Structure and function of the eye

Question 1

Summarise the bony orbit

Answer 1

Bony Orbit: maxilla, zygomatic, frontal, ethmoid, lacrimal, sphenoid and palatine bones create a pyramidal shaped orbit, with the wide base opening anteriorly to the face

Question 2

Describe the structure of the bony orbit

Answer 2

Roof: orbital part of the frontal bone
Medial wall: maxilla, lacrimal, ethmoid and sphenoid bones
Floor: orbital part of the maxilla (small contributions from the zygomatic and palatine bones)
Lateral wall: zygomatic bone anteriorly and greater wing of the sphenoid posteriorly

With fissures and foramina so vessels and nerves supplying the orbit can enter and leave

Question 3

What are the different type of tears that the lacrimal system can produce

Answer 3

– Basal Tears (constant fluid secretion by the lacrimal gland)
– Reflex Tears –in response to irritation • Afferent – Cornea – CN V1 (Ophthalmic Branch of Trigeminal Nerve) • Efferent – Parasympathetic • Neurotransmitter – Acetylcholine
– Crying (Emotional) Tears

Question 4

Where is the major contributor of tear production located (contributes to the water component of tears)

Answer 4

The lacrimal gland

Which is located supero-laterally to the orbit

Question 5

Summarise the lacrimal system

Answer 5

– Tear produced by the Lacrimal Gland
– Tear drains through the two puncta, opening on medial lid margin
– Tear flows through the superior and the inferior canaliculi
– Tear gathers in the Tear Sac
– Tear exits the Tear Sac through the tear duct into the nose cavity

Question 6

Describe the drainage of tears

Answer 6

basal tears move across the eyeball as the eyelids blink, accumulating medially in the lacrimal lake

Tears are drained into two puncta via two tiny holes in the upper and lower medial lid margins
It then drains into superior and inferior canaliculi à common canaliculus à tear sac
Tear sac then drains through the nasolacrimal duct, which opens up in the inferior meatu

Question 7

Summarise the functions of the tear film

Answer 7

• Tear film maintains smooth cornea-air surface
• Oxygen Supply to Cornea – Normal cornea has no blood vessels
• Removal of Debris (Tear film and Blinking)
• Bactericide

Question 8

Describe the components of the tear film

Answer 8

– Superficial Oily Layer to reduce tear film evaporation (produced by a row of MeibomianGlands along the lid margins) - tiny amounts secreted every time you blink- upper eyelids and head
– Aqueous Tear Film (Tear Gland)
– Mucinous Layer on the Corneal Surface to maintain surface wetting- goblet cells of cornea

Question 9

What are the roles of each of these three layers?

Answer 9

Superficial Oily Layer
· Reduces tear film evaporation
Aqueous Tear Film
· Oxygen and nutrients
· Bactericide
Mucinous Layer
· Ensures that tear film sticks to the eye
· Conjunctiva is a transparent layer above the cornea that is very vascular
· The conjunctiva has goblet cells that produce mucin

Question 10

What is the ultimate role of the tear film

Answer 10

Maintains clear vision and removes surface debris

Question 11

Which layer in the tear film protects the tear film from rapid evaporation?

Answer 11

Lipid layer (oily layer)

Question 12

Summarise the conjunctiva

Answer 12

The conjunctiva is the thin, elastic and 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.
§ It is nourished by tiny blood vessels that are nearly invisible to the naked eye.

Question 13

Describe the formation of the conjunctival sac

Answer 13

This membrane covers the full extent of the posterior surface of the eyelid before reflecting onto the outer surface (sclera) of the eyeball. With this membrane in place, a conjunctival sac is formed when the eyelids are closed, and the upper and lower extensions of the sac are the superior and inferior conjunctival fornices.

If you were to place something between the eye and eyelid- the conjunctiva will prevent it from reaching the back of the eye

Question 14

When will the vessels of the conjunctiva become visible

Answer 14

Conjunctivitis
Uveitis- inflammation of uvea and iris
Conjunctival hyperaemia- specific (diffuse redness of eye is related to conjunctivitis)
Uveitis- red ring around cornea

Question 15

Summarise the choroid

Answer 15

Layer that sits just outside the retina- full of blood vessels

Question 16

What is the role of the vitreous

Answer 16

Give shape to the eye

Question 17

What is the average anterior-posterior diameter of the orbit

Answer 17

24mm

Question 18

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

Answer 18

Sclera 
· Hard and opaque 
· Maintains the shape of the eye 
Choroid 
· Pigmented and vascular 
· Provides circulation to the eye 
· Shields out unwanted scattered light 
Retina 
· Neurosensory 
· Converts light into neurological impulses 

3 concentric layers

Question 19

Why is it important that the retina is transparent

Answer 19

So that the incoming light can pass through it and reach the receptors

Question 20

Describe the sclera

Answer 20

§ The sclera, commonly known as “the white of the eye,” is the tough, opaque tissue that serves as the eye’s protective outer coat. § High water content
Tough, white, fibrous tissue

Question 21

How much of the outer surface of the eye does the sclera coat

Answer 21

2/3rds

At the front of the eye- the sclera is transformed into the cornea.

