Ocular Anatomy + Physiology Flashcards
Describe the orbit and its function
Orbit: bony socket containing the eye
Formed from a number of fused bones
Serves to:
protect the delicate eye
provide a mounting point for the six striatal muscles that control ocular movement
What is the internal eye made up of? (7)
Sclera Lens Cilliary body Iris Choroid Retina Optic nerve
What is the Sclera?
It is the tough, outer protective coat
What is the Lens?
It is a transparent biconvex structure suspended by lens zonules
What is the Cilliary body?
provides attachment for lens zonules. Changes the focusing power of the lens
What does the Iris do?
regulates light entering eye attached by cilliary body
What does the Choroid do?
maintains the eye’s blood supply
What is the Retina?
inner layer containing visual photoreceptors (rods & cones)
What is the function of the optic nerve?
transmits images from photoreceptors to the brain
Where are tears produced from?
Lachrymal gland
What are the two types of tears?
Basic tears (to maintain a tear film on the corneal surface for optical, metabolic and lubricant purposes)
Reflex tears are induced in response to chemical or mechanical irritation, cold, light, etc)
What does precorneal tear film consist of? What is the three layers theory?
The precorneal tear film consists of a superficial lipid layer, a central aqueous layer and an inner mucus layer.
Describe the lipid layer in the precorneal tear film
It consists mainly of sterol esters, triacylglycerols and phospholipids, free sterols and free fatty acids
It spreads over the aqueous layer during eye opening
The lipids play an important role in reducing the evaporation rate to maintain the normal tear osmolality
What components make up the aqueous layer of the tear film?
Inorganic salts Glucose Urea Retinol Ascorbic acid Various proteins Lipocalins (tear-specific pre-albumins) Immunoglobulins Lysozyme Lactoferrin Glycoproteins
Describe the function of the precorneal tear film (4)
Prevention of corneal drying
Transport of oxygen and nutrients to the cornea
Maintenance of cornea clean and smooth
Protection against infection
What is the pH of normal tears? and when is it at its lowest?
pH 7.3 -7.7
Lowest on awakening
Why are tears more acidic in contact lens wearers?
impediment (hinderance) of the efflux (flowing out) of carbon dioxide
In what cases do tears become more alkaline?
Dry eye
Severe ocular rosacea
Lacrimal stenosis (tear duct narrows)
Under normal conditions what volume of tear can be produced?
7-9 microlitres
What is the maximum amount of fluid that can be held in the lower eye sac? how much of this is incorporated into the tear film without causing it to destabilise?
25-30 microlitres
only 3microlitres can be incorporated into the film
What is the function of blinking?
Important defence mechanism - brisk blinking reflex
Essential for periodic reforming of the tear film
Also activates a pumping mechanism for the drainage of tears through the lacrimal drainage apparatus
What are the functions of the cornea?
Passage of light
Refraction of light
Protection
What is the aqueous humour?
a transparent viscous fluid located in the anterior chamber of the eye
What is the aqueous humour composed of? (6)
98% water amino acids electrolytes ascorbic acid glutathione immunoglobulins
What are the functions of the aqueous humour?
Maintains the intraocular pressure and inflates the globe of the eye
Provides nutrition for the avascular ocular tissues (posterior cornea, trabecular meshwork, lens, and anterior vitreous);
May serve to transport ascorbate in the anterior segment to act as an antioxidant agent;
Potential immune response to defend against pathogens (immunoglobulins);
Light refraction
How does the lens change depending on the distance of the object?
The lens is elastic and can change shape, getting fatter to focus close objects and thinner for distant objects;
What does the lens consist of? (3)
lens capsule,
the lens epithelium, and the lens fibers
What is the vitreous humour?
clear gel that fills the space between the lens and the retina
What are the functions of the vitreous humour?
helps to maintain normal anatomic expansion of the globe;
impedes the diffusion of substances between the retina and the anterior segment of the eye.
small water-soluble substances in the retina may diffuse into the vitreous across the blood-retina barrier.
- acts as a reservoir of substances such as oxygen, glucose and ascorbic
acid, which support the metabolism of adjacent structures
- acts as a reservoir for metabolic waste products
What is the composition of the vitreous humour?
water (99%); a network of collagen fibrils; large molecules of hyaluronic acid; peripheral cells (hyalocytes); inorganic salts; glucose; ascorbic acid
What are the photoreceptors?
Photoreceptors located on retinal surface
Rods act in low light levels, do not provide colour and have little spatial resolution
Cones operate at higher light levels, perceive colour and allow good spatial resolution
What is the function of the optic nerve?
Rod/cone (photoreceptors) activation causes AP generation in the optic nerve.
Optic nerve is a bundle of ~1.25108 nerves
Pass into skull interior via optic canal to meet at optic chiasma.
Nerves project to visual cortex in occipital lobes
