Lecture 1 - Part 1: Retinal Organisation Flashcards
Eye components
- lens system
- aperture (pupil)
- dark chamber (pigmented)
- photoreceptors
- communication to central processors (optic nerve)
Focussing light onto retina
Light = focussed at:
- cornea (80% refraction)
- entering lens
- exiting lens
What allows fine focussing of an image?
Lens curvature and shape
Why do we need to focus the image?
For the image to land on the back of the eye (central retina).
Myopia - lens correction
- In myopia, focussing needs to be corrected via lenses.
+ Why? - image/ focal plane lands behind or in front of the eye.
Retina
Photosensitive part of the eye ( placed at the back of the eye).
It is made up of many different layers.
What does the retina do?
- Receives light focussed by lens and coverts it into an electrical signal via network of nerve cells.
Sclera
White of the eye - protective outer layer comprising of collagen and elastin fibres.
Humans = more sclera > other species - IMP for eye movement communication
Choroid
Vascular layer- provides O2 and nutrients to outer retina ( especially fovea)
Retinal pigment epithelium
Pigmented layer for light absorption and reducing oxidative stress.
- tight junctions form blood brain (retina) barrier
- supports photoreceptors
Photoreceptors
2 types rods + cones:
Cones: 6 million ( Red, blue and green)
- Concentrated at fovea
- High acuity
- Day (photopic) vision
- Colour vision
Rods: 100 million
- Dark (scotopic) vision
- Not present in central retina ( especially fovea = 0 rods)
Horizontal cells
Inter neutrons connecting photoreceptors laterally
Bipolar cells
Connect photoreceptors to retinal ganglion cells + facilitate sensory processing through horizontal and Amacrine cells.
Amacrine cells
Inter neutrons connecting bipolar laterally
Retinal ganglion cells
- Output cells from the retina.
4 types in humans: - parvocellular
- magnocellular
- koniocellular
- photosensitive ganglion cells
Retinal ganglion axons form the optic nerve.
Optic nerve head
Where all retinal ganglion cell axons pile up and information is taken from the eye to the brain.
Fundus image
Image of retina + back of eye
Comprising of:
- optic nerve head
- blood vessels
- macula
- fovea
OCT
Optical coherence tomography: takes detailed image of the retinal layers
Fovea
- Pit in the back of the eye ( retinal centre) needed for sharp vision.
- Densely packed with cones for high acuity vision.
- Inner regional layers pulled apart at fovea, less layers = easier for light to penetrate through and provide sharpened image.
- No rods at all
- Humans: only red, blue and green cones at fovea
Blind spot
At the optic nerve head the image leaves the eyes - there are no photoreceptors present leaving a blind spot
Core outer segment
Long narrow core outer segments allow high density packing
Henle fibres
- Axons from cone photoreceptors
- Run obliquely allowing info at foveal pit ( inner layers deflected sideways )
- Ganglion cells heap up around fovea to allow high spatial vision
Visual acuity
Measure of fine spatial (foveal vision), equivalent to 1-2 cone diameter in normal vision.
2 chart types:
- Shellen
- logmar = more scientific as there is equal gaps between levels and there are same no. of letter on each line.
Why do we move our eyes?
- To improve the sensory information input to our eyes.
- Photoreceptors most densely packed at fovea 0.25mm across.
Eyes move to: - bring image onto fovea
- keep the image there
