How we see Flashcards
what wavelengths of light will our retina respond to?
Our retina will only respond to images of light rays between 400-800 nanometres
How many layers does the retina have?
10
Where do the rods/cones sit within the retina layers?
they sit pretty deep in the outermost layer called the outer segment
light has to travel through all the layers to hit them
what contains rods/cones
discs/lamellae contain photoreceptors (rods and cones)
what is each lamellae made up of?
a cell membrane
integrated into this membrane is the visual pigment (proteins) rhodopsin in rods and cone opsins
how many types of cones are there? and what are they called?
3
S, M and L
Short, medium (green) and long (red) wavelength types
What is the only difference between rods and cones?
they respond to different wavelengths of light
What is chromophore formed from?
dietary vitamin A
What happens when light hits rod?
11-cis retinal (vit A compound) changes from cis to trans - alters shape
what is rhodopsin made up of? (2)
opsin and 11-cis retinal (helical protein)
when vit A changes shape what happens to rhodopsin?
rhodopsin splits and bleaches as not all trans retinal can fit in the opsin
Bleaching of the visual pigment results in what?
phototransduction cascade
What role does Vitamin A play in the visual pigment?
visual pigment regeneration
To correct the cascade after 1 photon of light we need energy (ATP) – channel opens again and ready for business again. Need to get Vit A to convert back to normal shape and latch onto rhodopsin so that rhodopsin is ready for next photon of light
Describe the phototransduction cascade
When light hits vit A – changes from cis to trans
Vit A changing shape results in a cascade of reactions - these spread through until they meet an optic nerve cell
ultimately causes hyperpolarisation (Na+ channels close) and AP produced in optic nerve cell
Then pigment epithelial cells convert trans –retinol (vit A) back to cis-retinol this is then transported back to rod/cone to reform opsin
what is different about photoreceptor cells in comparison to other cells in the body
photoreceptor cells are, at rest (in the dark), kept in a depolarised state by open Na+/Ca+ channels (not closed like normal)
What are pigment epithelial cells and what is their function?
Cells in the deepest part of the retinal layer
All-trans retinol taken up here and converted back to cis retinol which can then go back to make rhodopsin
Want to absorb it so that it’s not bouncing back and making multiple images on the retina
What happens to vitamin A along the regeneration pathway
At certain points along the way – lose a bit of Vit A which can’t be reincorporated into the regeneration process so we need to keep replenishing it through our diet
E.g Fish oil, carrots
Vitamin A deficiency
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.
can occur in conditions such as malnutrition, malabsorption syndromes such as coeliac disease, sprue (immune (allergic) reaction to gluten).
Clinical signs of vit A deficiency
Bitot’s spots in conjunctiva - silvery triangles
corneal ulceration
More serious is corneal melting which can progress to opacification
What is refraction of light?
bending of light when it passes from one optical medium to another ie air to liquid
When light comes into your eye – in our eye there is transparent media – this helps to bend the light and the image you see forms on the retina
What are the transparent parts of the eye?
cornea
Aqueous Humor
Lens
Vitreous Humor
which parts don’t help bend light in the eye
Aqueous humor
Vitreous humor
function of the choroid
provides retina with nutrition
Why does light need to fall in the centre of the eye to work?
Light needs to fall in the centre in order to hit the cornea then it will pass through the AH, lens and VH which are all transparent – light will travel and bend
What happens with your eyes when an object is close up
only divergent (less parallel) rays form on cornea so the eye has to work much harder to bend the light rays in order to form image on retina – need to make lens more biconvex (by using ciliaris muscle) so you can bend light more and image still forms on retina
What happens with your eyes when an object is far away
light rays are more parallel so the eye has to work a little (but not as much as if up close) for image to be formed
which part of the eye can change its power?
the lens - you can make it more or less powerful through movement of the ciliaris muscle
Which is the most powerful ‘bender’ of light?
the cornea however its power can’t be altered, it is static
Which 2 parts of the eye do help bend light
cornea
lens
What is accommodation
The process of increasing the power of eye (lens) in order to see/focus better from distant objects to close objects
What are the 3 things that happen simultaneously during accommodation?
Lens changes shape (becomes thicker & more spherical)
Pupil constricts
Eyes converge
How does the lens thicken?
Ciliary body contraction (parasympathetic - by oculomotor nerve) and suspensory ligament relax - lens is no longer under stretch - causes lens to become thicker & more spherical
thicker lens = more powerful and better focus up close
How do pupils constrict during accommodation
To sharpen focus the pupil constricts to allow only a few rays (those from the object) to pass through so you can focus up close
Pupillary constrictor (sphincter pupillae) is a concentric muscle around the border of the pupil which gets parasympathetic innervation.
Why do your eyes converge during accommodation
When focussing 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. (Oculomotor of both sides)
Name some refractive errors (4)
Myopia – short-sightedness
Hyperopia – long-sightedness
Astigmatism – non-spherical curvature of cornea (or lens)
Presbyopia – long-sightedness of old age
what is perfect vision called?
emmetropia
What is myopia?
Short sightedness
Usually because the eyeball is too long so image forms in front of the retina
Far away – hazy image because it doesn’t reach the retina and forms before it
Close up – eye is more powerful – no accommodative power needed
Symptoms of myopia
Headaches, Complain of not being able to see board/ distant objects.
Infants & preverbal children - divergent squint
Toddlers - loss of interest in sports/people. More interest in books, pictures (can see well close up)
Teachers may notice child losing interest in class.
Myopia correction
Glasses - diverge light rays so image forms on retina - irritating as smaller field of vision
Contacts
Laser eye surgery
What is Hyperopia
far sightedness
Eyeball is too short or cornea and lens are too flat
Image forms behind the retina - close up looks hazy, far away objects appear clear
person starts to use accommodative power naturally to see far off things that they should normally see without this
for close up they use more and more power until ultimately, their power is all used up so they can’t see close up
Hyperopia symptoms
Symptoms of 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 a “lazy eye”.
Correction of hyperopia
Biconvex glasses alleviates use of glasses for focusing distant objects and ‘rests’ the accomodative power
Contact lenses
Laser Eye surgery
What is Astigmatism?
unequal curvature of the eye
close and distant objects appear hazy - blurry vision due to having more than one image forming and can form in front or behind the retina - any combination
How do we correct astigmatism
need special glasses – called cylindrical glasses (which are curved in only one direction).
Laser eye surgery can also be used to correct the defect.
Need special contact lenses called toric lenses.
What is presbyopia
long-sightedness of old age (blurry up close)
Lens becomes more rigid with age (starts 40-49) – even if muscles contracting lens isn’t moving much
corrected by reading glasses
