Eye: Structure and Function Flashcards
What is the average anterior-posterior diameter of the orbit?
24 mm
What are the three layers of the eye? Describe their properties and function.
Sclera - Hard and opaque, Maintains the shape of the eye (High water content)
Choroid - Pigmented and vascular, 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
What are the layers of tear film?
Superficial oily - Reduce tear film evapouration (Produced in Meibomian glands along lid margins)
Aqueous tear film - From tear gland
Mucinous layer - Prevents wetting of cornea, produced by goblet cells
Explain the lacrimal system.
Lacrimal gland lies superior to upper lid (Supertemporal?), provides watery component to tears
The two puncta (Sing. Punctym) stenose and collect the secretions, travel through canaliculi to tear sac and tear duct. (Tear duct ends in inferior meatus in pharynx - valve prevents reflux)
What covers the outer surface of the eye?
Conjunctiva - thin, transparent tissue (also lines inside of eyelid)
Has many small blood vessels.
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, provides mechanical support for the eye.
In ageing 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 changes can be seen in the retina in glaucoma?
Retinal ganglion cell death
Enlarged optic disc cupping
Leads to Progressive loss of peripheral vision, Blindness
What are two causes of Glaucoma? What is the first line treatment?
Primary open angle glaucoma It is caused by a functional blockage of the trabecular meshwork
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 (Small eyes (hypermetropic) Treatment: peripheral laser iridotomy)
Prostaglandin analogue - only 20% pressure reduction max
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
What are the consequences of prolonged contact lens wear?
Reduce the oxygen supply to the cornea
What are the layers of the cornea?
1 - Epithelium - large turnover, tight barriers
2 - Bowman’s membrane
3 - Stroma - No bloodvessels but has nerve endings, transparent
4 - Descemet’s membrane
5 - Endothelium (Density of cells decreases with age, pumps water out of cornea)
Where is the lacrimal gland located? What does it do?
Superio-laterally to the orbit, produces tears.
Types
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)
What two things regulate how much light reaches the retina?
Pupil
Pigmented Uvea
What are the types of vision?
Emmetropia - axial length match refractive power, rays fall on retina, no accomodation
Amemtropic - Myopia (Near-sight), Hyperopia (Far-sight), Astigmatism, Presbyopia
What causes Myopia and hyperopia?
Excessive globe length (axial myopia) common
Excessive refractive power (refractive myopia)
Treat with concave lens
Insufficient globe length (axial hyperopia) common
Insufficient refractive power (refractive hyperopia)
Treat with convex lens
What are symptoms of hyperopia?
Asthenopic - eyepain, frontal headache, buring sensation in eyes, blepharoconjunctivitis
Early blurring in vision acuity (nature varies)
Amyblyopia - uncorrected hyperopia, one eye takes dominance
What causes astigmatism?
Non-spherical cornea - light refracts differently along one perpendicular meridian leading to two focal points - unfocused vision
What are the symptoms/treatment for astigmatism?
Symptoms - blurring, asthenopia, head tilting
Treatment - Reg: Cylinder lens or other lens Irreg: Rigid CL, surgery
What causes Presybyopia?
Natural associated with aging and cell death onset around 40, only affects near vision so treated with convex lenses
What are some different types of lenses?
Spectacle
Contact – less distortion, higher infection risk
Interocular – Cataract lens replacement
What are some types of occular surgery?
Keratorefractive surgery – laser??
Interocular surgery
What is the difference between central and peripheral vision
Central - Day vision, colour, detail (CONE)
Peripheral - Night vision, movement (ROD)
What are the retinal neural layers?
Outer - Photoreceptors
Middle - Bipolar cells (Improve contrast, regulate sensitivity)
Inner - Retinal ganglion cells
State some differences between cone and rods
Rod - longer outer segments, 100x more light sensitive, slower response 120 mil cells
Cones - 6 mil cells, faster response, phototopic colour vision
What are different forms of cone cells?
S cones - sensitive to blue light
M cones - Sensitive to green light
L cones - Sensitive to red light
Rods are senstive at about 500 nm
How is colour blindness tested?
Ishihara test (only for red-green deficinencies)
How long do cells take to adapt to light/dark?
Dark: Cones 7 mins, Rods 30 mins (rhodopsin regeneration)
Light: 5 mins, bleach photo-pigments, Neuroadapt to inhibit rod and cone function
(Pupil adaptation has minor effect)
What is the macula?
Small highly sensitive part of retina responsible for detailed central vision
(centre of macula is fovea - responsible for level of detail for reading etc)
Fovea appears as a foveal pit due to the absence of overlying retinalganglion cells in macula cross section.
Where are highest concentration of rod and cone receptors found?
20-40 degrees from Fovea, for cones it’s IN fovea
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
Where is the physiological blind spot located?
20 degrees temporal to central vision
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
