13. Structure and function of the eye Flashcards
(92 cards)
1
Q
What are the two foramina found above and below the orbit?
A
Supraorbital foramen
Infraorbital foramen
2
Q
Which bones form the medial part of the orbit?
A
Lacrimal
Orbital plate of the ethmoid
Lesser wing of sphenoid
Frontal process of the maxilla
3
Q
Which bones form the roof of the orbit?
A
Lesser wing of the sphenoid
Orbital plate of the frontal bone
4
Q
Which bones form the lateral wall of the orbit?
A
Zygomatic process of the frontal bone
Greater wing of the sphenoid
Zygomatic bone
5
Q
Which bones form the floor of the orbit?
A
Orbital surface of the maxillary
Zygomatic bone
6
Q
What is the limbus in the external eye?
A
A dark ring that separates the iris from the sclera. It is where the corneal stem cells sit in
7
Q
What is the tear film?
A
A thin layer of fluid that covers the cornea
It maintains a smooth cornea-air surface
It is important for maintaining clear vision and removing debris during blinking
It is a bactericide
8
Q
What are the functions of the tear film?
A
To protect and lubricate the eye
Contribute to the optical properties of the eye
Supply oxygen and nutrients to the cornea
Remove debris
9
Q
What are the three layers of the tear film?
A
Superficial Oily Layer
Aqueous Tear Film
Mucinous Layer
10
Q
What is the function of the superficial oily layer of the tear film?
A
Reduces tear film evaporation – prevents the layers underneath from evaporating
It allows smooth blinking
It is produced by the Meibomian Glands along the lid margin
11
Q
What is the function of the aqueous tear film layer?
A
Main bulk of the tear film
Delivers oxygen and nutrients to the surrounding tissue
It contains bactericide
12
Q
What is the function of the mucinous layer of the tear film?
A
Produced by goblet cells
Maintains surface wetting
Ensures that the tear film sticks to the eye surface
The mucin molecules act by binding water molecules to the hydrophobic corneal epithelial cell surface
13
Q
Where is mucin produced from?
A
The conjunctiva - this is a transparent layer on top of the cornea that is very vascular. It has goblet cells that produce mucin
14
Q
Where is the lacrimal gland located?
A
The lacrimal gland is located within the orbit, latero-superior to the globe
15
Q
What are the three types of tears produced by the lacrimal gland?
A
Basal Tears - These are tears that are produced at a constant level, even in the absence of irritation or stimulation Reflex Tears - This refers to increased tear production in response to irritation. The tear reflex is made up of an afferent pathway, CNS and efferent pathway and the lacrimal gland Emotional Tears (crying)
16
Q
How are reflex tears produced?
A
The cornea is one of the most sensitive tissues in the body
Irritation is detected within the cornea which is innervated by sensory nerve fibres via the ophthalmic branch of the Trigeminal Nerve (CN V)
The efferent pathway is mediated by a parasympathetic nerve which innervates the lacrimal gland
17
Q
How are tears drained?
A
Tears drain through two puncta from tiny openings on the upper and lower medial lid margins
The puncta form the opening to the superior and inferior canaliculi within the eyelids
The two canaliculi converge as one single common canaliculus which drains the tears into the tear sac
The tears are drained out of the tear sac via the tear duct (nasolacrimal duct) which opens up in the nasal cavity in the inferior meatus
There is a valve between the canaliculi and the tear sac which does not allow the reflux of tears onto the surface of the eye
18
Q
What is the conjunctiva?
A
A thin, 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
19
Q
What does the ciliary body produce?
A
Aqueous humour
20
Q
What can be seen when you look into the back of the eye?
A
The optic disk and optic nerve
21
Q
What is the vitreous body?
A
Clear gel that fills the space between the lens and the retina
22
Q
What is found between the lens and the cornea?
A
A transparent fluid called the aqueous humour
23
Q
What are the three layers of the coat of the eye and their functions?
A
Sclera - this is the white of the eye. Hard and opaque
Outermost layer so protects the eye and maintains its shape
High water content
Choroid - pigmented and vascular
Provides circulation to the eye
Shields out unwanted scattered light
Provides nutrients for the retina
Retina - neurosensory tissue
Converts light into neurological impulses to be transmitted to the brain via the optic nerve
Deepest in the eye so is the most protected
24
Q
What is the cornea?
