Structure and Function of the Eye (A) Flashcards
What is the average anterior-posterior diameter of the orbit?
24 mm
What are the three layers of the eye? Describe their propertiesand function.
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
What are the two segments of the eye separated by?
Lens separates anterior and posterior segments
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 behind the iris. The fluid moves anteriorly around the iris and is drained in front of the iris via 2 pathways:
Either the trabecular meshwork into the canals of Schlemm (80-90%, active process)
Or uveal scleral outflow (20%, passive)
What is the role of aqueous humour?
Provides nutrients to the cornea and other tissues in the anterior chamber
Describe the composition of vitreous humour.
99% of vitreous humour is water, trapped in a jelly like 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 (can shrink basically).
Vitreous detachment in seen as floaters in the eyes and is a common problem as people age
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 is the uvea
collective term for the choroid, iris and ciliary body
What is the normal range for intraocular pressure?
12-21 mm Hg
What is glaucoma?
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
What changes can be seen in the retina in glaucoma?
Retinal ganglion cell death
Enlarged optic disc cupping
(optic cup is the yellowy part of the optic disc)
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
Caused by trabecular network dysfunction.
Outline the classification of glaucoma
glaucoma can be open angle or closed angle. If the glaucoma is an intrinsic problem then its primary if caused by something else then its secondary (eg eye injury in a fight)
In what type of patients does closed angle glaucoma tend to occur and what is the treatment?
Small eyes (hypermetropic/long sighted. Light focused behind retina) This is because the structures in the eyes are more crowded and so more likely to block the drainage of aqueous humour. Treatment: peripheral laser iridotomy -otomy means cutting into. Cutting into the iris to create a hole. This allow aqueous humour to flow through the new hole and pushes the iris back so the angle is opened again
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
What layer of the eye is the cornea continuous with?
Sclera
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 are the 5 layers of the cornea?
- Epithelium
- Bowman’s membrane (basement membrane for epithelium)
- Stroma
- Descemet’s Membrane (basement membrane of endothelium)
- Endothelium
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?
Nutrition of and oxygen supply to the cornea
Gliding layer for the eyelids (lubrication)
Protection against foreign bodies (dust, pollen, etc.)
Smoothing of the optical surface of the eye for clear vision
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?
1.Superficial Oily Layer
Reduces tear film evaporation
2.Aqueous Tear Film
Oxygen and nutrients
Bactericide
3.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 which forms part of the tear film
what is a closed angle glaucoma
Vicious cycle of increased pressure which pushes the iris/lens forwards, blocking the route of drainage of aqueous humour (trabecular meshwork etc)
Which fluid is used in the measurement of intraocular pressure
aqueous humour
vitreous humour is fixed so does not really affect pressure that much whereas aqueous humour is affected by the rate of drainage and production so needs to be monitored
3 types of tears
Basal tears - constantly produced and make the tear film that is always there
reflex tears- irritation causes reflex
crying tears
outline the reflex pathway of the reflex tears
afferent- V1 of trigeminal (opthalmic)
Efferent- parasympathetic nerves
neurotransmitter - ACh
Outline the pathway of tear production and tear drainage
–Tear produced by the Lacrimal Gland
– Tear drains through the two puncta (opening of lacrimal duct located medially)
– Tear flows through the superior and the
inferior canaliculi (aka lacrimal duct)
– Tear gathers in the lacrimal Sac
– Tear exits the lacrimal sac into the nasolacrimal duct then into the nasal cavity below the inferior nasal concha
Does the conjunctiva cover the cornea?
NO it does not cover the cornea. It only covers the visible part of the sclera and also lines the inside of the eye lids (search up a pic)
Sclera water content high or low?
High
Cornea water content high or low
low
Are there blood vessels in cornea
no
Outline the 3 functions of lens
Transparency due to regular structure Refractive power (1/3) Accomodation (its quite elastic)
Anterior vs posterior chamber
Anterior segment vs posterior segment
Anterior and posterior chambers are part of the anterior segment:
Anterior chamber: between cornea and iris
Posterior chamber: between iris and lens
Anterior segment= everything in front of lens and suspensory ligaments (all aqueous humour)
Posterior segment (vitreous chamber)= everything behind lens and suspensory ligaments (all vitreous humour)
look at pic in word file
