Vision Flashcards
What is refraction?
Light rays bend to form a sharp image on the retina
What is accommodation?
The changes occurring in both eyes as it changes focus from a distant object to a close object
When do refractive errors occur?
When there is a mismatch between how much we bend light rays
When does bending of light occur?
When light passes from one optical medium to another
How much of the bending of light depends on what? What is this called?
Density of the medium - the refractive medium
Two main benders of the eye
Cornea
Lens
What happens when an object comes closer?
The eye needs more bending power to focus on an object
The lens becomes thicker and hence more powerful
A clear image is formed on the retina again
What is the most powerful bender of light and its dioptres?
Cornea
45D
What bender has the capacity to change its bending power and what is its dioptres?
Lens
15D
What distance is considered as infinity and what does this mean for the light rays received in the eye?
> 6 metres
Only parallel rays will be received
3 things that happen to make up accommodation
- lens changes shape (becomes thicker and more spherical)
- Pupil constricts
- Eyes converge
How does the lens thicken in accommodation?
Ciliary body contraction (parasympathetic) causes lens to become thicker and more spherical
When contracts the space in the middle decreases and the suspensory ligaments become lax
When lens isn’t being pulled it flops down and therefore becomes more spherical
How does the pupil constrict in accommodation?
The pupil constricts to allow only a few rays (those from the object) to pass through
Pupillary constrictor muscle is the muscles around the border of the pupil which gets parasympathetic innervation
What muscle constricts the pupil?
Sphincter pupillae
What muscle causes both eyes to converge and what is this supplied by?
Medial rectus muscle
IIIn of both sides
Which of the lateral recti muscles and the medial recti muscles are thicker? Why?
Medial recti muscles
Humans spend a lot of time doing ‘close work’
Definition of myopia
Short sightedness
Definition of hyperopia
Long sightedness
What is astigmatism?
Non spherical curvature of cornea (or lens)
Definition of presbyopia
Long sightedness of old age
What is the name for perfect vision?
Emmetropia
What do you see in myopia?
Close objects look clear
Distant objects look hazy
Most common cause of myopia
Eyeball too long
Pathology of myopia
When cornea and lens bend ray of light, the image is formed in front of the retina
Therefore bending power of cornea and lens is too much for that eyeball
Symptoms of myopia
Headaches Cant see distant objects Children - lose interest in class Infants and preverbal children - divergent squint Toddlers - loss of interest in people, more interest in books / pictures
What type of lenses are used to correct myopia lenses?
Biconcave
Treatment of myopia
Biconcave spectacles
Lenses
Laser eye surgery
What is seen when have hyperopia?
Close objects look hazy
Distant objects appear clear
Cause of hyperopia
Eyeball too short
Cornea and lens too flat
Pathology of hyperopia
Image of the distant object is formed behind the retina
Then accommodative power is used to make the lens thicker which causes the image to form on the retina
When seeing closer objects, he uses more and more power until ultimately all his power is used up
Symptoms of hyperopia
Symptoms of eyestrain after reading/working
Convergent squint in children/toddlers
Treatment of hyperopia
Spectacles
Contact lenses
Layer eye surgery
In children/toddlers with divergent squint
- needs immediate correction with glasses to preserve vision in both eyes and prevent a lazy eye
What is seen in astigmatism?
Close and distant objects appear hazy
Pathology of astigmatism
Surface has different curvatures in different meridians
Bending of light rays along one axis will never be the same as the other axis
So the image formed is always hazy, whatever the distance of the object
Treatment of astigmatism
Cylindrical glasses
Laser eye surgery
Toric contact lenses
What type of lens is used to treat hyperopia?
Biconvex
What are cylindrical glasses?
Glasses which are only curved in one axis
Pathology of presbyopia
Lens gets less mobile / elastic with age
So when ciliary muscle contracts, it is not as capable as before to change shape
When does presbyopia usually start?
5th decade of life
Treatment of presbyopia
Biconvex “reading glasses”
What is visible light?
An electromagnetic wave
How does photo transduction work?
The energy in light waves need to stimulate photoreceptor cells on the retina
Definition of phototransduction
The conversion of light energy to an electrochemical response by photoreceptors (rods and cones)
What do phototransduced rods and cones need to activate?
Optic nerve neurones (generate an AP)
Each lamellae in rods and cones is made up from what?
Cell membrane
What is integrated into the cell membrane of lamellae in rods?
Visual pigment rhodopsin
What is integrated into the cell membrane of lamellae in cones?
Opsins S, M and L
What happens to visual pigement rhodopsin in rods?
Inside the opsin - Vit A compound
This compound absorbs photos to form 11-cis retinal which is quite bendy
It is very sensitive to light - changes chemical form to form an isomer called ALL TRANS RETINAL (same chemical composition but is straight)
How does bleaching of the visual pigment result in photo transduction?
Photo transduction cascade
What role does vitamin A play in the visual pigment?
Visual pigment regeneration
What can vit A deficiency result in?
Night blindness
Abnormal conjunctiva
Abnormal corneal epithelium
Causes of vit A deficiency
Malnutrition
Malabsorption syndromes e.g. coeliac
Sprue
Presentation of vit A deficiency (in the eyes)
Bitot’s spots
Corneal ulceration
Corneal melting - leading to future opacification of the cornea
