12. Structure and Function of the Eye

Question 1

Through what structure does the optic nerve pass?

Answer 1

Throught the bony orbit nasally

Question 2

Describe the location and function of the lacrimal gland?

Answer 2

• Lacrimal gland located within orbit, latero-superior to globe
• Gland function = tear production
  
  • Basal tears - these are tears that are produced at a constant level, even in the absence of irritation or stimulation
  • Reflex tears - These are tears that are produced in response to irritation
    
    • Afferent - Cornea - CN VI
    • Efferent - Paraympathetic
  • Neurotransmitters = ACh
  • Crying (emotional) tears

Question 3

Describe the Lacrimal system.

Answer 3

• Tear made by Lacrimal Gland
• Drains through two puncta= opening on medial lid margin
• Flows through superior + inferior canaliculi
• Gather in tear sac
• Exits sac via tear duct (nasolacrimal duct, opens into inferior meatus) into nose cavity

Question 4

What is the function of the tear film?

Answer 4

• Tear film maintains smooth cornea-air surface
• Oxygen supply to cornea b/c normal cornea has no blood vessels
• Removal of Debris (Tear film + Blinking)
• Bactericide

Question 5

Describe the structure of the tear film.

Answer 5

1. Superficial Oily Layer to reduce tear film evaporation (produced by a row of Meibomian Glands along lid margins)
2. Aqueous Tear Film made by tear gland
3. Mucinous Layer on Corneal Surface to maintain surface wetting

Question 6

Describe the conjunctiva.

Answer 6

• = the thin, transparenttissue covering outer surfaceof eye (including cornea)
• Begins at outer edge of cornea, covers visible part of eye, and lines inside of eyelids.
• Nourished by tiny blood vessels nearly invisible to naked eye.
• Ciliary body produces aqueous humour

Question 7

Describe the layers of the eye.

Answer 7

3 layers:

1. Sclera - Hard + opaque, protects eye + maintains shape of the eye –> “the white of the eye” - tough + has high water content
2. Choroid - Pigmented + Vascular –> shields out unwanted scattered light + supplies blood
3. Retina - Neurosensory Tissue converting light –> impulses to the brain via the optic nerve

Question 8

Define the cornea.

Answer 8

Transparent, dome-shaped window covering front of the eye.

Question 9

Describe the structural features of cornea.

Answer 9

• Front-most part of Anterior Segment
• Continuous with sclera
• Low water content
• Convex Curvature
• Higher refractive index than air

Question 10

Describe the function of the cornea. What does prolonged contact-lens wear?

Answer 10

• Powerful refracting surface, providing 2/3 of eye’s focusing/refracting power
  
  • It has a convex curvature and a higher refractive index than air
• Acts as a clear window to look through
• Physical + infection barrier

Cornea relies on tear film + aqueous fluid for nutrients + oxygen supply

Prolonged contact-lens wear –> reduces oxygen supply to cornea + increases risk of corneal infection

Question 11

Describe the physical 5-layer structure of the cornea.

Answer 11

1. Epithelium - stratified
2. Bowman’s Membrane (specialised basement membrane)
3. Stroma - regularity contributes to transparency, thickest
   
   • ​​Corneal nerve ending provide sensation and nutrients for healthy tissue
   • No blood vessels in normal cornea so transparent
4. Descemet’s Membrane (specialized basement membrane)
5. Endothelium – pumps fluid out of corneal and prevents corneal oedema,
   
   1. Only single layer (simple)
   2. No regenerative ability
   3. Endothelial cell density decreases w/ age
   4. Endothelial cells pump out excess fluid from cornea.
   5. Thus, dysfunction –> corneal oedema+ cloudiness

Question 12

Define the uvea and describe its composition.

Answer 12

• Vascular coat of eye ball between sclera + retina.
• Composed of three parts:
  
  • Iris - Coloured part of the eye:
    
    • Controls light levels inside eye(like aperture on a camera)
    • Pupil = round opening in centre of iris
    • Iris embedded w/ tiny muscles that dilate + constrict pupil size.
  • Ciliary body
  • Choroid - Between retina + sclera; composed of layers of blood vessels nourishing back of eye.
• These three portions intimately connected –> disease of one also affects others, though not necessarily to the same degree.

