Visual defects Flashcards
Why is the pupil black?
Pupil is the opening that allows light to enter the eye; appears dark because of the light absorbing pigment epithelium in the retina.
What does the iris contain and what is its function?
Iris is pigmented to give the “eye’s colour”; contains two muscles to vary the size of the pupil.
Does Cornea contained blood vessels and if not what supports it?
Cornea lacks blood vessels and is nourished by aqueous humour.
What is the Cornea continous with?
The sclera of the eye
What is Conjunctiva?
Conjunctiva is a membrane that folds back from the inside of the eyelids and attaches to the sclera
What controls the movement of the eyeball in the orbit of the skull?
Extraoccular muscles
3 pairs
What is the vitreous humour?
Vitreous humour is a viscous jellylike substance that lies between the lens and the retina; it keeps the eye spherical.
What is the fovea?
Is the point of highest visual acuity in the retina, where light can reach photoreceptors directly.
What is the lens attached to and how is it suspended?
Lens is a transparent structure suspended by ligaments (zonule fibres) attached to the ciliary muscles which control the shape of the lens.
What is refraction?
Refraction is the bending of light rays.
When does refraction occur?
Refraction occurs when light passes from one transparent medium (e.g. air) to another (e.g. cornea).
Refraction occurs when light is slowed down as it changes from one media to another.
What occurs when light passess through the cornea?
Light rays that strike the curved surface of the cornea bend so that they converge on the back of the eye.
Light rays that enter the centre of the eye pass straight to the retina
Why does lens also refract light?
The lens also refracts light rays passing through it to produce a sharp image.
How does lens bring close images into focus?
Close images are brough into focus by the lens changing shape (accommodation)
What is the outcome of rounding the lens?
increases the curvature of The lens surface and increases the refractive power.
What changes occur to the lengs with change in age?
The ability to accommodate the lens changes with age.
Accomodation occurs due to what structure?
Accommodation is due to contraction of the ciliary muscles which relieves the tension on the zonule fibres, allowing the lens to become rounder due to its natural elasticity.
How does the lens change its shape?
By the contraction of the ciliary muscles
What is the optic disc?
origin of blood vessels; where optic nerve axons exit eye (blind spot)
What is the route of light and how is it focued?
Light is focused by the cornea and the lens and then passes through the vitreous humour to the retina.
What does the pigmented epithelium contain?
Cells in the pigment epithelium are filled with the black pigment melanin which absorbs any light not absorbed by the retina.
What are the two types of photoreceptors in retina?
Cones – colour system – daylight.
Rods – no colour – low light levels
Absorption occurs in what part of the photoreceptors?
The outer segment contains a stack of membranous disks which contain light sensitive photopigments.
What is the function of photoreceptors?
Photoreceptors transduce light energy into changes in membrane potential.
In mesopic –> twilight what type of photoreceptor is active?
Both rods and cones
What is the pigment in rods called?
rhodopsin
How many types of pigments are there in cones?
In each cone there is one of three types of opsins
What is another name for colour blindness?
Dichromat
What is hyperopia?
“farsightedness” – close-up objects = blurry
How do you treat hyperopia?
Convex Lens glasses
What occurs in hyperopia?
Light focuses beyond retina (too far = hyper)
What is myopia?
nearsightedness” – far away objects = blurry
What occurs in myopia?
Light focuses infront of retina
What is the treatment of myopia?
Concave Lens glasses
What is normal colour vision?
trichromat–> create all the colours
Is rod cells specialised for night or day vision and give acount of its characteristics
- High sensitivity to light and more photopigment – captures more light e.g at dusk so specialized for night vision
- low acuity
- more sensitive to scattered light
- Achromatic – black & white
Is cones cells specialised for night or day vision and give acount of its characteristics
- Low sensitivity to light so day vision
- high acuity – concentrated in Foeva
- More sensitive to direct light rays
- Trichromatic – colour
What is the function of photoreceptors?
Photoreceptors - convert light energy to neural activity
what is the function bipolar cells?
Create the direct pathway from photoreceptors to ganglion cells.
What is the function of horizontal/amacrine cells?
Indirect pathways i.e. modulators
What is the fucntion of retinal ganglion cells?
Axons leave the eye, forming the optic nerve
On bipolar cells depolarise in response to what?
Response to light
Off bipolar cells depolarise in response to what?
Response to dark
Direct input to the bipolar cells comes from?