A
The cornea is in front of the sclera. It is the transparent, dome-shaped window covering the front of the eye. It is continuous with the scleral layer
25
What are the functions of the cornea?
Powerful refracting surface, providing 2/3 of the eye's focusing power
due to its convex curvature and refractive index that is greater than air
Physical Barrier – it is a very tough layer and is very hard to be damaged. Blowout fractures can occur when the bony orbit is broken but the eye does not break
Infection Barrier
26
Outline the 5 layer structure of the cornea
Epithelium – has a turnover of 36-48 hours so corneal injuries heal really quickly
Bowman’s Membrane
Stroma – regularity contributes towards transparency. This is the thickest layer
Descemet’s Membrane
Endothelium – pumps fluid out of corneal stroma and prevents corneal oedema
27
Why is the endothelial layer that forms the cornea so important?
It has no regeneration power
Endothelial cell density decreases with age
Endothelial cell dysfunction may result in corneal oedema and corneal cloudiness
28
What happens if you hydrate the cornea?
It goes opaque and you are unable to see
29
What is the Uvea and what is its structure?
This is the vascular coat of the eyeball and lies between the retina and the sclera
Is composed of three parts: The iris, cilliary body
and choroid
30
What is uveitis and what does it affect?
An inflammation of the eye or uvea is called uveitis
Anterior uveitis mainly affects the iris, intermediate uveitis mainly affects the cillary body and posterior uveitis affects the choroid
31
What is the function of the choroid?
Lies between the retina and the sclera (as it is a part of the uvea)
Made up of blood vessels that nourish the back of the eye
Nourishes mainly the outer part of the retina as the central retinal artery nourishes the inner retina
32
What are the functions of the inner and outer retina?
The outer retina mainly transforms light signals into electrical signals and the inner retina will modulate these signals and send them back to the brain
33
What is the ciliary body and where is it located?
Ciliary body is a ring shaped tissue, surrounding the lens
It is between the anterior and posterior segments of the eye and is located behind the iris. It is medial to the ciliary muscle
34
What is the function of the iris and what is it made of?
The Iris is the coloured part of the eye that controls the light levels that can enter. The iris is embedded with several muscles that restrict the pupil size. Also allows focussing of the eye.
It is composed of TWO layers:
Anterior Layer - Stromal Layer containing muscle fibres
Posterior Layer - Epithelial
35
How do the ciliary body and iris arise from the retina and the choroid?
The retina gives rise anteriorly to the ciliary body epithelium and the posterior (epithelial) layer of the iris
Choroid gives rise anteriorly to the ciliary body stroma and the anterior (stromal) layer of the iris
36
What is the structure of the lens?
Outer acellular capsule
| Regular inner elongated cell fibres - transparency
37
What is cataracts?
Loss of transparency of the lens with age
| It is the commonest preventable cause of blindness worldwide
38
What is the function of the lens?
Transparency - because of its regular structure
Refractive Power:
The lens is responsible for 1/3 of the refractive power of the eye
It has a higher refractive index than the aqueous and vitreous fluid
The lens can change shape so its refractive power and ability to focus can change
Accommodation - allows you to focus on near and distant objects by changing their shape
39
What are Lens Zonules?
The lens is suspended and held in place by fibrous bands called lens zonules
They consist of passive connective tissue
They anchor the lens by attaching to the ciliary body
The fibres of the zonules don't stretch at all - they merely transmit force from the contraction of the ciliary muscles
40
How does the lens let you focus on near and distant objects?
The ciliary muscles, which are part of the ciliary body, are attached to the lens and contract or release to change the lens shape
The lens becomes more rounded to focus on near objects
The lens becomes more stretched) to focus on objects that are far away
The ciliary body is attached to the lens by the zonules which are ligaments
41
What is the retina and what is its function?
The retina is a thin layer of tissue that lines the inner part of the eye
Is responsible for capturing the light rays that have entered the eye - from here the impulses travel via the optic nerve to the brain to get processed
42
What is the optic disk?
