Vision 1 Flashcards
What is refraction?
Bending of light when it passes from one optical medium to another
Light rays bending to form a sharp image on the retina
What are refractive errors?
Mismatch between how much we bend light rays
What parts of the eye does light pass through and therefore contribute to refraction
Cornea
Aqueous humor
Lens
Vitreous humor
All these transparent to allow light to fall on retina
How does a share image form on the retina?
Light waves from an object bend at the cornea, bend some more at the lens to form a clear image on the retina.
This bending of light waves is called Refraction.
When an object comes closer, the eye needs more bending power to focus on an object (the lens becomes thicker and hence more powerful, and a clear image is formed on the retina again)
What is better at refraction cornea or lens?
Cornea is the most powerful “bender” of light (45D) but lens (15D) has the capacity to change its “bending power”
Explain accommodation
Our eyes have the capacity to change the focus from distant objects (infinity) to close objects (20cm)
The changes occurring in both eyes as it changes focus from a distant to a close object is called accommodation.
What 3 things happen simultaneously and comprise accommodation?
- LENS CHANGES SHAPE (becomes thicker & more spherical)
- Pupil constricts
- Eyes converge
Describe the process of lens thickening
- Ciliary muscle contracts making the ciliary body bulge (parasympathetic).
- Space in the middle decreases
- Suspensory ligaments become lax
- Lens is no longer under stretch
- LENS BECOMES THICKER
Describe the process of pupil constriction
- When we are looking at an object up close, we need a sharp focus.
- To sharpen focus the pupil constricts to allow only a few rays (those from the object) to pass through.
- Pupillary constrictor (sphincter pupillae) is a concentric muscle around the border of the pupil which gets parasympathetic innervation IIIn.
Describe the process of eye convergence
When focusing on an object up close, our eyes have to turn in to look at the object (convergence)
We use our medial rectus muscles of both eyes to converge (IIIn)
Humans spend a lot of time doing “close” work, so our Medial recti muscles are thicker than our lateral recti muscles.
What are the 4 refractive errors?
- Myopia: short sightedness
- Hyperopia: long sightedness
- Astigmatism: non-spherical curvature of cornea (or lens)
- Presbyopia: long-sightedness of old age
If you are lucky enough to have perfect vision you are an emmetrope (i.e. you have emmetropia)
What is myopia?
Shortsightedness: Close objects look clear, distant objects appear hazy.
Picture focuses in front of retina
Basically the bending power of cornea + lens are too much for that eyeball
When the object is brought closer the rays coming from it are divergent; and then this “bending power” comes to use. The image is formed on the retina without needing to increase curvature of lens.
What is the most common cause of Myopia?
Eyeball too long
So the cornea and lens, when the bend rays of light, make the image to form IN FRONT of the retina. So far off objects not seen clearly.
When the object is brought closer the rays coming from it are divergent; and then this “bending power” comes to use. The image is formed on the retina without needing to increase curvature of lens.
What are the symptoms of myopia?
Headaches, Complain of not being able to see blackboard/ distant objectsInfants & preverbal children divergent squint
Infants and preverbal children -> divergent squint
Toddlers -> loss of interest in sports/people. More interest in books, pictures.
Teachers may notice child losing interest in class
How to correct myopia
Bending power needs to be decreased.
Biconcave lenses
- Spectacles
- Contact lenses
- Laser eye surgery
What is hyperopia?
Farsightedness
Image focuses behind the retina
Close objects look hazy, distant objects appear clear.
What causes hyperopia?
Eyeball too short or cornea + lens too flat
So when an image of a distant object is formed on the retina it lies BEHIND THE RETINA.
The person then automatically starts to use his accommodative power and makes the lens thicker.
This causes the image to form on the retina.
So he is using the lens power to see far off thing (that he should normally be seeing without using any power).
When seeing closer objects, he uses more and more power until ultimately, his power is all used up.
What are the symptoms of hyperopia?
Eyestrain after reading/ working on the computer in a young individual.
Convergent squint in children/ toddlers - needs immediate correction with glasses/lenses to preserve vision in both eyes and prevent lazy eye
What is the correction of hyperopia?
