Eye Flashcards
what is the difference between the anterior and posterior segment
anterior:
cornea (light hits eye most refraction occurs here)
aqueous humour (water and nutrients)
lens (also refracts light, but less than cornea- water to lens has a smaller change)
FOCUSING
posterior:
retina
SENSING part
what is the refractive index
measure of bending of ray of light from one medium to another
index of air is lower than cornea index
change is greatest here, so more
describe the cornea
what it does
layout (layers)
major light focusing element of the eye
40D of fixed power
tear film
epithelium- constantly being replenished (damaged day to day)
stroma (90% thickness of cornea- gives structure and keeps it transparent)
endothelium (keep it transparent but DON’T REPLENISH) as you age the numbers reducde
discuss the refraction of the eye
CORNEA
-largest element (40D)
-interfaces with air (low RI0- big difference
LENS
-lesser element (20D)
-interfaces with aqueous- small difference
-vary in power
WHOLE EYE BALL
-about 60D
what is accommodation
what happens to ciliary muscles and lens during far and near accommodation
the act of focusing from distant to near and vice versa
DISTANCE
ciliary muscles are relaxed
zonular fibers under tension
lens flattens
CLOSE
ciliary muscles contract
zonular fibers relaxed
lens rounds
what does it mean to have dual innervation of the iris
sympathetic- dilation
parasympathetic- constriction
what is the near triad
- miosis
- convergence
- accommodation
what happens if you lose parasympathetic supply to the eye
3rd nerve palsy
-causes a big pupil
aneurysm in the posterior circulation of the brain
(eg. pca)
what is presbyopia
it is a refractive error
occurs in older people when they need glasses for reading
AGE RELATED
lose ability to focus on close
as lens gets thicker and less plasticity
muscles not working as well
what is myopia vs hypermetropia
MYOPIA
SHORT SIGHTED
eye too powerful (rays focused on front of retina)
HYPERMETROPIA
LONG SIGHTED
eyes not powerful enough (rays focused behind the retina)
what are some risks with myopia
-OPEN ANGLE GLAUCOMA
-RETINAL DETACHMENT
what are some risks with hypermetropia
-ANGLE CLOSURE GLAUCOMA
-ISCHAEMIC OPTIC NEUROPATHY
also associated with squint and lazy eye
what is the main cause of visual impairment world wide
UNCORRECTED REFRACTIVE ERROR (cataract is second type)
-errors that can be correct by glasses/spectacles but are not
PRESBYOPIA- main type
what is the commonest cause of blindness worldwide
CATARACT
what is the visual acuity assessment
standard way to measure someone’s vision
6/6
patient/normal
for example
20/80
patient sees it at 20 feet but a person with normal vision could see that at 80 metres
what is the standard distance for ‘big’ Snellen
6 metres/ 20 feet
what is the reduced visual acuity chart
use the chart at 3 metres
top line is still called the 60 line
what are the steps to testing distance visual acuity
- measure out 3m
- cover left eye
- ask patient to read from top of chart
- chart perpendicular, good lighting, smoothed out and flat
- record number of smallest line
- if can’t read top line go to 1.5m
- if cant read go to 0.5m
- if cant read try counting fingers at 1m, hand movements then perception of light and classify with projection or with no projection of perception light then finally no perception of light
repeat for other eye
repeat with both eyes together
repeat with pinhole and with glasses
what is driving vision (cars- not HVG)
6/12
What is blindness 3
3/60
what is blindness 4
1/60
what is blindness 5
no light perception
what is moderate visual impairment
6/18
what is severe visual impairment
6/60
what is TRACHOMA
infectious disorder caused by Chlamydia
transmitted by flies
causes lashes to go wrong way and abrasion of eye- blindness
cornea scarring
preventable ad treatment
eradicate fly
antibiotics
surgery
describe the retinal structure
outer nuclear layer
(with rods and cones)- photoreceptors AT BACK produce signal to back of brain
plexiform layers
ganglion cell layers
outer nuclear layer
info from rods and cones is converged to ganglion cells, lateral cells and amacrine act to provide initial processing of visual signal
describe difference between rods and coes
rods
far more - 120 million
HIGH CONVERGENCE to ganglion cells
VISION IN GREYSCALE- night vision
very light sensitive
widespread distribution in retina
broad spectral sensitivity
cones
6 million
LOW CONVERGENCE to ganglion cells
BLUE GREEN RED
1/30th sensitivity of rods
concentrated in macula
narrow spectral sensitivity
photoreceptors are depolarised in the dark
hyperpolarised in the light
what is the most sensitive part of the eye in terms of visual acuity
FOVEA- high cones
sensitivity decreases as move away from centre
what is found where the blind spot is
where optic nerve is
discuss role of opsin
chromophore RETINAL (derived from vitamin A) is found in all rods and cones
changes shape when light hits it
bound to protein (opsin)
cascade
PHOTOTRANSDUCTION
there are different opsin types (each specific to a different type of cone)
- 3 colours with rod retinal binding an opsin– RHODOPSIN
when light hits retinal it changes conformation- appears bleached
what does vitamin a deficiency cause
night blindness
then total corneal blindness
then death
is retina/ photoreceptors active during sleep
metabolically more active when you are asleep in DARK
keep eye in hyperpolarised state which requires energy
therefore can exploit as therapy for retinal disease
what adaptations do photoreceptors have
detect levels of light
-vision can be created in bright sunlight and star lit sky
adapt to different ambience light
dependent upon changes in Ca++ and cGMP levels within the cell altering the sensitivity of membrane channels
what is cone fatigue
when you stare at a specific colour for too long, the cells that detect that frequency of light get fatigued
after image is a result of photoreceptors not bein gin balance
when they become less tired the balance is recovered and the after image disappears
what happens due to age related macular degeneration
age related macular degeneration
- eye does not readjust well to ambience light
-retinal cell connectivity
retinal cell connectivity to see objects and moving objects
how photoreceptors connect to ganglion cells by amacrine cells and bipolar cells
what are bipolar cells
on cells- when light is shone they turn on
– NT release in the light- switch light on
off cells– opposite
allows when something is moving u can see shadow and edge (via on/off bipolar cells)
what do horizontal and amacrine cells do
connect bipolar cells and allow for summation of info to allow detection of edges and contrast
RECEPTIVE FIELDS– convergence of info from FR into bipolar cells than a ganglion cell
what conditions cause problems to the retina
age related macular degeneration
diabetes
what is the optic nerve
collection of all the ganglion cells
exit back of the eye through a hole in the sclera
front focus light into retina, retina detects and changes it into electrical signal – brain
seen at the back of the eye (optic disc)
does processing happen at the retina
yes
convergence- 100 million photoreceptors but only 1 million ganglion cells
ganglion cell transfers retinal info to the brain, optic nerve-chiasm-optic tract
first synapse is at the lateral geniculate nucleus
part of thalamus
major relay station for sensory info