Question 22

How tough is the sclera

Answer 22

If the eyeball is hit by a non-sharp structure that is roughly the same size as the orbit (squash ball)- the sclera will absorb the shockwaves and pass it on to the orbit- leading to a blow out fracture

Question 23

What is the cornea

Answer 23

§ The corneais the transparent, dome-shaped window covering the front of the eye. It is a powerful refracting surface, providing 2/3 of the eye’s focusing power. Like the crystal on a watch, it gives us a clear window to look through § Low water content

Question 24

Describe the properties of the cornea

Answer 24

Cornea – Front-most part of Anterior Segment – Continuous with the Scleral Layer
– Transparent
– Refraction –2/3 of power
• Convex curvature • Higher refractive index than air - light will bend towards normal upon entering
– Physical Barrier
– Infection Barrier

Question 25

Describe the 5 layers of the cornea

Answer 25

Epithelium
Bowman’s membrane (basal membrane for the epithelium)
Stroma (nerve endings for sensation/nutrition)
Descemet’s membrane (basal membrane for the endothelium)
Endothelium (removes fluid and prevents corneal oedema)

Question 26

Describe the epithelium of the cornea

Answer 26

An epithelial layer of stratified squamous non-keratinizing cells richly innervated with sensory nerves from cranial nerve V and continuously bathed in tear fluid.
The basement membrane and Bowman’s membrane, which gives strength to the cornea.

Question 27

Describe the corneal stroma

Answer 27

regularity contributes towards transparency • Corneal nerve endings provides sensation and nutrients for healthy tissue • No blood vessels in normal cornea

which occupies 90% of the thickness of the cornea. It consists of thin sheets of collagen fibrils that are oriented parallel to each other in the same sheet and at right angles to fibrils in sheets on either side. The spacing and arrangement of the fibrils gives the cornea its transparency. The stroma is avascular.

Question 28

Describe the endothelial layer of the stroma

Answer 28

The endothelial layer, which is a single layer of squamous cells, providing mechanisms for metabolic exchange between the aqueous humour and the cornea. It regulates the water content of the corneal stroma, preventing oedema and consequent opacity.

Question 29

What is the role of the endothelial layer of the stroma

Answer 29

Endothelium –pumps fluid out of corneal and prevents corneal oedema, • Only 1 layer of endothelial cell • No regeneration power • Endothelial cell density decreases with age • Endothelial cell dysfunction may result in corneal oedemaand corneal cloudiness

Question 30

What happen if you hydrate the cornea

Answer 30

It becomes whiter

Question 31

Summarise the uvea

Answer 31

Vascular coat of eye ball and lies between the sclera and retina. Uvea is composed of three parts. • Iris • ciliary body • choroid. These three portions are intimately connected and a disease of one part also affects the other portions though not necessarily to the same degree
Iris- will see cells in vitreous in region between cornea and iris- when you dilate the pupil it will be transparent- inflammatory cells interfere with opticl transparency
So uveitis can affect different regions or uvea
Topical steroids to treat iris
Systemic drugs or intra-vitreal treatment to treat intermediate or posterior regions

Question 32

Describe the dural layer of the cornea

Answer 32

Its integrity could be used to assess whether corneal transplants will be successful or not

Question 33

Summarise the choroid

Answer 33

The choroidlies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye.

Retina needs two sets of vessels- inner retinal vessels from central retinal artery
And an outer blood supply from the choroid

Question 34

Describe the choroid

Answer 34

Largest component of the uveal tract
Composed of a rich capillary bed that nourishes the retinal photoreceptors; a major feature is a high melanin in what is called the pigment epithelium.

