The eye Flashcards
(27 cards)
- Sclera
- cornea
- aqueous humour
- iris and pupil
- lens
- ciliary muscle
- vitreous humour
Sclera: white; around whole eye
Front: sclera gives way to cornea - At front: aqueous humour Behind this: iris (colour part) with a black hole (pupil) •Black because no light coming out of it
Behind iris and pupil: lens (transparent)
Proteinaous structure: Sits on ciliary muscle
Rest of eye filled with vitreous humour
Back of eye: how we actually see: retina
Behind retina: dark structure: choroid
(Dark in humans: any light that gets in and doesn’t get absorbed by retina is absorbed by choroid)
-Stops light from bouncing around
- In many animals, choroid reflective (nocturnal animals)
Light not absorbed by retina
o Seeing light (no matter where from) is better than not seeing light
The back of retina: fovea: Fine focus
what gives rise to nerves and nerve axons which gather to form optic nerve
retina
major and minor blood supply
major: optic nerve blood vessels
minor: sclera blood vessels
the cornea
function and irrregular
Nerve ending: trigeminal nerve
Very sensitive to touch
Light coming in have to be bent
2/3 of this process is done by cornea (lens only adjusts focus)
Front of cornea irregular: focusing messed up
The pupil
- pinhole effect
Radial and circular muscles of iris control size
Pinhole effect
- Because of pinhole, light from one part of object only reaches on part of the eye (focusing)
- Distance of object from pinhole doesn’t matter
Still get sharp image
Depth of focus
- Smaller pupil = better focusing on back of the eye
The iris
Haemoglobin in blood is very red contributes to eye colour
- Eye colour very difficult to predict (even if genetics known)
- Many complications in iris colour
- Motion and level of attention can change size of pupil
The lens
Adjusts focus
Distant: Thin because being stretch out by zonule fibres (pulling it)
Near vision:
- Circular ciliary muscles contract which causes lens to relax
Only activating focusing power when looking at distant things when lens get older, it gets rigid so can’t relax as easily so can’t see things in near vision (far-sightedness)
Accommodation based on distance
Distant: thin lens
Near: bending is done by making lens thicker (relaxed)
accommodation and vergence
As an object moves farther and closer, lens must change shape (accommodate) to maintain focus
o Ciliary muscles do this
Close object: eye converge (turn in)
o Degree of convergence used by brain to determine how far or clos object is
This is binocular depth cue
Cataract
- Accommodation is lost: lens has fixed depth focus after surgery because not reconnected to zonule fibres
Aqueous and vitreous humour
- Called humour: body fluid
- Vitreous tension: experience effects as age
Aqueous humour and glaucoma
- IOP
- Glaucoma
IOP: measured by pushing against front of eye (more ways to measure as well)
- Measured by: high pressure associated with glaucoma
- But there are a group of glaucoma patients that don’t have raised IOP
Glaucoma
- Visual world starts to “shrink”
- Most patients don’t realise anything is wrong until already lost a lot peripheral vision
If canals get locked, do surgery to make tiny hole to try and cause drainage (more procedures for glaucoma too)
The Retina
Choroid absorbs most of the light
o So ophthalmic scope has to be very bright so can get some light to retina during eye exams
Fovea is in centre of macula
o Light accurately focused
Because blood vessels and nerves in optic disk, there is no retina
o We don’t notice blind spot because two blind spots are different (one eye fills in for the other)
o Also, brain fills in the image (compensates for missing area by blind spot)
Light
EM can be seen over huge range (gamma to microwaves)
We see 400nm to 700nm
o Restrictive
o Nobody can tell what anyone else sees (in terms of colour)
Wavelength gets longer, energy becomes lower
o A lot of animals can see beyond 400nm
o Not many can see past 700nm (can’t see low energy light)
o Humans don’t see past 400nm because of lens, fovea, retina structure
Find it very difficult to see in extreme light and in extreme lack of light
o This only applies if don’t allow time for adaptation
o Allow time for adaptation, can see better in these conditions
Retinal cells
Choroid absorbs light not use
Rods and cones detect light
Rods and cones to horizontal cells
Bipolar cells synapse from rods and cones to ganglion
oMultiple bipolar on one ganglion
Ganglion has axons that form optic nerve
Light has to hit all these neurons before it is seen by rods and cones
Retinal regions
Fovea is depression in macula
Within fovea, smaller depression called foveola
Outside macula: periphery
o Many rods, few cones
o A lot of convergence
Fovea vs Foveola
Fovea: Mainly cones
Foveola:
- Only cones
- Very thin cones packed close together
- Camera with a lot of pixels (analogous to pixels in camera)
- Each cone in foveola have direct connection to brain
Rods and cones
-impossible to make rod more sensitive than it already it
Cones are less sensitive -> need more light t see colour
at night mainly use rods.
Rods
-Cilium is greatly enlarged to form rod structure
End of rod: disks
- Made of membrane
- First 10-20 disks formed by membrane invaginations
- Later, separate disks
- Light passes through cell body and into disk region to be absorbed by rhodopsin
Have a lot of disks so that is photon passes first without being absorbed, next disk will absorb it or next will, etc.
Cones
All of the disks are invaginations of outside membranes
In some animals (NOT HUMANS) find oil droplets full of brightly coloured pigments
- Birds and some amphibians or reptiles: have these to enhance colour vision
Rhodopsin turnover
Band of radioactivity appears at beginning of rod until it gets to choroid and is chewed up
Rhodopsin doesn’t move around in eye
If disks aren’t degraded, get build up of disks
Accumulation of disk
- Rod or cone degradation
- And slowly loss of vision
Rhodopsin structure
Two things make it up: opsin and retinal
- Retinal is the molecule that actually absorbs light
Retinal is constant, opsins are variable
Retinal surrounded by amino acids (opsins) which shifts vibration frequencies of the double bonds in retinal which gives different light absorption wavelength
We absorb 4 different colours
- Human females may see colours better because has two X chromosomes and some opsins are on X which increases ability to see colour (may be able to see more colours)
Butterflies: have 8-9 opsins (see a lot of colours)
Some animals have less opsins
Transduction in rods
Rhodopsin absorbs light, forms metarhodopsin to cGMP
Normally enough cGMP in resting rod for Na+ to enter but when light comes through closes sodium channels and cells hyperpolarise
- In Human rods, stimulus (light) causes hyperpolarising change
Ion currents in rods
Calcium concentration inside goes up when cell stimulate
Have Na+/Ca2+ exchanger which extrudes Ca2+ that comes in which Na+
When light falls, cell hyperpolarises and changes amount of calcium and this change causes adaptation
