Structure and function of the eye

Question 1

Macula

Answer 1

• ~centre of retina
• temporal to optic nerve
• small and highly sensitive
• responsible for detailed central vision
• fovea (necessary for sharp, central vision) at the centre of the macula
• macula allows appreciation of detail and ability to perform tasks requiring central vision (eg: reading)

Question 2

Tear film

Answer 2

• maintains smooth cornea-air surface
• oxygen supply to cornea (normal cornea has no blood vessels)
• debris removal (achieved by tear film and blinking)
• bactericide

Question 3

Conjunctiva

Answer 3

• thin, transparent tissue covering outer surface of the eye
• begins at outer edge of cornea, covers visible eye part and lines inside of eyelids
• nourished by tiny blood vessels (nearly invisible to the naked eye)

Question 4

3 layers of the coat of the eye

Answer 4

• Sclera
• Choroid
• Retina

Question 5

Schlera

Answer 5

• ‘white of the eye’
• hard, opaque tissue
• eye’s protective outer coat
• high water content
• maintains eye shape

Question 6

Choroid

Answer 6

• pigmented
• vascular (layers of blood vessels nourish back of the eye)
• between retina and sclera

Question 7

Retina

Answer 7

• neurosensory tissue

- converts lights to neurological impulses that are transmitted via optic nerve

Question 8

Cornea

Answer 8

• transparent, dome-shaped window
• covers front of the eye (front-most part of anterior segment)
• continuous with scleral layer
• powerful refracting surface (2/3 of the eye’s focusing power)
• low water content
• physical barrier and infection barrier

Question 9

5 layers of the cornea

Answer 9

(outer) 1) Epithelium
2) Bowman’s Membrane
3) Stroma
4) Descemet’s Membrane
(inner) 5) Endothelium

Question 10

Stroma of the cornea

Answer 10

• regularity contributes towards transparency
• corneal nerve endings provide sensation and nutrients for healthy tissue
• no blood vessels in normal cornea

Question 11

Endothelium of the cornea

Answer 11

• pumps fluid out of corneal and prevents corneal oedema
• 1 cell thick
• no regeneration power
• decrease in endothelial cell density with age
• endothelial cell dysfunction-> corneal oedema and corneal cloudiness

Question 12

Parts of the uvea

Answer 12

1) iris
2) ciliary body
3) choroid

Question 13

Iris

Answer 13

• coloured part of the eye
• controls light levels inside eye (amount of light reaching the retina)
• round opening in centre of iris is the pupil
• iris embedded with tiny muscles that dilate and constrict pupil size (iris controls size and diameter)

Question 14

Structure of the lens

Answer 14

• outer acellular capsule
• regular inner elongated cell fibres
• may loose transparency with age (cataracts)

Question 15

Functions of the lens

Answer 15

• transparency
• refractive power
• accommodation

Question 16

Lens zonules

Answer 16

• fibrous ring suspending the lens

- consists of passive connective tissue

Question 17

Retina

Answer 17

• very thin tissue layer

- lines inner part of eye

Question 18

Functions of the retina

Answer 18

-capturing light rays entering the eye-> light impulses then sent to the brain for processing via the optic nerve

Question 19

Optic nerve

Answer 19

• transmits electrical impulses from retina to brain

- connects to the back of the eye near the macula

Question 20

Optic disc

Answer 20

Visible portion of the optic nerve

Question 21

Blind spot

Answer 21

Point where the optic nerve meets the retina (no light sensitive cells so insensitive to light)

Test:

• draw dot on piece of paper with ‘X’ 10cm to the left of the dot
• close right eye and hold paper at arms length
• look at dot and move paper towards you
• ‘X’ disappears into blind spot

Question 22

Fovea test

Answer 22

• Paper has star in centre and series of letters on either side of star
• Look at star and try to read letters

