Lecture 15- The Eye: How we see Flashcards
light must reach andf be focused onto the
macula
features of the eye that help us to see
- transparent
- regulate light entry
- refract to bring into focus
- shape of eye ball
transparency of the eye
nothing interferes with light getting through
- Tear film
- Cornea
- Anterior and posterior chamber fluid
- Vitreous jelly
Any pathology effecting the transparency of these will impede light
Regulate light entry and retract to bring into focus
- Light will only be able to pass through via the lens once it has entered the pupil–> iris will not allow light to pass through
- Strongest point of refraction- the cornea –> tear film air interface –> scattered eye waves will be bent
- Focused through the pupil
- Further refraction as it passes through the lens
Shape of eyeball
- Too long – short sighted- myopic
- Too short- long sighted- hypermetropic
myopic
short sited- eyeball too long
hypermetropic
too short- long sighted
Accommodation reflex aim
Focusing near objects requires greater refraction of light
outline the Accommodation reflex
light from near-objects more divergent- greater refraction required to focus onto retina, beyond capability of the cornea (which is in a fixed shape)
Eye accomodates
- pupils constrict (limits amount of light coming through
- eyes converge (to ensure image remains focused on same point of retina in both eyes)
- lens becomes more biconvex (fatter) by contract of ciliary muscles
presbyopia
harder to focus on close object
Lens tend to stiffen with age- less able to change shape presbyopia age related inability to focus near object
When the ciliary muscle is relaxed
*
- Ligaments pulled tight
- Pulls the lens thinner good for looking at distant objects
When ciliary muscle is constricted (parasympathetic fibres of oculomotor nerve)
- Tension removed from ciliary muscles
- Rounder, fat lens
- Good for looking at nearby objects
Age related macular degeneration
Thinning and atrophy of the macula affects central vision
- Visual loss
- Gradual
- Some types are more rapidly
- When light received by the retina (rods and cones) ligght waves are
are converted to action potentials and received by the ganglion cells which converge to form the optic nerve
- Rods
- Active at lower light levels, do not mediate colour vision
- Abundant in peripheral parts of retina
- Cones
- High definition
- Colour vision
- Active at high light levels
- Concentrated within macula of retina
- Fovea= only cones
When light hits the retina
- Photoreceptors convert light signals into Ap
- Interneurons communicated with ganglion cells
- Action potentials propagated via retinal ganglion cells
- RGC axons collect in area of optic disc–> form optic nerve
- Blind spot= no photoreceptors
- AP propagated along visual pathway to occipital lobe for interpretation
optic disc-
-> form optic nerve
Blind spot= no photoreceptors
decreased visial acuity… pathology affecting…
- transparency of structures anteiror to retina e..g opcity of lens such as a cataract
- refractive ability of strutures anteiror to retina e..g irregularity of corneal surface (astigmatism), ability of lens to change shape (presbyopia), shape of eyeball
- retina (inc macula) or optic nerve e.g. retinal detachment, age-related macular degeneration, optic neuritis
cataracts
cloudy lens- replaced with artificial lens
astigmatism
irregularity of corneal surface
what can be used to assess vision
snellen chart
- stand 6m away
- Record the line they can read to for both eyes
- E.g 6/8 (if read up to 8th line)
snellen chart can be used to tell if problem is a
refractive or non-refractive problem
refractive or non-refractive problem using Snellens chart
- Refractive – corrected with pin-hole testing
- Non-refractive- not corrected with pin-hole (optic nerve problem
error of redraction is correted with
pin hole testing
not corrected with pin hole indicates
pathology involving retina or optic nerve
acute angle closure glaucoma
(closed angle glaucoma)
opthalmological emergency
- older patient 55+
- acutely painful red eye
- irregular oval shaped pupil (fixed)
blurring of vision - Halos around light (due to corneal oedeme)
- nausea and vomiting
fluid in the eye provides
- Provides intraocular pressure
- If problem with drainage If pressure rises in the eye -> can be transferred to the back of the eye- damage to the back of eye
Production of aqueous humour
- From the ciliary processes in ciliary body
- Passes into posterior through the pupil into anterior –> provide cornea nourishment
Drainage of aqueous humour
- Drains via the iridocorneal angle
- Via trabecular meshwork into canal of Schlemm
- Circumferential venous channels draining into venous circulation
glaucoma is related to
- Problem with fluid drainage
outline glaucoma
- Problem with fluid drainage
- Causes optic nerve damage secondary top raised intraocular pressure
Can develop chronically or acutely
chronic glaucoma
- most common (open-angle glaucoma)
- Normal iridocorneal angle
- Trabecular meshwork deteriorates as age
- Many asymptomatic (eye screening)
- Increased IOP increased optic nerve cupping
Gradual loss of peripheral vision
- Acute – closed angle glaucoma
- Narrowing of iridocorneal angle
- Ophthalmological emergency
