adaptation has minor effect)

Question 41

What is the macula?

Answer 41

Small highly sensitive part of retina responsible for detailed central vision (centre of macula is fovea - responsible for level of detail for reading etc) Fovea appears as a foveal pit due to the absence of overlying retinalganglion cells in macula cross section.

Question 42

Where are highest concentration of rod and cone receptors found?

Answer 42

20-40 degrees from Fovea, for cones it's IN fovea

Question 43

What are the four branches of vessel arcades radiating from the optic disc?

Answer 43

Superior Temporal Inferior Temporal Superior Nasal Inferior Nasal

Question 44

Describe the difference in perfusion between the outer and inner parts of the retina.

Answer 44

Inner 2/3 of the retina = retinal arteries | Outer 1/3 of the retina = choroidal vasculature

Question 45

Where is the physiological blind spot located?

Answer 45

20 degrees temporal to central vision

Question 46

What is the most common colour vision deficiency and what is it caused by?

Answer 46

Deuteranomaly It is caused by the shifting of the M-cone towards the L-cone

Question 47

What is the term given to shifted peaks?

Answer 47

Anomalous trichomatism