Question 35

Summarise the iris

Answer 35

The coloured part of the eye is called the iris. It controls light levels inside the eye similar to the aperture on a camera. § The round opening in the centre of the iris is called the pupil. § The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.
Two sets of muscles under neural control to the control not only how much light enters the eye but also the depth of field
Wider pupil- smaller depth of field

Question 36

Describe the structure of the lens

Answer 36

Structure: outer acellular capsule, with regular inner elongated cell fibres (regularity = transparency) - contains collagen fibres

Question 37

Why does the pupil appear black

Answer 37

Because it is transparent

Question 38

What happens to the lens over time

Answer 38

May loose transparency with age –Cataract
With time the properties of the lens change. The elasticity of the lens becomes reduced and, therefore, the lens loses the ability to accommodate. Furthermore, transparency is lost either because of changes in the constituent proteins of the fibres or because of dehydration of the lens. Opacity in the lens is known as a cataract (accounts for half the cases of blindness)

Question 39

Summarise the functions of the lens

Answer 39

– Transparency
• Regular structure – Refractive Power • 1/3 power • Higher refractive index than aqueous fluid and vitreous – Accommodation • Elasticity

Question 40

What are the lens zonules

Answer 40

Lens is suspended by a fibrous ring known as lens zonules, consists of passive connective tissue
These suspend the lens from the ciliary bodies (muscle)

Question 41

Summarise the ciliary body

Answer 41

Extending from the choroid near the front of the eye is the ciliary body, a ring of tissue that encircles the lens and consists of two parts: a muscular component that adjusts the refractive power of the lens, and a vascular component (the so-called ciliary processes) that produces a fluid that fills the front of the eye

Question 42

What is the lens held in place by and what determines the shape of the lens

Answer 42

The lens is held in place by radially arranged connective tissue bands called zonule fibres that are attached to the ciliary muscle.
The shape of the lens is determined by two opposing forces: the elasticity of the lens, which tends to keep it rounded up ( removed from the eye, the lens becomes spheroidal); and the tension exerted by the zonule fibres, which tends to flatten the lens.

Question 43

Describe accommodation of the lens

Answer 43

The refractive power of the lens is changed by accommodation (which refers to a dyamic change in the shape of the lens). When the ciliary muscle contracts, it moves downwards and forwards. This reduces the tension in the suspensory ligament and allows the elastic lens to become fatter and shorter. This has the effect of increasing the refractive power of the lens allowing light from near targets (which reaches the surface of the eye as divergent rays) to be focused by convergence onto the retina. The lens is an elastic structure in young people, but it gradually hardens with age and can no longer readily change shape. Near vision, therefore, becomes impaired with increasing age, a phenomenon called presbyopia or long-sightedness
The inherent elasticity of the lens becomes the dominant force..

Question 44

Describe the contraction the ciliary muscle

Answer 44

Sphincter-like contraction
Ciliary muscle forms a ring around the lens, when the muscle contracts, the attachment points of the zonule fibres move toward the central axis of the eye, reducing the tension on the lens

Question 45

Summarise the retina

Answer 45

§ The retinais a very thin layer of tissue that lines the inner part of the eye.
§ 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.
Converts the photons to electrons

Question 46

Summarise the optic nerve

Answer 46

The optic nerve transmits electrical impulses from the retina to the brain. § It connects to the back of the eye near the macula. § The visible portion of the optic nerve is called the optic disc.

Question 47

Describe the optic disk

Answer 47

Numerous blood vessels, both arteries and veins, fan out over the inner surface of the retina. These blood vessels arise from the opthalamic artery and vein, which enter or leave the eye through a whitish circular area known as the optic disk, or optic papilla.
The optic disk is also where retinal axons leave the eye to reach targets in the thalamus or midbrain via the optic nerve.

Question 48

Describe the clinical importance of the optic disk

Answer 48

Useful gauge for intracranial pressure
The subarachnoid space surrounding the optic nerve is continuous with that of the brain; as a result, increases in intracranial pressure- a sign of serious neurological problems such as a space-occupying lesins or brain swellings due to trauma- can be detected as protuberance of the optic disk
Blood vessels thinner- further away from optic disk- can look at blood vessels to look at diabetic retinopathy

Question 49

Summarise the macula

Answer 49

The maculais located roughly in the centre of the retina, temporal to the optic nerve. § It is a small and highly sensitive part of the retina responsible for detailed central vision. § The fovea is the very centre of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such reading.

Question 50

Describe the role of the macula

Answer 50

Circular region containing yellow pigment (xanthophyll)
Supports high visual acuity (the ability to resolve fine details)
Acuity is greatest at the centre of the macula, a small depression or pit in the retina called the fovea.
The pigment xanthophyll has a protective role, filtering U.V light that could be harmful to photoreceptors.
Damage to this region, as occurs in age-related macular degeneration, has a devastating impact on visual perception
Region of highest acuity as this region has the highest concentration of cone receptors, and is also where the ratio of photoreceptors and ganglion cells is 1:1. The receptive field is the widest of the occipital cortex