Vision 1 Flashcards
What is refraction?
Light rays bending to form a sharp image on the retina
What is accommodation?
Adapting focus between near and far away objects by changing how much we bend the light rays
What are refractive errors?
A mismatch between how much we bend light rays
What are the refractive media of the eyeball?
Cornea, lens, aqueous humour, vitreous humour
What media has the most powerful refractive properties?
The cornea - 45D
What media has the ability to alter its degree of refraction?
The lens
What is the shortest range at which human eye can focus?
20cm
What three actions take place to allow accommodation?
lens changes shape
pupil constricts
eyes converge
Describe the process of the lens thickening
Ciliary muscle contracts making ciliary body bulge Space in the middle decreases Suspensory ligaments become lax Lens is no longer under strain Lens becomes thicker
Describe the process of the pupil constricting
The pupillary constrictor is a concentric muscle around the border of the pupil which gets parasympathetic innervation. The parasympathetic innervation causes the muscle to contract and constrict the pupil
Describe the process of the eyes converging
When looking at a close up object we use medical recited muscles of both eyes to converge. Converging is done more often than distance so the medial rectus muscles are much thicker then the laterals
What is myopia?
Short-sightedness. Bending power of the eye is overused causing close objects to be seen clearly but far away objects to be out of focus
What is hyperopia?
Long-sightedness
What is astigmatism?
Non-Spherical curvature of the cornea or lens
What is presbyopia?
Long-sightedness of old age
What is emmetropia?
Perfect vision
What is the most common cause of myopia?
Eyeball being too long
Where do images form in relation to the retina in myopia?
In front of the retina
Symptoms for myopia?
Headaches, inability to focus at distance, divergent squint in infants and preverbal children
Correctional action for myopia?
Biconcave lenses - spectacles, contact lenses, laser eye surgery
Causes of hyperopia?
Eyeball too short or cornea and lens too flat
Where does an image form in relation to the retina in hyperopia?
Behind the retina therefore accommodation has to be used to see distant objects which should not need any increased bending power
Hyperopia symptoms?
Eyestrain after working on computer, convergent squint in infants/children
What can happen in children if hyperopia is not corrected?
Some vision in one or both eyes may be lost and the child may develop ‘lazy eye’
Correctional action for hyperopia?
Biconvex glasses, contact lenses, laser eye surgery
Correctional action for astigmatism?
Cylindrical glasses, toric contact lenses, laser eye surgery
Aetiology of presbyopia?
Loss of mobility and elasticity in the lens with age, less malleability in the lens when the ciliary body contracts, seeing near objects becomes difficult
Correctional action for presbyopia?
Biconvex reading glasses
What is phototransduction?
The stimulation of photoreceptor cells on the retina by the energy in light waves - the conversion of light energy to an electrochemical response by the photoreceptors (rods and cones)
Describe rods and cones
Rods and cones (the names reflect their respective shapes) contain light sensitive pigments. Each photoreceptor consists of an outer segment which contains hundreds of thin plates of membrane (lamellae). The outer segment is connected by a cilium to an inner segment which contains a nucleus. Rods are about 500 times more sensitive to light than cones, but cones give us colour vision.
Which photoreceptors give us colour vision?
Cones
Which photoreceptors are more sensitive to light?
Rods (~500times)
Where in photoreceptors does pigment reside?
Within the cell membrane of the lamellae of the photoreceptor
What pigment is found in rods?
Rhodopsin
What pigments are found in cones?
Cone opsins S, M and L
What makes up the pigment rhodopsin?
Opsin + 11-cis Retinal
How does bleaching occur?
When light falls on rhodopsin, the 11-cis retinal isomerises to all-trans retinal. All-trans retinal cannot fit into the opsin and so rhodopsin splits. This results in the bleaching of the colour purple.
How does bleaching of the visual pigment result in phototransduction?
The phototransduction cascade
What role does vitamin A play in the visual pigment?
Key for visual pigment regeneration
Give some conditions as a result of which Vitamin A deficiency is common
Malnutrition, malabsorption - coeliac disease, sprue
What is the effect on the eyes of Vitamin A deficiency?
Any condition that affects Vit A absorption will affect vision and cause night blindness. Vit A is also essential for a healthy epithelium so the conjunctiva and corneal epithelium will also become abnormal
What are some signs of Vitamin A deficiency seen in the eye?
Bitot’s spots in the conjunctiva, corneal ulceration, corneal melting which leads to further opacification of the cornea