From the receptive field centre
Indirect input to the bipolar cells comes from?
From the receptive field surround via. Horizontal/Amacrine cells
Eeach bipolar recieves input from how many photoreeptors?
At least 1
What are the 3 types of retinal ganglion cells?
Magnocellular (M-type
Parvocellular (P-type)
Non-M non-P (K-Type)
What is Magnocellular and what is its function? How common is it?
Larger cell type
5% of population
Large receptive field
Important for detection of stimulus movement.
What is Parvocellular (P-type) and what is its function? How common is it?
Smaller cell type
90% of population
Sensitive to stimulus form and fine detail
What is Non-M non-P (K-Type) and how common is it?
Medium cell type
5% of population
What is receptive field?
Area of retina which alters (depolarizes) Voltage Magnitude (Vm) of a single bipolar cell
How is On centre ganglion cells depolarized?
ON-centre ganglion cell will be depolarized when a small spot of light is projected onto the middle of its receptive field.
What do Off centre ganglion cells response to?
OFF-centre ganglion cell will respond to a small dark spot presented to the middle of its receptive field.
Via what does neurones from the Lateral geniculate neurone of the thalamus project onto the visual cortex?
Neurons of LGN project to visual cortex via optic radiation
What is the striate cortex?
Striate cortex, is the first region of visual processing in the cortex.
What are the two cortical streams of visual processing?
One towards parietal lobe
One towards temporal lobe
What is the cortical stream towards parietal lobe invovled in?
Visual motion
What is the cortical stream towards temporal lobe invovled in?
Recognition of objects (memory)
What are the different visual processes of the cortex? What are there function and what layer are they in?
M-Channels – detect movement (Layer III)
P-IB Channels – detect object shape (Layer III)
Blobs – analysis of colour (Layer III)
Orientation Collums – oreination (Layer V)
Occular Dominance – receive information from left or right eye
What are the 6 symptoms of visual loss?
Glare
Distorted vision
Things look pale
Shadow
Floater–> are small pieces of debris that float in the eye’s vitreous humour.
What is the features of blurred vision?
Out of focus / not sharp
No distortion
Refractive problem
Macular problem
What are the features of glare?
Difficulty seeing in bright light
Low sun
Driving at night
Fluorescent light in supermarkets
Corneal or lens problem
Often due to cataract
Vision may be good otherwise
What are the features of distrotion of vision?
Things look wavy, jumbled up. Lines not straight. Kink in lines
What are the features of things looking pale?
Things look darker / less colour
Optic nerve disease
Optic neuritis
Compressive optic nerve disease
Condition affecting retina
Wet macular degeneration
Central serous retinopathy
What is catarcts ?
Opacity of the lens
Common aging change
What are the symptoms of cataracts?
Gradual onset
Symptoms depend on type of cataract
Blurred vision, glare, change in refraction
Common and may be asymptomatic
Difficult to judge effect on vision from examination
What is the treatment of cataract and what is the advtange and disadvantages?
Phacoextraction with lens implant
Adv:
Vision could probably be improved to near normal with operation
Can be performed under local anaesthetic as day case
Rapid rehabilitation
Disadvantage
Risk of serious complication leading to loss of vision (1-2%)
What are the signs of Dry Age-related macular degeneration?
Drusen which is yellow deposits under the retina
Retinal pigment epithelium pigmentation
Retinal pigment epithelium atrophy
What type of vision does Dry Age-related macular degeneration effect?
Reading vision
What would sudden detrioration of Dry Age-related macular degeneration indicate?
Wet Age-related macular degeneration?
What are the treatments of Dry Age-related macular degeneration?
No effective medical treatment Stop smoking Dark glasses to protect from bright sunshine Dietary changes / supplement Low visual aids
Causes of central retinal artery occlusion?
Embolic
Carotid artery disease
Most common cause
Heart disease
Valve disease
What is the symptoms of giant cell arteritis?
Systemic vasculitis of unknown cause Elderly Headache Malaise Visual loss Needs urgent treatment
How does Giant cell arteritis cause visual loss and is it treatable?
Causes visual loss due to Anterior Ischemic Optic Neuropathy or Central Retinal Artery Occlusion
It is not treatable
What is a further risk of Giant cell arteritis and how is it detected?
Have high risk of Anterior Ischemic Optic Neuropathy or Central Retinal Artery Occlusion to fellow eye
Confirm diagnosis with temporal artery biopsy