Question 13

Describe the lens structure and function.

Answer 13

Lens Structure:

• Outer Acellular Capsule
• Capsule encases regular inner elongated cell fibres– transparency
• May lose transparency w/ age –> Cataract (opaque lens) – quite common

Function

• Transparency due to regular structure
• Refractive Power = 1/3 of overall power
  
  • Higher refractive index than aqueous fluid + vitreous
• Accommodation – elasticity

Question 14

Describe the Lens zonules (suspensory ligaments).

Answer 14

• Lens suspended by fibrous ring known as lens zonules, anchoring lens to ciliary body.
• Consists of passive connective tissue
• Surface of lens normally held flat + tort by tension along stretched lens zonules.

Question 15

Describe the function of the optic nerve

Answer 15

• Transmits electrical impulses from retina –> brain.
• Connects to back of eye near macula.
• Visible portion of optic nerve = optic disc.

Question 16

Describe the blind spot test.

Answer 16

Where optic nerve meets retina, no light sensitive cells present –> blind spot.

1. On paper, draw a dot, then an “X” 10 cm to L
2. Close R eye + hold paper at arm’s length.
3. Look at dot + move paper towards you –> X disappears into blind spot!

Question 17

Describe the macula

Answer 17

• Located roughly in centre of retina, temporal (closer to temples) to optic nerve.
• Small + highly sensitive part of retina allows for detailed central vision+ perform tasks that require central vision e.g. reading.
• Fovea= very centre of macula. Allows us to appreciate detail and perform tasks that require central vision such reading

Question 18

Describe the anterior and posterior segments if the eye.

Answer 18

Anterior segment - between cornea and lens (filled with aq. fluid) - supplies nutrients

Posterior segment - posterior to lens

Question 19

Describe the function ciliary body

Answer 19

• Secretes aqueous fluid/humour in eye
• Intraocular Aqueous Fluid flows anteriorly into Anterior Chamber
• Aqueous Fluid supplies nutrient
• Trabecular Meshwork(between ciliary body + cornea) drains fluid out of eye
• Normal IOP = 12-21mmHg

Question 20

What does glaucoma primarily affect?

Answer 20

Glaucoma primarily affects the retinal ganglion cells

Leads to retinal ganglion cell death + enlarged optic disc cupping = loss of ganglion nerve fibres –> hollowing out of optic nerve head

Question 21

Describe the types of glaucoma.

Answer 21

Patients with untreated glaucoma lose peripheral vision progressively.

Types

• Primary Open Angle Glaucoma (on the left) - commonest
  
  • ​Trabecular Meshwork Dysfunction
  • Generally asymptomatic until advanced stages
• Closed Angle Glaucoma - acute or chronic
  
  • ​Increased IOP –> iris/lens pushed forward –> blocking trabecular meshwork - vicious cycle
  • Risk factors = small eye (hypermetropia), narrow angle at trabecular meshwork
  • May present with sudden painful red eye with acute drop in vision
  • Can be treated with peripheral laser iridotomy –> create drainage hole on iris

Question 22

What are the risk factors of glaucoma?

Answer 22

• HIGH PRESSURE DOES NOT MEAN GLAUCOMA àIT JUST MEANS HIGHER RISK OF GLAUCOMA
• Risk factors:
• Family history
• Age

Question 23

Describe the fovea test.

Answer 23

• Fovea is most sensitive part of retina.
• Fovea has highest [cone cells], but low [rods]
  
  • Explains why stars out of corner of eye brighter than when look directly.
• But only fovea has high-enough [cones] to perceive in detail –> try to read letters using peripheral vision, but can’t

Question 24

Describe the difference in central and peripheral vision.

Answer 24

• Central
  
  • ​DetailDay Vision,Colour vision - fovea has highest conc. of cone cells
    
    • Reading, Facial recognition
  • Assessed by Visual Acuity Assessment
    
    • Loss of Foveal Vision –> poor visual acuity
• Peripheral (by rod cells)
  
  • 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 25

Describe the retinal structure.

