Vision + Visual Field Flashcards
What happens when light enters eye
Travels through all layers of retina to reach rod and cones (photoreceptor)
Transduction occurs to convert light -> chemical
Travels back and synapse at nerve fibre layer
Joint optic nerve to generate AP (photoconduction)
What is fovea centralis
Area packed with cone cells for max visual acuity
Located within the macula
High concentration of cone cells
Exposed so don’t have to pass through all layers of retina
Eyes will move to focus on this
What is the macula
Small area at centre of retina for central field of vision
Contains highest conc of cone cells
How do rod and cone cells work in the dark
At rest
Depolarised by open Na / Ca) channel
What do rod and cone cells do different
Different visual pigment
Rhodopsin pigment in rods = low light
Opsin in cones = daylight / colour
Colour blindness
Defect in cone cells or pathway to brain
What happens when light enters aye
Changes vit A in pigment to all-trans-retinal (ActivateD)
Molecule can’t fit into rod / cone
Splits = bleaching and activation of visual pigment (rhodopsin / opsin)
Picked up and changed back to original but only if have diet vitamins A
What is activated rhodopsin
No vit A present 11’cis-retinal
What happens when activated
Causes channels to close
Hyperpolarisation
AP forms and travels to optic nerve in nerve fibre layer
What happens if vit A deficiency
Can't see in the dark Unhealthy conjunctival and cornea Bitots spots (triangular silver foam) in conjunctiva Ulceration Corneal melting + opacification
What is refraction
Bending of light rays to form a sharp image on retina when changing from one optic medium to another
What are main mediums
Cornea + lens
AH / VH = refract slightly
When do errors occur
If mismatch in bending
What happens if object close
Eye needs to bend more to focus
Lens becomes thicker
If object >6m
Light is parallel
If <6m
Divergent rays so bend
What is convex
Makes light converge (bend in)
What is concave
Makes light diverge (bend away)
What is accommodation
Bending light to focus on far off or near objects
What happens
Lens thicker to focus on close object
Pupil constrict for sharp focus
Eyes converge - MR (III)
What are intrinsic ocular muscles and nerve
Pupillary constrictor - para III - Ach
Pupillaey dilator - sympathetic - NA
What happens if increased light and what is the mechanism
Pupil constrict
Light on retina
Travels to optic nerve -> chiasma -> tract
Do not go to LGB
Go to mibrain and synapse on III on both sides
What happens if decrease
Pupil dilates
What are extrinsic ocular muscles
Superior rectus Inferior rectus Lateral rectus - CN6 Medial rectus Superior oblique - CN4 Inferior oblique All other CN3
LOOK AT EYE
OK
Where is L half of visual field seen
R side of retina
What is the pathway of visual field
Optic nerve -> optic chiasma
Medial fibres (nasal) cross to opposite side
Lateral stay the same
Travel in optic tract and synapse in LGB of thalamus
Optic radiation travels through internal capsule
Goes to visual cortex in occipital lobe
What does process at chiasma mean
Optic tract contains fibres from lateral half of ipsilateral and medial from contralateral
so ALL fibres from opposite half of visual field = optic tract
L visual field = R optic tract
What does L side of brain receive
L visual field of both eyes
What happens if R optic nerve damage
No vision in R eye
Direct reflex is absent but indirect will remain intact
What happens if optic chiasm disrupted due to pituitary tumour
Affects temporal vision (nasal fibres) as they cross over at chiasma
Bitemporal hemianopia
What happens if optic tract or radiation is damaged
One side of visual field is lost in both eyes
Contralateral homonymous hemianopia
If R tract = L visual field lost in both eyes
What happens if occipital cortex is damaged
Contralateral homonymous hemianopia with macular sparing
If optic tract lesion in parietal lobe
Inferior qaudranopia
If lesion in temporal lobe
Superior quadranopia
How do you remember
PITS
In opposite side of Brain
What type of bitemporal hemianopia if pituitary
UQ as inferior chasm compressed
What type of bitemporal hemianopia if craniopharyngioma
LQ as superior chasm compressed
What are causes of visual field defects
Ischaemia - TIA / migraine / stroke Glioma Menigioma Abscess AV malformation Drugs