Question 23

Hyperopia symptoms

Answer 23

• visual acuity at near tends to blur relatively early (Nature of blur varies from inability to read fine print to clear near vision but suddenly and intermittently blur. More noticeable blurred vision if tired patient, printing is weak or light adequate)
• asthenopic symptoms (eyepain, headache in frontal region, burning eye sensation, blepharoconjunctivitis etc)

Question 24

Amblyopia

Answer 24

LAZY EYE

-uncorrected hyperopia >5D

Question 25

Types of lenses

Answer 25

- Converging lens (convex) | - Diverging lens (concave)

Question 26

Myopia treatment

Answer 26

Concave lens to decrease refraction on the light

Question 27

Hyperopia treatment

Answer 27

Convex lens to increase refraction power

Question 28

Astigmatism

Answer 28

- Parallel rays come to focus in 2 focal lines rather than a single focal point - heredity aetiology

Question 29

Spectacle lenses

Answer 29

- Monofocal lenses (spherical lenses, cylindrical lenses) | - Multifocal lenses

Question 30

Contact lenses

Answer 30

- higher optical image quality and less influence on retinal image size compared to spectacle lenses - indications: cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease - disadvantages include expense and careful daily cleaning/disinfection - complications include infectious keratitis, giant papillary conjunctivitis, corneal vascularisation, severe chronic conjunctivitis

Question 31

Intraocular lenses

Answer 31

- cataract crystalline lens replacement - best optical correlation for aphakia - avoid significant magnification and distortion caused by spectacle lenses

Question 32

Surgical correction

Answer 32

- keratorefractive surgery: RK, AK, PRK, LASIK, ICR, Thermokeratoplasty - intraocular surgery: clear lens extraction (with or without IOL), phakic IOL

Question 33

Photoreceptors in the retina

Answer 33

- Rod photoreceptor | - Cone photoreceptor

Question 34

Rod photoreceptor (rods)

Answer 34

- longer outer segment with photo-sensitive pigment - 100 times more sensitive to light than cones - slow response to light - responsible for night vision (Scotopic Vision) - 120 million rods

Question 35

Cone photoreceptor (cones)

Answer 35

- less sensitive to light compared to rods - faster response to light than rods - Responsible for day light fine vision and colour vision (Photopic Vision) - 6 million cones

Question 36

Anterior segment

Answer 36

Ocular structure anterior to the lens

Question 37

Posterior segment

Answer 37

Ocular structure posterior to the lens

Question 38

Anterior chamber

Answer 38

- between cornea and lens - filled with clear aqueous fluid - supplies nutrients

Question 39

Glaucoma

Answer 39

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Question 40

Types of Glaucoma

Answer 40

- Primary Open Angle Glaucoma | - Closed Angle Glaucoma

Question 41

Primary Open Angle Glaucoma

Answer 41

- commonest type | - due trabecular meshwork dysfunction (meshwork cannot drain properly leading to fluid build-up)

Question 42

Closed Angle Glaucoma

Answer 42

- acute or chronic - increased pressure pushes iris/lens complex forwards which blocks the trabecular meshwork (inability to drain aqueous humour)->vicious cycle - risk factors include small eyes, narrow angle at trabecular meshwork etc - patients may present with sudden painful red eye with an acute drop in vision - treatment with peripheral laser iridotomy (laser beam creates drainage hole on the iris for the aqueous humour)

Question 43

Fovea

Answer 43

- most sensitive part of the retina - highest concentration of cone photoreceptors but low concentration of rod photoreceptors (why stars out of corner of eye are brighter than looking directly at stars) - only fovea has concentration of cone photoreceptors to perceive in detail

Question 44

3 layers of the tear film

Answer 44

- Superficial oily layer (reduces tear film evaporation and is produced by row of Meibomian Glands along lid margins)=lipid layer - Aqueous tear film (Tear Gland)=water layer - Mucinous layer on the corneal surface (maintains surface wetting)=mucin layer