Answer 25

• Outer layer = Photoreceptors (1^st Order) - light detection
• Middle layer = Bipolar Cells (2^nd Order) - Local Signal Processing to improve contrast sensitivity, regulate sensitivity
  
  • Sound amplifier
• Inner layer = Retinal Ganglion Cells (3^rd Order) - transmission of signal from eye –> brain

Retinal pigment epithelium transports nutrient from choroid –> photoreceptors and removes metabolic waste from retina

Question 26

Define the macula lutea and fovea

Answer 26

Macula - Yellow patch at centre of retina of about 6 mm in diameter

Fovea - forms pits at centre of macula due to absence of overlying ganglion cell layer

• Clinically assessed with OCT scan (Optical Coherence Tomography)

Question 27

Describe the difference in Rod and Cone photoreceptors

Answer 27

• Rod
  
  • Longer outer segment with photo-sensitive pigment
  • 100X more sensitive to light than cones
  • Slow response to light
  • Responsible for night vision (Scotopic Vision)
  • 120M rods
• Cone
  
  • Less light-sensitive, but faster response
  • Responsible for day light fine vision and colour vision (Photopic Vision)
  • 6M cones
  • 3 different types of cone cells for different colours:
    
    • S for blue, M for green, L for red

Question 28

Describe the distribution of photoreceptors.

Answer 28

• Rods widely distributed all over retina; highest density just outside macula.
• Density of rod photoceptors gently tails off towards periphery.
• Rod photo-receptors completely absent in macula.
• Cone photo-receptors distributed only in macula.
• More rod cells à more pigment à higher Spatial and Time Summation

Question 29

Explain the peak light sensitivities of different photoreceptors. What photoreceptors are stimulated by yellow light?

Answer 29

• Rods vision has single peak light sensitivity at 498 nm
• S-Cones w/ pigment sensitive to short wavelength –blue
• M-Cones w/ pigment sensitive to medium wavelength –green
• L-Cones w/ pigment sensitive to long wavelength –red
• Yellow light wavelength between peak sensitivities of M and L
  
  • Yellow light stimulates M + L equally
  • Experience yellow = green + red combo

Question 30

Describe Deuteranomaly.

Answer 30

• Commonest form of colour deficiency
• Caused by shifting of M-cone sensitivity peak towards L-cone curve –> red-green confusion.
• Colour vision deficit higher in Males than Females

Question 31

Describe Anomalous Trichromatism.

Answer 31

colour vision deficits due to shift in photo-pigment peak sensitivity.

Question 32

Describe Monochromatism and Dischromatism.

Answer 32

• Colour Vision deficits also caused by absence of cone photo-pigments type(s).
  
  • Dichromatism = two cone photo-pigment sub-types present.
  • Monochromatism = no colour vision.
• Rod Monochromatism = no cone photo-receptors –> no functional day vision
• Blue Cone Monochromatism –> normal daylight visual acuity

Question 33

Describe the Isihara Test.

Answer 33

colour perception test for R-G deficiencies only

Question 34

Describe Light Dark Adaptation.

Answer 34

• Dark Adaptation
  
  • Increase in light sensitivity when moving from light to dark (in daylight, rod cells sensitivity suppressed as cone receptors are responsible for daylight vision)
  • Biphasic Process:
    
    • Cone adaptation = 7 minutes to adapt
    • Rod adaptation = 30 minutes – regeneration of rhodopsin
• Light Adaptation
  
  • Adaptation from dark to light
  • Occurs over 5 minutes
  • Bleaching of photo-pigments
  • Neuro-adaptation
  • Inhibition of Rod/Cone function
    
    • rod function is greatly suppressed and cone function takes over within a minute
• Pupil Adaptation (minor) = constriction with light

Question 35

Describe refraction (refraction index) and reflection

Answer 35

• Index of Refraction = speed of light in first medium/ speed of light in new medium
• As light goes from one medium to another –> velocity CHANGES
• As light goes from one medium to another –> path CHANGES
• When light reflected, angle of incidence = angle of reflection
• How do we change refraction? CHANGE THE LENS
  
  • Concave – diverging
  • Convex – converging - has a real focal point

Question 36

Define Emmetropia.