Where the optic nerve meets the retina. This is the physiological blind spot as it is where retinal ganglion cells exit via the optic nerve
43
Outline the structure and function of the optic nerve
The optic nerve is formed from collections of axons from the retinal ganglion cells. It is the visible part of the optic nerve
It transmits impulses from the retina to the brain
Connects to the back of the eye near the macula
Visible portion of the optic nerve is called the optic disk
44
What is the macula?
An area of the retina that has the highest concentration of photoreceptors so is vital for fine vision
It is highly sensitive and responsible for central vision
45
What is the fovea?
The flovea is the centre of the macula. This is why you have peripheral vision – whenever light falls on the fovea you can see the image clearly but any objects offset from your visual axis are blurry because the image does not fall on the fovea
The fovea has a high concentration of cones but a low concentration of rods (or even no rods)
46
What are the four branches of vessels radiating from the optic disc?
Superior Temporal
Inferior Temporal
Superior Nasal
Inferior Nasal
47
Outline the blood supply to the retina
The retinal arteries and veins provide circulation to the inner 2/3 of the retina
The outer 1/3 of the retina is supplied by the choroidal vasculature
48
What is the structure of the retina?
It consists of an outer layer of retinal pigment epithelium, immediately in front of the choroid and an inner thicker layer called the neuroretina (made up of photoreceptors and neurones)
The retinal pigment epithelium transports nutrients from the choroid to the photo-receptor cells and removes metabolic waste from the retina
49
What are the divisions of the neuroretina?
Outer Layer - photoreceptors (rods and cones)
Middle Layer - bipolar cells (because their axons project in both directions). The photoreceptors in the outer layer can either link directly to the retinal ganglion cells or connect to bipolar cells before connecting to ganglion cells.
Inner Layer - retinal ganglion cells (have their axons running into the optic nerve)
50
How are the anterior and posterior segments of the eye formed?
The two segments are separated by the lens
51
What are the two anatomical spaces in the anterior segment and what is found in each of the segments?
```
There are also two anatomical spaces within the anterior segment:
Anterior Chamber (cornea to iris)
Posterior Chamber (iris to lens)
```
Anterior Segment = aqueous humour
Posterior Segment = vitreous humour
52
What is the significance of the aqueous fluid in the anterior segment?
Aqueous fluid is produced by the ciliary body and it passes into the anterior chamber and then out through the angle via the trabecular meshwork
This drainage maintains the intraocular pressure
The role of the aqueous fluid is to supply nutrients to the cornea and other tissue in the anterior chamber
There are no blood vessels in the middle of the eye because you need a clear window for the light to pass through
53
What is the significance of the vitreous humour in the posterior chamber of the eye?
Vitreous humour is composed of 99% water, trapped inside a jelly matrix
The jelly substance provides mechanical support to the eye
There are some collagen and GAGs in the vitreous humour
The regular structure of the vitreous allows it to be transparent
54
What happens to vitreous humour as we get older and why can this be a problem?
As we get older, the vitreous humour loses its jelly consistency, liquefies and detaches from the retina
The vitreous detachment is experienced as seeing FLOATERS
Normally this is harmless but sometimes it may lead to a small tear in the peripheral retina
The liquid vitreous can then pass into the subretinal space and lead to retinal detachment which can lead to BLINDNESS
55
What does the ciliary body secrete?
Aqueous fluid (aqueous humour)
56
What happens to the fluid secreted by the ciliary body?
Ciliary body secretes aqueous fluid
The aqueous fluid then travels anteriorly into the anterior chamber
The trabecular network drains the fluid out of the eye
The fluid can be absorbed passively through uveoscleral outflow or actively through canal of Schlemm
57
What is the schlemm canal?
The Schlemm canal is a modified vein that sits around the trabecular meshwork where the aqueous is actively absorbed
This is why prostaglandin analogues are the first line of treatment for glaucoma as they act on the uveal-scleral outflow
58
What is the range for normal intraocular pressure?
11-12 mm Hg
59
What is glaucoma?
Definition: Optic neuropathy with characteristic structural damage to the optic nerve, associated with progressive retinal ganglion cell death, loss of nerve fibres and visual field loss
It is a condition of sustained raised intraocular pressure – patients with an intraocular pressure of over 24 mmHg are more at risk of developing glaucoma
60
What happens to patients with untreated glaucoma?
