Ocular Anatomy + Physiology

Question 1

Describe the orbit and its function

Answer 1

Orbit: bony socket containing the eye

Formed from a number of fused bones
Serves to:
protect the delicate eye
provide a mounting point for the six striatal muscles that control ocular movement

Question 2

What is the internal eye made up of? (7)

Answer 2

Sclera
Lens
Cilliary body
Iris
Choroid
Retina
Optic nerve

Question 3

What is the Sclera?

Answer 3

It is the tough, outer protective coat

Question 4

What is the Lens?

Answer 4

It is a transparent biconvex structure suspended by lens zonules

Question 5

What is the Cilliary body?

Answer 5

provides attachment for lens zonules. Changes the focusing power of the lens

Question 6

What does the Iris do?

Answer 6

regulates light entering eye attached by cilliary body

Question 7

What does the Choroid do?

Answer 7

maintains the eye’s blood supply

Question 8

What is the Retina?

Answer 8

inner layer containing visual photoreceptors (rods & cones)

Question 9

What is the function of the optic nerve?

Answer 9

transmits images from photoreceptors to the brain

Question 10

Where are tears produced from?

Answer 10

Lachrymal gland

Question 11

What are the two types of tears?

Answer 11

Basic tears (to maintain a tear film on the corneal surface for optical, metabolic and lubricant purposes)

Reflex tears are induced in response to chemical or mechanical irritation, cold, light, etc)

Question 12

What does precorneal tear film consist of? What is the three layers theory?

Answer 12

The precorneal tear film consists of a superficial lipid layer, a central aqueous layer and an inner mucus layer.

Question 13

Describe the lipid layer in the precorneal tear film

Answer 13

It consists mainly of sterol esters, triacylglycerols and phospholipids, free sterols and free fatty acids

It spreads over the aqueous layer during eye opening

The lipids play an important role in reducing the evaporation rate to maintain the normal tear osmolality

Question 14

What components make up the aqueous layer of the tear film?

Answer 14

Inorganic salts
Glucose
Urea
Retinol
Ascorbic acid
Various proteins
Lipocalins (tear-specific pre-albumins)
Immunoglobulins
Lysozyme
Lactoferrin
Glycoproteins

Question 15

Describe the function of the precorneal tear film (4)

Answer 15

Prevention of corneal drying
Transport of oxygen and nutrients to the cornea
Maintenance of cornea clean and smooth
Protection against infection

Question 16

What is the pH of normal tears? and when is it at its lowest?

Answer 16

pH 7.3 -7.7

Lowest on awakening

Question 17

Why are tears more acidic in contact lens wearers?

Answer 17

impediment (hinderance) of the efflux (flowing out) of carbon dioxide

Question 18

In what cases do tears become more alkaline?

Answer 18

Dry eye
Severe ocular rosacea
Lacrimal stenosis (tear duct narrows)

Question 19

Under normal conditions what volume of tear can be produced?

Answer 19

7-9 microlitres

Question 20

What is the maximum amount of fluid that can be held in the lower eye sac? how much of this is incorporated into the tear film without causing it to destabilise?

Answer 20

25-30 microlitres

only 3microlitres can be incorporated into the film

Question 21

What is the function of blinking?

Answer 21

Important defence mechanism - brisk blinking reflex
Essential for periodic reforming of the tear film
Also activates a pumping mechanism for the drainage of tears through the lacrimal drainage apparatus

Question 22

What are the functions of the cornea?

Answer 22

Passage of light
Refraction of light
Protection

Question 23

What is the aqueous humour?

Answer 23

a transparent viscous fluid located in the anterior chamber of the eye

Question 24

What is the aqueous humour composed of? (6)

Answer 24

98% water
amino acids
electrolytes
ascorbic acid
glutathione
immunoglobulins

Question 25

What are the functions of the aqueous humour?

Answer 25

Maintains the intraocular pressure and inflates the globe of the eye Provides nutrition for the avascular ocular tissues (posterior cornea, trabecular meshwork, lens, and anterior vitreous); May serve to transport ascorbate in the anterior segment to act as an antioxidant agent; Potential immune response to defend against pathogens (immunoglobulins); Light refraction

Question 26

How does the lens change depending on the distance of the object?

Answer 26

The lens is elastic and can change shape, getting fatter to focus close objects and thinner for distant objects;

Question 27

What does the lens consist of? (3)

Answer 27

lens capsule, | the lens epithelium, and the lens fibers

Question 28

What is the vitreous humour?

Answer 28

clear gel that fills the space between the lens and the retina

Question 29

What are the functions of the vitreous humour?

Answer 29

helps to maintain normal anatomic expansion of the globe; impedes the diffusion of substances between the retina and the anterior segment of the eye. small water-soluble substances in the retina may diffuse into the vitreous across the blood-retina barrier. - acts as a reservoir of substances such as oxygen, glucose and ascorbic acid, which support the metabolism of adjacent structures - acts as a reservoir for metabolic waste products

Question 30

What is the composition of the vitreous humour?

Answer 30

``` water (99%); a network of collagen fibrils; large molecules of hyaluronic acid; peripheral cells (hyalocytes); inorganic salts; glucose; ascorbic acid ```

Question 31

What are the photoreceptors?

Answer 31

Photoreceptors located on retinal surface Rods act in low light levels, do not provide colour and have little spatial resolution Cones operate at higher light levels, perceive colour and allow good spatial resolution

Question 32

What is the function of the optic nerve?

Answer 32

Rod/cone (photoreceptors) activation causes AP generation in the optic nerve. Optic nerve is a bundle of ~1.25108 nerves Pass into skull interior via optic canal to meet at optic chiasma. Nerves project to visual cortex in occipital lobes