Structure and function of the eye Flashcards Preview

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Flashcards in Structure and function of the eye Deck (50)
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1
Q

Summarise the bony orbit

A

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

2
Q

Describe the structure of the bony orbit

A

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

3
Q

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

A

– 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

4
Q

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

A

The lacrimal gland

Which is located supero-laterally to the orbit

5
Q

Summarise the lacrimal system

A

– 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

6
Q

Describe the drainage of tears

A

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

7
Q

Summarise the functions of the tear film

A
  • 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
8
Q

Describe the components of the tear film

A

– 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

9
Q

What are the roles of each of these three layers?

A

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

10
Q

What is the ultimate role of the tear film

A

Maintains clear vision and removes surface debris

11
Q

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

A

Lipid layer (oily layer)

12
Q

Summarise the conjunctiva

A

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.

13
Q

Describe the formation of the conjunctival sac

A

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

14
Q

When will the vessels of the conjunctiva become visible

A

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

15
Q

Summarise the choroid

A

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

16
Q

What is the role of the vitreous

A

Give shape to the eye

17
Q

What is the average anterior-posterior diameter of the orbit

A

24mm

18
Q

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

A
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

19
Q

Why is it important that the retina is transparent

A

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

20
Q

Describe the sclera

A

§ 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

21
Q

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

A

2/3rds

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

22
Q

How tough is the sclera

A

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

23
Q

What is the cornea

A

§ 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

24
Q

Describe the properties of the cornea

A

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

25
Q

Describe the 5 layers of the cornea

A

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)

26
Q

Describe the epithelium of the cornea

A

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.

27
Q

Describe the corneal stroma

A

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.

28
Q

Describe the endothelial layer of the stroma

A

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.

29
Q

What is the role of the endothelial layer of the stroma

A

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

30
Q

What happen if you hydrate the cornea

A

It becomes whiter

31
Q

Summarise the uvea

A

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

32
Q

Describe the dural layer of the cornea

A

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

33
Q

Summarise the choroid

A

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

34
Q

Describe the choroid

A

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.

35
Q

Summarise the iris

A

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

36
Q

Describe the structure of the lens

A

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

37
Q

Why does the pupil appear black

A

Because it is transparent

38
Q

What happens to the lens over time

A

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)

39
Q

Summarise the functions of the lens

A

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

40
Q

What are the lens zonules

A

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)

41
Q

Summarise the ciliary body

A

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

42
Q

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

A

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.

43
Q

Describe accommodation of the lens

A

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..

44
Q

Describe the contraction the ciliary muscle

A

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

45
Q

Summarise the retina

A

§ 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

46
Q

Summarise the optic nerve

A

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.

47
Q

Describe the optic disk

A

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.

48
Q

Describe the clinical importance of the optic disk

A

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

49
Q

Summarise the macula

A

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.

50
Q

Describe the role of the macula

A

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