Answer 36

• Adequate correlation between axial (eye) length + refractive power (of 0)
• Parallel light rays fall on retina (no accommodation)

Question 37

Define Ametropia and types of them.

Answer 37

• Mismatch between axial length + refractive power
• Parallel light rays don’t fall on retina (no accommodation); types:
  
  1. Near-sightedness (Myopia)
  2. Farsightedness (Hyperopia)
  3. Astigmatism
  4. Presbyopia

Question 38

Describe Hyperopia

Answer 38

Parallel rays converge at focal point posterior to retina

• Aetiology: not clear, inherited; causes:
  
  • Excessive short globe (axial hyperopia), more common
  • insufficient refractive power (refractive hyperopia)
• Symptoms: (originally compensated by accommodation until certain point)
  
  • Near visual acuity blurs relatively early
    
    • Blur varies in degree
    • blurred vision more noticeable if person tired, printing is weak or light inadequate
  • Asthenopic symptoms: eye pain, headache in frontal region, burning sensation in eyes, blepharoconjunctivitis
  • Lazy Eye = Amblyopia – uncorrected hyperopia > 5 diopters
• Treatment = convex glasses/contacts or remove lens + put in IOLs (cataract extraction)

Question 39

Describe myopia.

Answer 39

Parallel rays converge at focal point anterior to retina

• Aetiology not clear, genetic factor; Causes:
  
  • Excessive long globe/eye (axial myopia), more common
  • Excessive refractive power (refractive myopia)
• Symptoms:
  
  • Blurreddistancevision à squint to improve uncorrected visual acuity
  • Headache
• Treatment = concave glasses or contacts or surgery

Question 40

Describe Astigmatism.

Answer 40

• Parallel rays focus in multiple focal lines, NOT single focal point
• Aetiology = heredity; causes:
  
  • Refractive index/cornea not evenly shaped, not spherical à refract differently along one meridian than along meridian perpendicular to it à 2 focal points (punctiform object is represent as 2 sharply defined lines)
• Symptoms
  
  • Asthenopic symptoms (headache, eye pain)
  • blurred vision
  • distortion of vision
  • head tilting + turning
• Treatment
  
  • For regular astigmatism = cylinder lenses w/ or w/o spherical lenses (convex or concave), Sx
  • Irregular astigmatism = rigid CL, surgery

Question 41

Describe Presbyopia.

Answer 41

• Naturally occurring loss of accommodation (focus for near objects)
• Onset from age 40 years
• Distant vision intact
• Treatment = corrected by convex glasses in near vision à increase refractive power
  
  • E.g. reading, Bi/trifocal, Progressive power glasses

Question 42

Describe the near response triad.

Answer 42

For adaptation for near vision:

1. Pupillary Miosis (Sphincter Pupillae) –> increase depth of field
2. Convergence (medial recti from both eyes) to align both eyes to near object
3. Accommodation (Circular Ciliary Muscle) –> increase refractive power of lens

Question 43

Describe the types of optical correction.

Answer 43

• Spectacle lenses
  
  • Monofocal: spherical or cylindrical
  • Multifocal
• Contact lenses
  
  • Higher quality of optical image + less influence on size of retinal image than spectacle lenses
  • Indication: cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease
  • disadvantages: careful daily cleaning + disinfection, expense
  • Complication: infectious keratitis, giant papillary conjunctivitis, corneal vascularization, severe chronic conjunctivitis
• Intraocular lenses:
  
  • Replacement of cataract crystalline lens
  • Give best optical correction for aphakia, avoid significant magnification, and distortion caused by spectacle lenses
• Surgical correction:
  
  • Keratorefractive surgery: RK, AK, PRK, LASIK, ICR, thermokeratoplasty
  • Intraocular surgery: clear lens extraction (with or without IOL), phakic IOL

Question 44

Describe the accommodation mechanism.

Answer 44

Mediated by efferent oculomotor nerve

1. Contraction of Circular Ciliary Muscle in Ciliary Body
2. Relaxes zonules, which are normally stretched between ciliary body attachment and lens capsule attachment
   
   1. Zonules are passive elastic bands with no active contractile muscle
3. When no zonular tension, lens returns to natural convex shape due to innate elasticity
4. Increases refractive power of lens