Patients with untreated glaucoma lose peripheral vision progressively
Untreated glaucoma will eventually lead to blindness
There is retinal ganglion cell death and ENLARGED optic disc cupping
61
What is the lamina cribrosa?
The nerves of the eye pass out through the scleral canal which is occupied by a mesh-like structure called the lamina cribrosa. It is formed by a multilayered network of collagen fibers that insert into the scleral canal wall.
62
Outline the two theories associated with glaucoma
Mechanical theory of glaucoma – there is stiffening of the sclera so there is stretching of the lamina cribrosa which causes physical injury to the axons causing ganglion cell death
Vascular theory of glaucoma – overnight there is a reduced blood supply to the retina which causes hypoxic conditions and so the death of ganglion cells
63
What are the two types of glaucoma?
Primary open angle glaucoma
| Closed angle glaucoma
64
What is primary open angle glaucoma caused by?
MOST COMMON
| It is caused by a functional blockage of the Trabecular Meshwork
65
What happens in closed angle glaucoma?
Can be acute or chronic
It is caused by the forward displacement of the iris/lens complex - narrowing the trabecular meshwork
It occurs commonly is patients with small eyes (hypermetropic)
Can be treated with peripheral laser iridotomy to create a drainage hole in the iris
Vicious Cycle - the increase in intraocular pressure pushes the iris and lens forward thus narrowing the angle and reducing the drainage so the intraocular pressure is raised even more
66
What is central vision?
Detail day vision, colour vision - FOVEA
The fovea has the highest concentration of cones
This vision is responsible for reading and facial recognition
It is assessed by a visual ACUITY assessment
Loss of foveal vision leads to poor visual acuity
67
What is peripheral vision?
Shape, movement, night vision
Is mainly due to the rods
Assessed by a visual FIELD assessment
Extensive loss of visual field means you are unable to navigate the environment
68
What are the two types of photoreceptors and their characteristics?
Rods:
Longer outer segment with photosensitive pigment
100 time more sensitive to light than cones
Slow response to light
Responsible for night vision (scotopic vision)
20 times more rods than cones
Cones:
Less sensitive to light
FASTER response to light
Responsible for day light fine vision and colour vision (photopic vision)
69
How does photopigment regeneration take place?
Photopigments are synthesised in the inner photo-receptor segment and are then transported to the outer segment discs
The distal discs with deactivated photo-pigments are shredded from the tips and phagocytosed by the retinal pigment epithelial cells
The deactivated photopigments are regenerated inside the retinal pigment epithelial cells and are then transported back to the photo-receptors
70
What are the two different photopigments and their compositions?
Rod Photopigment (Rhodopsin):
Opsin is the transmembrane protein
Cofactor = 11 cis retinal (vitamin A derived)
Cofactor reacts to photons
Causes a conformational change to rhodopsin
This activates a G-protein pathway
Results in a nerve action potential
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Cone photopigments:
There are THREE subtypes of photopsin that react maximally to THREE different light frequencies:
S-cone - short wavelength (blue)
M-cone - medium wavelength (green)
L-cone - long wavelength (red)
This forms the basis of colour vision
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71
Where are rod photoreceptors located and what are they responsible for?
Rods are responsible for night vision (scotopic vision)
Rod receptors are widely distributed across the retina but the highest density is just outside the macula
The rod photoreceptors are completely absent within the macula
The density of rod photoreceptors tails off towards the periphery
72
Where are cone photoreceptors located and what are they responsible for?
Cone photoreceptors are responsible for day-time vision -PHOTOPIC VISION
They are ONLY found within the macula
73
Where is the physiological blind spot located?
About 20 degrees temporal to the central vision, corresponding to the location of the optic disc
74
What is the normal visual field in each eye
60 degrees nasally
100 degrees temporally
60 degrees above
70 degrees below
75
What is the most common form of colour vision deficiency?
Deuteranomaly (red-green colour blindness)
| This is caused by the shifting of the M-cone sensitivity peak towards the L-cone peak causing red-green confusion
76
What are colour vision deficits caused by?
