Structure and Function of the Eye Flashcards
What is the average anterior-posterior diameter of the eye in adults?
24 mm
What are the three layers of the coat of the eye? Describe their properties and function.
Sclera:
- Hard and opaque
- High water content - makes it opaque
- Continuous with the cornea
- Protects eye and maintains its shape
Choroid:
- Pigmented and vascular
- Provides circulation to the eye
- Shields out unwanted scattered light
Retina:
- Neurosensory tissue
- Converts light into neurological impulses
What are the 5 layers of the cornea?
Epithelium Stroma Endothelium
What are the two segments of the eye separated by?
Two segments:
- Anterior segment
- Posterior segment
The two segments are separated by the lens
Which humours are found in the two segments of the eye?
Anterior = aqueous humour Posterior = vitreous humour
What name is given to the fibrous strands that suspend the lens from the ciliary bodies?
Zonules
Describe the production and drainage of aqueous humour.
Aqueous humour is produced by the ciliary body It is drained via the trabecular meshwork into the canals of Schlemm
What is the role of aqueous humour?
Provides nutrients to the cornea and other tissues in the anterior chamber
Describe vitreous humour.
It is 99% water trapped inside a jelly matrix
What is the function of vitreous humour?
Mechanical support for the eye
Describe how the vitreous humour changes with age.
It loses its jelly consistency, liquefies and can become detached from the retina Vitreous detachment in seen as floaters
What are the potentially disastrous consequences of vitreous humour detachment?
Detaching from the retina could cause a small tear in the peripheral retina If there is a small tear, liquid vitreous could seep into the sub-retinal space and lead to retinal detachment If untreated, it can lead to blindness
What are the two layers of the iris?
Anterior – stromal layer containing muscle fibres Posterior – epithelium
Describe how the retina and choroid contribute to the different parts of the iris and ciliary body.
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 layer of the iris (stromal layer)
What is the collective term for the choroid, iris and ciliary body?
Uvea
What is the normal range for intraocular pressure?
11-12 mm Hg
What is glaucoma?
Condition of sustained raised intraocular pressure
What changes can be seen in the retina in glaucoma?
Retinal ganglion cell death Enlarged optic disc cupping
What are the consequences of untreated glaucoma?
Progressive loss of peripheral vision Blindness
What is the most common type of glaucoma and what is it causedby?
Primary open angle glaucoma It is caused by a functional blockage of the trabecular meshwork
State another relatively common type of glaucoma. What is it caused by?
Closed angle glaucoma This can be acute or chronic It is caused by the forward displacement of the iris-lens complex –narrowing the trabecular meshwork
In what type of patients does closed angle glaucoma tend to occur and what is the treatment?
Small eyes (hypermetropic) Treatment: peripheral laser iridotomy
Describe the structure of the lens.
It has an outer acellular capsule There are regular inner elongated fibres, which give the lens its transparency NOTE: may lose transparency with age
Which two structures provide the majority of the refractive power of the eye?
Cornea = 2/3 Lens = 1/3
Other than its role in refracting light, what else is the corneanecessary for?
Physical barrier – protects the eye from opportunistic infection
What are the consequences of prolonged contact lens wear?
Reduce the oxygen supply to the cornea
What is the role of the endothelial layer of the cornea?
It pumps out fluid from the stroma and prevents stromal oedema
What is the role of tear film?
Maintains clear vision and removes surface debris
What are the three layers of the tear film?
Superficial oily layer Aqueous tear film Mucinous layer
What produces the superficial oily layer?
Meibomian glands
What are the roles of each of these three layers?
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
Where is the lacrimal gland located?
Superio-laterally to the orbit
What are the three types of tears?
Basal Tears – produced at a constant level in the absence of irritation Reflex Tears – increased tear production in response to irritation Emotional Tears – crying
Describe the innervation of the cornea.
The cornea is very sensitive and it is innervated by the ophthalmic branch of the trigeminal nerve (CN V)
Describe the drainage of tears.
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 –> common canaliculus –> tear sac Tear sac then drains through the nasolacrimal duct, which opens up inthe inferior meatus
What two things regulate how much light reaches the retina?
