Vision 1 Flashcards
Refraction in the eye
Light rays bend to form a sharp image on the retina
Accommodation
We can focus on far off or near objects by changing how much we bend the light rays
Refractive errors
Sometimes there is a mismatch between how much we bend light rays
Refraction
Bending of light when it passes from one optical medium to another
What is transparent to allow light to fall on the retina?
- Cornea
- Aqueous humor
- Lens
- Vitreous humor
What are the 2 main areas of bending of light?
The cornea is the most powerful bender of light (45D) but the lens (15D) has the capacity to change its bending power
What happens to the lens as an object comes closer?
The eye needs more bending power to focus on the object, therefore the lens becomes thicker and hence more powerful.
What accommodative power does our eyes have?
To change focus from distant objects (infinity) to close objects (20cm)
What are the 3 components of accommodation?
- Lens changes shape
- Pupil constricts
- Eyes converge
What shape change does the lens undergo during accommodation?
Becomes thicker and more spherical
Why does ciliary body contraction result in the ability to focus on close objects?
- Ciliary body contraction (parasympathetic) causes lens to become thicker and more spherical
- A thicker lens is more powerful: can focus close objects
How does the lens become thicker?
- Ciliary muscle contracts making the ciliary body bulge
- Space in the middle decreases
- Suspensory ligaments become lax
- Lens is no longer under stretch
- Lens becomes thicker
Why does the pupil constrict when focussing on a close object?
We require a sharp focus
How is a sharp focus created?
Pupillary constriction to allow only a few rays (those from the object) to pass through
What muscle is responsible for constriction of the pupil?
Pupillary constrictor (sphincter pupillae) is a concentric muscle around the border of the pupil which gets parasympathetic innervation
What muscles are responsible for convergence of our eyes?
We use our medial rectus of both eyes to converge
Why are our medial recti thicker than our lateral recti muscles?
Humans spend a lot of time doing close work
Convergence
Turning inwards of our eye to focus on a close up object
Myopia
Short sightedness
Hyperopia
Long sightedness
Astigmatism
Non-spherical curvature of cornea(or lens)
Presbyopia
Long sightedness of old age
Give 4 examples of refractive errors.
- Myopia
- Hyperopia
- Astigmatism
- Presbyopia
Emmetropia
Perfect vision
Describe myopic vision.
- Close objects look clear
- Distant objects appear hazy
- Image formed short of the retina
What is the most common cause of myopia?
Eyeball is too long
Why can people with myopia see close up objects clearly?
- The bending power of the cornea and lens is too much
- Therefore, when an object is brought closer the rays coming from it are divergent and then this bending power comes to use
What are the symptoms of myopia?
- Headaches
- Complain of not being able to see the board at school
- Divergent squint in infants and preverbal children
- Loss of interes in sports/ people with greater interest in books and pictures in toddlers
- Teachers may notice child losing interest in class
What is the treatment for myopia?
The bending power needs to be decreased by biconcave lenses
- Spectacles
- Contact lenses
- Laser eye surgery
Describe hyperopic vision.
- Close objects look hazy
- Distant objects appear clear
- Image formed behind retina
What are the main causes of hyperopia?
- Eyeball too short
- Lens too flat
Why are longsighted people unable to see close up objects?
- They automatically start to use up their accommodative power to see far away things.
- They use more and more of this power up as the object moves closer until there is no more accommodative power
- Therefore the find it difficult to see clos up objects
What are the symptoms of hyperopia?
- Symptoms of eyestrain after reading/working on computer in the young
- Convergent squint in children/toddlers
If a child has a convergent squint what must be done?
Immediate correction to preserve vision in both eyes and prevent a lazy eye
What is the treatment for hyperopia?
Biconvex glasses alleviates use of glasses for focussing distant objects and rests the power for accommodative power
- Contact lenses
- Laser eye surgery
Describe astigmatic vision.
Close and distant objects appear hazy
Why is the image formed always hazy in astigmatism?
- Surface has different curvatures in different medians
- So the bending of light rays along one axis will never be the same as that of the other axis
- Therefore image is always hazy
What is the treatment for astigmatism?
Cylindrical glasses
- Laser eye surgery
- Toric contact lenses
What causes presbyopia?
- With age the lens gets less mobile/elastic
- So when the ciliary muscle contracts it is not as capable as before to change shape
- Therefore it becomes difficult to see near objects
When does presbyopia usually occur?
Around about the 5th decade
How is presbyopia treated?
Biconvex glasses
What kind of wave is visible light?
Electromagnetic
What does the energy in light waves need to stinumlate?
Photoreceptor cells on the retina
Phototransduction
The conversion of light energy to an electrochemical response by the photoreceptors
What cells in the human eye are the photoreceptors
Rods and cones
What do the phototransduced rods and cones need to activate?
Optic nerve neurons to generate an action potential
Describe the structure of rod and cone cells.
- Synaptic terminal
- Axon
- Nucleus forming (outer nuclear layer of retina)
- Inner segment containing cell organelles
- Outer segement containg discs/ lamellae
What is each lamellae made up of?
Cell membrane
What is integrated into the cell membrane of the lamellae?
The visual pigment rhodopsin in rods and opsins S,M and L in cones
How do opsins and rhodopsins differ?
In their spectral sensitivity
What happens to 11-cis retinal when light falls on it?
Isomerises to all-trans retinal
The chromophore nesting in the posit is formed from what?
Dietary vitamin A
What is rhodopsin formed from?
- Opsin
- 11-cis retinal
How does bleaching of the visual purple occur?
-All trans retinal cannot fit into the opsin so rhodopsin splits resulting in bleaching
How does bleaching of the visual pigment result in photo transduction?
Phototransduction cascade
What role does vitamin A play in the visual pigment?
Visual pigment regeneration
How do the photoreceptors differ form other cells in the body?
When at rest (in the dark) they are kept in a depolarised state by open Na/Ca channels
If there is not a constant supply of vitamin A what will be former?
Retinyl esters
What will any condition affecting vitamin A absorption result in?
- Affect vision: night blindness
- Essential for healthy epithelium so can cause abnormalities in conjunctiva and corneal epithelium
What conditions can vitamin A deficiency occur in?
- Malnutrition
- Malabsorption syndromes such as coeliac disease, sprue
What is sometimes the first indication of vitamin A deficiency?
Bitot’s spots in the conjunctiva