Can be caused by a shift in the photo-pigment peak sensitivity
When it is caused by a shifted peak it is called anomalous trichromatism
Can also be caused by the absence of one or more of the three cone photopigment subtypes
Dichromatism - only 2 cone photopigment subtypes are present
Monochromatism - complete absence of colour vision
77
What test is used for red-green colour perception deficiencies
Ishihara Test
| People with normal red-green vision will be able to see a pattern amongst the dots
78
What is light adaptation and dark adaptation?
In dark adaptation, the retina increases its light sensitivity when moving from a LIGHT TO DARK environment
Cone photoreceptors adapt much quicker than rod photoreceptors
Light adaptation is the suppression of light sensitivity when moving FROM DARK TO LIGHT environments
79
What happens in light adaptation?
It is mediated by photopigment bleaching by bright light and neuro-adaptation inhibiting rod and cone function
In light adaptation - rod function is greatly suppressed and cone function takes over within a minute
The pupil also provides some light and dark adaptation by acting as an adjustable aperture regulating the amount of light that passes into the eye
80
What is refraction and total refraction?
The light refracts at the boundary between the two mediums
Total refraction – the angle of incidence is so steep that none of the light is refracted through. Instead it is all reflected
Lenses are used to refract light
81
What is emmetropia?
This is perfect focusing - in emmetropia, parallel rays (from a distance) converge exactly on the fovea forming a clear image on the retina
82
What is focusing power? What parts of the eye have the most focusing abilities?
Ability to focus light to form an image on the retina
Division of focusing power:
Cornea - 2/3
Lens - 1/3
83
What us ametrioia? Give some examples of ametropia
There is a mismatch between the axial length and refractive power
Parallel light rays don't fall on the retina – the eye cannot accommodate
Examples of ametropia include:
Nearsightedness (myopia)
Farsightedness (hyperopia)
Astigmatism
Presbyopia
84
What is hypermetropia/hyperopia and what is it caused by?
Long sightedness
The eye doesn't have enough focusing power to focus the rays on the retina - it can only focus it behind the retina
This can be corrected with a convex lens to provide additional converging power
Commonly caused by a short eyeball and, more rarely, a flat corneal surface
You can't see nearby objects
85
Why is hypermetropia worse in the elderly?
When people are young they are able to accommodate so they can constrict their ciliary muscles and contract their lens, causing it to pull back and become more refractive, making the focal point on the surface of the retina
However, older people are not able to accommodate and are more likely to face hyperopia and its symptoms
86
What is myopia and what is it caused by?
Light rays coming from distant objects focus in front of the retinal surface - focal point is in front of the retina
The cornea and lens have excessive refractive power
This is commonly caused by having a long eyeball or, occasionally, a highly curved cornea
Patients require concave lenses to see clearly at a distance
Symptoms include squinting to see at a distance and headaches
87
What is amblyopia?
If the two eyes are slightly shaped differently or have different optical properties then one eye will be able to see better than the other.
The brain will prefer one of the eyes and only begin to accept signals from one of the eyes
The eye that is hyperopic will not be able to see even if a lens is put in front of it
88
What is astigmatism?
In astigmatism, the cornea is OVAL rather than a spherical shape
The refractive power varies along different planes
So the eye is myopic in the vertical plane and mildly hypertropic in the horizontal plane
There will therefore be two different focal points
89
What is used to help with astigmatism?
Special astigmatic glasses are required with correction for the different planes. They cause the focal points to align so that a clear image can be formed
90
How is the near response mediated?
Constriction of the sphincter pupillae increases the depth of field (pupillary meiosis)
Convergence where both medial recti contract to adduct both eyes
Accommodation - the circular ciliary muscle contracts to fatten the lens and increase the refractive power to focus on a near object
91
What is presbyopia?
Naturally occurring loss of accommodation with age (unable to focus on near objects)
Onset from age 40 years
Distant vision intact
Corrected by reading glasses (convex lenses) to increase the refractive power of the eye
92
What are the different types of optical correction?
Spectacle lenses - monofocal lenses, spheracle lenses
Contact lenses - higher quality of image and less change in size of the image
Intraocular lenses
Surgical correction - e.g. intraocular surgery