Pupil Pigmented Uvea
What is the term given to perfect focusing ability?
Emmetropia
What is the technical term for long-sightedness?
Hypermetropia
How can long-sightedness be corrected and what is it caused by?
Convex lenses It is usually caused by having short eyeballs It is occasionally caused by a flat corneal surface
What is the technical term for short-sightedness?
Myopia
How can short-sightedness be corrected and what is it caused by?
Concave lenses It is usually caused by having a long eyeball It is occasionally caused by having a highly curved cornea
What is astigmatism?
The cornea is oval rather than round This means that the refractive power varies in different planes (in some planes you will be hypermetropic, and in others you would be myopic)
Which nerve is responsible for accommodation?
Oculomotor Nerve (CN III)
What is the near response triad?
Pupillary Miosis (constriction of sphincter pupillae) – increases the depth of field Convergence – the medial recti of both eyes contract Accommodation – ciliary muscles contract to thicken the lens and increase its refractive power
What is the term given to naturally occurring loss of accommodation with age?
Presbyopia
What are the four branches of vessel arcades radiating from the optic disc?
Superior Temporal Inferior Temporal Superior Nasal Inferior Nasal
Describe the difference in perfusion between the outer and inner parts of the retina.
Inner 2/3 of the retina = retinal arteries Outer 1/3 of the retina = choroidal vasculature
What part of the retina is responsible for central vision?
Fovea (it has the highest concentration of cones)
What is peripheral vision responsible for?
Shape, movement, night vision
Describe the structure of the retina.
Just inside the choroid you have the retinal pigment epithelium Then you have the neuroretina, which consists of: Outer Layer – photoreceptors (rods and cones) Middle Layer - bipolar cells Inner Layer – retinal ganglion cells
What is the function of the retinal pigment epithelium?
Transports nutrients from the choroid to the photo-receptor cells and removes metabolic waste from the retina
Describe how the fovea appears on a cross-section of the macula.
It appears as a foveal pit due to the absence of overlying retinalganglion cells
State the two classes of photo-receptor and their properties.
Rods Longer outer segment with photo-sensitive pigments 100 times more sensitive to light than cones Slow response to light Responsible for night vision (scotopic vision) 120 million rods Cones Less sensitive to light Faster response to light Responsible for daylight vision and vision and colour vision (photopic vision) 6 million cones
Describe the recycling of photopigments.
Photopigments are synthesised in the inner photo-receptor segment and then are transported to the outer segment discs The distal discs with deactivated photo-pigments are shredded from the tips and phagocytosed by retinal pigment epithelial cells The deactivated photo-pigments are regenerated inside the retinal pigment epithelial cells and are then transported back to the photo-receptors
What is the rod photopigment?
Rhodopsin It is a G-protein coupled receptor system
What is the co-factor for this photopigment?
11 cis retinal (vitamin A derived)
To what wavelength does rhodopsin react maximally in humans?
498 nm
What is another term for night vision?
Scotopic vision
What is another term for day-time vision?
Photopic vision
Describe the distribution of rods and cones across the retina.
Rods have the highest density just outside the macula They decrease in density the further you move away from the macula There are NO rods in the macula Cones are ONLY found in the macula The highest density of cones is in the fovea
What does a hill of vision represent?
Sensitivity of vision over a visual field
Where is the physiological blind spot located?
20 degrees temporal to central vision
What are the three types of cone photopigment and which colours do they respond maximally to?
S-cone – short wavelength – BLUE M-cone – medium wavelength – GREEN L-cone – long wavelength – RED
What is the most common colour vision deficiency and what is it caused by?
Deuteranomaly It is caused by the shifting of the M-cone towards the L-cone
What is the term given to shifted peaks?
Anomalous trichomatism
What test is used to diagnose colour blindness?
Ishihara Test
Describe how light sensitivity changes in dark adaptation.
Light sensitivity increases in dark adaptation
How does retinal light change in light adaptation and what is responsible for this effect?
Light sensitivity decrease in dark adaptation This suppression of light sensitivity is caused by photopigment bleaching and neuro-adaptation inhibiting rod and cone function
