Define refraction accomadation and refractive errors?
Refraction - Bending of light rays as they move from one medium to another
Accomadation - Altering lens shape, pupil size, eye convergance to alter how much we refract the light
Refractive Errors - Mismatches in light ray refraction
What kind of lens causes conversion and what kind causes divergence?
A biconvex lens converges light rays
A biconcave lens diverges light rays
What refractive media are in the eye?
- Aqueous Humor
- Vitreous Humor
The cornea is the biggest bender but the lens is capable of varying its refractive power (accomadation)
At what distance will all light rays reaching the eyes be parallel?
6m and beyond
How does lens thickness change as the object moves closer to the eye?
It gets thicker because:
- The close the object the more divergent the rays reaching the eyes
- So the greater the refractive power required
How does the lens thicken?
- > Parasympathetic stimulus
- > Ciliary muscle contracts
- > Makes ciliary muscle bulge
- > Causes suspensory ligaments to go lax
- > Lens stretch decreases and lens gets thicker
How does pupil constriction affect focus?
The closer the object the more the pupil constricts to minimize the rays getting through
How does the pupil constrict?
Cranial Nerve III (Oculomotor)
- > Parasympathetic fibres
- > Sphincter Pupillae Muscle Contracts
What causes our eyes to converge to focus on closer objects?
Cranial Nerve 3 (Oculomotor)
-> Stimulates Both Medial Recti
Because we do so much close work our medial recti have become thicker than our lateral recti
What do we call the condition of having perfect eyesite?
Emmetropia i.e. you are an emmetrope
List some common refractive errors?
Myopia - Short Sightedness
Hyperopia - Long Sightedness
Astygmatism - Non-spherical cornea/lens
Presbyopia - Age related long-sightedness
What causes myopia?
The persons eyeball is slightly longer than normal
Causes the refractive power of the eye to be excessive and the rays converge before hitting the retina
How does myopia affect close vision?
It doesn’t because close up the excess refractive power of the eye does the job without needing to use your lens
How does myopia affect distance vision?
Further away objects are seen with less divergent rays so the over-refractive nature of the myopic eyeball means you cant focus the image on the retina even with the ciliary muscle fully relaxed
How would you detect a myopia in kids?
- Infants may show a divergent squint
- Toddlers may lose interest in sports/people and instead focus on books/pictures
- Difficulty seeing or losing interest in class
How do you treat myopia?
Need to reduce refractive power:
- Biconcave lenses or glasses
What causes hyperopia?
Either the eyeball is too short or the cornea/lens is too flat
So the bending power of the eye is less than normal and the light rays converge behind the retina
How does hyperopia affect closeup/distance sight?
The person has to use more accomadative power (a more convex lens) than usual to see distant objects.’
By the time an obect gets close the person doesnt have enough accomadative power to focus the rays so close up objects are blurry
How would Hyperopia show up in a kid?
Symptoms of eyestrain after reading/working close up
How would we treat hyperopia?
- Biconvex Lens/glasses
- Laser Eye Surgery
Define an Astigmatism?
Non-spherical Curvature of the cornea or lens.
I.e. the eye is more like a rugby ball than a football
How does astigmatism affect sight?
Because the cornea/lens is no longer spherical, the bending of light will never line up in one axis with another, therefore all objects will appear hazy
How would we treat Astigmatism?
- Toric Lenses
- Laser Eye Surgery
- Cylindrical Glasses (curved in only 1 axis)
Age related long-sightedness
With age the lens gets less elastic/mobile so the ciliary muscle isnt capable of thickening it as much as before.
Therefore you cant focus close up
How do we treat presbyopia?
Biconvex “reading Glasses”
Most people in their 5th decade on have them.
The conversion of light energy to electrochemical signals
Explain the process of phototransduction?
Opsins (in cones) and rhodopsin (in rods) contain chromophore 11-cis retinal.
When struck by a photon 11-cis retinal isomerizes to all-trans retinal.
All-trans retinal leaves the opsin triggering a signal transduction cascade that leads to closure of na/ca channels.
Leads to hyperpolarisation of the photoreceptor cell which triggers an action potential
Explain the bleaching effect of phototransduction?
The visual pigment 11-cis retinal provides a purple colour to the retina.
When its all been struck by photons and converted/lost it leaves the retina temporarily bleached
How is the visual pigment regenerated?
All-Trans Retinal is recycled into 11-cis retinal.
However some is lost and some forms retinyl esters, this is replaced using dietary vit A
How would we detect vitamin A deficiency in the eye?
- Night Blindness
- Abnormal conjunctiva/corneal epithelium (Vit A important to epithelial health)
- Bitot’s spot (White/silvery) in conjunctiva
- Corneal Ulcer