Vision 1 Flashcards
What is meant by accomodation?
How much we bend light rays
When is an object considered to have parallel rays of light?
Anything beyond 6 metres
What is meant by refraction?
Bending of light when it passes from one optical medium to another
What are the transparent parts of th eye?
Cornea, lens, aqueous humour and vitreous humour
What is the power of the cornea and the lens?
Cornea - 45 D
Lens - 15 D
What three things comprise accomodation?
Lens shape - becomes thicker and more spherical
Pupil constricts
Eyes converge
What is responsible for the lens becoming thicker?
Ciliary muscle contracts making the ciliary body bulge. Ciliary body contraction is under parasympathetic control.
The lens is normally under stretch
Ciliaris muscle is under control by the third cranial nerve
Which muscle is repsonsible for the constriction of the pupil when looking close up at an object? What is the innervation
Constrictor pupillae
It is the concentric muscle around the border of the pupil. Gets parasympathetic innervation from cranial nerve 3
Why are our medial rectus muscles thicker than our lateral rectus muscles?
Because humans spend a lot of time doing close work and it is our medial retus muscles that are responsible for convergeance
What is the definition of the following?
- Myopia
- Hyperopia
- Astigmatism
- Presbyopia
- Emmetropia
- Myopia – short-sightedness
- Hyperopia – long-sightedness
- Astigmatism – non-spherical curvature of cornea (or lens)
- Presbyopia – long-sightedness of old age
- Emmetropia - perfect vision
What is the cause of myopia?
Most common cause is that the eyeball is too long
What are symptoms of myopia?
Headaches
‘Can’t see blackboard/distant objects’
Infants and preverbal children have a divergent squint
Toddlers have a loss of interest in sports and people - more intersted in books and pictures
Child loses interest in class
What is the treatment for short sightedness?
Spectacles
Contact lenses
Laser eye surgery
What is the cause of hyperopia?
Eyeball is too short or cornea and lens is too flat
Where is the image formed in hyperopia?
Image is formed behind the retina
Why can’t people who are long sighted see objects close up?
They are aalready suing a large portion of their accomodative power at a relatively long distance
Their accomodative power eventually expires at a longer distance than normal
What are symptoms of long sightedness?
Eyestrain after reading/working on the computer in a young individual.
If a toddler has a convergent squint, it needs immediate correction with glasses/lenses to preserve vision in both eyes and prevent a lazy eye
What type of lenses are indicated for people with long sight?
Biconvex
What is an astigmatism?
Surface of the eye has different curvatures in different meridians - bending of light rays on one axis will never be the same as that of the other axis
What type of lens is needed for an astigmatism?
Cylindrical glasses - which are curved only on one axis
What is treatment besides glasses for an astigmatism?
Laser eye surgery
Toric lenses
What is presbyopia, and why does it occur?
Longsightedness of old age
Happens as a result of the lens becoming less mobile and elastic, when the ciliary muscle contracts it is less able to change its shape
When does presbyopia occur?
5th Decade of life
What is corection of presbyopia?
Using boconvex reading glasses
What do the following columns indicate?
RE - means right eye
LE - left eye
BC - Base curve - the curve of your contact lens, measured in millimetres
DIA - diameter, so refers to the size of the lens
SPH - the lens power you need to achieve emmetropia
What is meant by phototransduction?
The conversion of light energy to an electrochemical response by the photoreceptors (rods and cones)
Where does phototransduction occur in the rods and cones?
The outer segments of the rods and cones (effectively their dendrites)
What is the structure of the rods and cones?
Inner segment contains organelles
Outer segment contains stacks of disks / lamellae which have photoreceptors
What is the visual pigment embedded in the cell membrane of the lamellae of rods?
Rhodopsin
What is the visual pigment embedded in the cell membrane of the lamellae of cones?
Cone opsins S,M and L
These molecules differ in their spectral sensitivity
Cones and rods respond to different wavelengths of light
Define chomophore
An atom or group whose presence is responsible for the colour of a compound.
What are the two main compnents of rhodopsin?
Opsin and 11-cis retinal
What happens when light falls on 11-cis retinal?
It isomerises to all-trans retinal
What happens to rhodopsin after the formation of all-trans retinal?
All- trans retinal cannot fit into the opsin. So rhodopsin splits. This results in BLEACHING of the visual purple chromophore nesting in the opsin. This molecule formed from dietary Vitamin-A.
What role does vitamin A play in the visual pigment?
Visual pigment regeneration
When the photoreceptor cells are at rest (in the dark), what state are they in?
Depolarized state - by open sodium and calcium channels
What is vitamin A used to form?
11 cis Rol from all trans retinol (all trans retinal) which is used to regenerate 11 cis retinal
How does phototransduction stimulate the retinal cell?
Rhodopsin is activated
Results in a sodium channel closing
Hyperpolarization of photoreceptor cell
This is transmitted by a flux of calcium ions to the synapse with bipolar cell
Which are more numerous rods or cones?
Rods
What is the effect of vitamin A deficiency?
Night blindness
Vitamin A is essential for a healthy epithelium, so conjunctivae and corneal epithelium are abnormal
When does vitamin A deficiency occur?
Conditions such as malnutrition
Malabsorption syndromes such as coeliac disease and sprue
What is often the first indication of vitamin A deficiency?
Bitot’s spots - present in conjunctivae
Corneal ulceration
Corneal melting which leads to future opacification of the cornea