Biconvex glasses alleviates use of glasses for focusing distant objects and “restS” the accommodative power
Contact lenses
Laser eye surgery
What is astigmatism?
Both close and distant objects appear hazy
Multiple images are formed
What causes astigmatism?
Surface of the lens has different curvatures in different meridians
So the bending of light rays along one axis will never be the same as that of the other axis.
So image formed is always hazy, whatever the distance of the object
How do you correct astigmatism?
Special glasses - called cylindrical glasses (which are curved in only one axis)
Laser eye surgery can also be used to correct the defect.
Need special contact lenses called topic lenses
Explain presbyopia
Longsightedness of old age.
With age the lens gets less mobile/elastic.
So when the ciliary muscle contracts, it is not as capable as before to change shape.
So seeing near objects/ reading the newspaper starts to become difficult -> needs glasses to read
Usually starts in 50s
How do you correct presbyopia?
Biconvex “reading glasses”
What is the process of us actually seeing called?
Phototransduction
What is phototransduction?
Visible light is an electromagnetic wave.
The energy in light waves need to stimulate photoreceptor cells on the retina (phototransduction)
Phototransduction is defined as the conversion of light energy to an electrochemical response by the photoreceptors (rods and cones)
The phototransduced rods and cones need to activate optic nerve neurones (generate an action potential)
Explain the structure of rods and cones
Outer segment contains stacks of discs/laminae which have photoreceptors
- Cones this is like an ice-cream cone shape
- Rods this is rod shaped
Inner segment just behind with cell organelles
Outer limiting membrane divides cell. Just inside this is nucleus connected to axon and synaptic terminal.
-This area is the muller cell
Explain the visual pigment in photoreceptor outer segments
Each lamellae is made up of cell membrane.
Integrated into this cell membrane is the visual pigment rhodopsin in rods and small, medium and large wave cone-opsins .
These molecules differ in their spectral sensitivity
What are the different visual pigments and what do they correspond to on the spectrum?
Small wave cone-opsin = blue cone
Rodopsin
Medium wave cone-opsin = green cone
Large wave cone-opsin = red cone
Explain the visual pigment Rhodopsin
Opsin + 11-cis Retinal
Integral transmembrane helical protein in the lamellae.
Chromophore nests in the opsin of this protein (11-cis retinal).
When light falls on 11-cis retinal, it isomerism to all- trans retinal
All-trans retinal cannot fit into the opsin. So rhodopsin splits.
this results in BLEACHING of the visual pigment
What is 11-cis retinal formed from?
Dietary Vitamin A
Involved in visual pigment regeneration
“Carrots help see in the dark”
How does bleaching of the visual pigment of cones and rods result in phototransduction?
Phototransduction cascade
Why are photoreceptor cells unique in terms of charge?
Unlike other cells in the body, the photoreceptor cells are, at rest (in the dark), kept in a depolarised state by open Na+/Ca+ channels
Explain the phototransduction cascade
Na channels open in cell resting state.
Photon of light activates Rhodopsin.
This causes the phototransduction cascade.
Na CHANNEL CLOSES.
Relative hyper polarisation of photoreceptor cell.
Hyperpolarisation transmitted by a flux of Ca ions to the synapse with bipolar cell.
Stimulates ultimately the retinal cell
Explain briefly visual pigment regeneration
Inside photoreceptor:
-Rhodopsin activated (breaks from opsin, All-trans retinol formed)
Inside pigment epithelial cell
- All-trans retinol converted to 11-cis retinol then 11-cis retinal
- Retinyl esters are given off
- Dietary vitamin A required
11-cis retinal combines with opsin to form rhodopsin again in photoreceptor
Describe vitamin A deficiency
since vitamin A is supplied through the diet, any condition that affects vitamin a absorption will affect vision-(night)blindness.
Vitamin A is also essential for healthy epithelium.
So conjunctiva and corneal epithelium are also abnormal.
Vitamin A deficiency can occur in conditions such as malnutrition, malabsorption syndromes such as coeliac disease, sprue
What is a clinical sign of vitamin A deficiency?
Bitot’s spots in conjunctiva
Corneal melting which leads to future opacification of the cornea
