Optic Nerves And Visual Pathways Flashcards
Which is the outermost layer of the tear film?
Lipid
What purpose does the lipid serve?
Prevent evaporation of the aqueous layer
What is the innermost layer of the tear film?
Mucus layer
Which layers are most commonly implicated in dry eye ?
Lipid and aqueous
What is the innermost layer of the tear film?
Mucus (aqueous is middle)
Which gland is is responsible for baseline tear formation?
Accessory lacrimal gland
How does light ravel through the eye? Label diagram
Retina
Light will go in
Stimulates light sensitive material such as rhodopsin
This creates a chemical change
Sends nerve impulse to retina
Rods and cones
Outer segment contains discs containing light sensitive photopigment
Inner segment made up of cell body, axon and synaptic terminals
Phototransduction – absorb light, send electrical signal
Different cones respond to different wavelengths of light
Phototransduction
Photopigment lies in discs of outer segments
In rods called rhodopsin, in cones called cone opsin
Opsins are transmembrane proteins which contain the light sensitive molecule retinal
Different opsin structures mean retinal absorbs different wavelengths of light
Phototransduction- effect of a photon on a rhodopsin molecule
Triggers conformation change to all-trans form
This change triggers changes in the opsin structure
This in turn triggers a cascade within the cell
Can have a graded response
https://www.youtube.com/watch?v=dhd2fja0LZ4
Signals in the retina
Horizontal and apocrine cells modulate signal
Retinal ganglion cells not graded response – action potential
But for the eye, there must be a mechanism to allow the eye to tell the difference between an intense light and a less intense light (can not just have all or nothing principle)
Photoreceptors allow this graded response
The horizontal and apocrine cells modulate the signal
Pupillary light reflex
Light is shone on the right eye only
Action potentials from right eye reach both the right and left pretectal nuclei
The pretectal nuclei stimulate both sides of the eddigner-westphal nucleus even though the light was perceived only in the right eye
The right and left sides of the eddinger westphal nuclei generate action potentials through the right and left occulomotor nerves, causes both pupils to constrict
Visual pathway: nerve —> chiasma —> optic tract —> LGN —> optic radiations
SLIDE 14
Nasal retina is responsible for temporal field
Temporal retina is responsible for nasal field
Superior retina is responsible for inferior field
Macula (central vision) is always temporal to the nerve
Nerve is always nasal
This gives you a blind spot
Nasal retina will cross over from both sides at the optic chiasma
Temporal retina do not cross over, stay on same side
After the optic chiasma, the optic nerves with the nasal retina are referred to as optic tracts
Difference between tracts and nerve is that, fibre is coming from one eye, tract is a mix up of fibres from both eyes (still retinal ganglion in both cases- same nerve fibres )
Optic tract travels to lateral geniculate nucleus in the thalamus
This is the first point of synapse for these neurones and nerves terminate
Second order neurones
2 major radiations from the LGN:
From each LGN theres a loop of fibres that will cross over the temporal lobe, going slowly anterior and then back to occipital lobe
There’s also fibres going superiorly through the parietal lobe
Optic radiations now reach occipital cortex, Romans area 17, The primary visual cortex, then Romans area 18 and 19 (secondary visual cortex)
As the fibres terminate in the primary visual cortex, the image is interpreted, colour and shape are then interpreted (all primary)
Secondary interpretation: depth
Tertiary interpretation: facial recognition
Fusion of image gives you stereopsis, 3D vision
Lesions to optic pathway
Meters loop lesion contains fibres from inferior left retina and inferior right nasal retina- Causes right homonymous superior quadronopiea
