Lecture 1 - Part 1: Retinal Organisation

Question 1

Eye components

Answer 1

• lens system
• aperture (pupil)
• dark chamber (pigmented)
• photoreceptors
• communication to central processors (optic nerve)

Question 2

Focussing light onto retina

Answer 2

Light = focussed at:
- cornea (80% refraction)
- entering lens
- exiting lens

Question 3

What allows fine focussing of an image?

Answer 3

Lens curvature and shape

Question 4

Why do we need to focus the image?

Answer 4

For the image to land on the back of the eye (central retina).

Question 5

Myopia - lens correction

Answer 5

• In myopia, focussing needs to be corrected via lenses.
  + Why?
• image/ focal plane lands behind or in front of the eye.

Question 6

Retina

Answer 6

Photosensitive part of the eye ( placed at the back of the eye).
It is made up of many different layers.

Question 7

What does the retina do?

Answer 7

• Receives light focussed by lens and coverts it into an electrical signal via network of nerve cells.

Question 8

Sclera

Answer 8

White of the eye - protective outer layer comprising of collagen and elastin fibres.

Humans = more sclera > other species - IMP for eye movement communication

Question 9

Choroid

Answer 9

Vascular layer- provides O2 and nutrients to outer retina ( especially fovea)

Question 10

Retinal pigment epithelium

Answer 10

Pigmented layer for light absorption and reducing oxidative stress.
- tight junctions form blood brain (retina) barrier
- supports photoreceptors

Question 11

Photoreceptors

Answer 11

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)

Question 12

Horizontal cells

Answer 12

Inter neutrons connecting photoreceptors laterally

Question 13

Bipolar cells

Answer 13

Connect photoreceptors to retinal ganglion cells + facilitate sensory processing through horizontal and Amacrine cells.

Question 14

Amacrine cells

Answer 14

Inter neutrons connecting bipolar laterally

Question 15

Retinal ganglion cells

Answer 15

• Output cells from the retina.
  4 types in humans:
• parvocellular
• magnocellular
• koniocellular
• photosensitive ganglion cells

Retinal ganglion axons form the optic nerve.

Question 16

Optic nerve head

Answer 16

Where all retinal ganglion cell axons pile up and information is taken from the eye to the brain.

Question 17

Fundus image

Answer 17

Image of retina + back of eye
Comprising of:
- optic nerve head
- blood vessels
- macula
- fovea

Question 18

OCT

Answer 18

Optical coherence tomography: takes detailed image of the retinal layers

Question 19

Fovea

Answer 19

• 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

Question 20

Blind spot

Answer 20

At the optic nerve head the image leaves the eyes - there are no photoreceptors present leaving a blind spot

Question 21

Core outer segment

Answer 21

Long narrow core outer segments allow high density packing

Question 22

Henle fibres

Answer 22

• 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

Question 23

Visual acuity

Answer 23

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.

Question 24

Why do we move our eyes?

Answer 24

• 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