The Eye and Vision

Question 1

What are the 2 two compartments of the eye?
Where are they?
What is their function?

Answer 1

Vitreous humour = between the lens and the retina, gives eyeball shape
Aqueous humour = between cornea and lens, nourishes cornea

Question 2

Where is aqueous humour produced and reabsorbed?

Answer 2

It is secreted into the posterior chamber and reabsorbed in the anterior chamber

Question 3

What is glaucoma?

Answer 3

Increased pressure in the aqueous humour due to increased production or blocked resorption

Question 4

What are the two types of glaucoma?
What are their causes?
Which is more serious?

Answer 4

1. Angle Closure —> blindness
   - iris adheres to cornea, blocks reabsorption from anterior chamber
2. Open Angle - common
   - sclerosis of veins where humour is reabsorbed

Question 5

What are the treatment options for glaucoma?

Answer 5

1. Surgery? - angle closure only
2. Medication
   - beta blocker, decrease production of aqueous humour
   - prostaglandin analogues –> vasodilation, increased reabsorption

Question 6

What are the five cell types in the retina?
How are they organised?
What are their functions?

Answer 6

Deep to superficial

• photoreceptors = detect light, photosensitive pigment
• horizontal = modilate transmissoin
• bipolar = transmit signal from photoreceptors to ganglion cells
• amacrine = modulate transmission
• ganglion = generate action potentials

Question 7

What is the optic nerve made up of?

Answer 7

axons of ganglion cells in the retina

- all the axons converge at the optic disc to form the optic nerve

Question 8

What are the two types of photoreceptors?

Answer 8

Rods (rhodopsin)

Cones (3 x opsin, red, blue, green)

Question 9

Describe the structure of a rod cell

Answer 9

Intracellular membrane discs stacked in the outer segment of the cell.
They do not interact with each other or the cell membrane
High density –> large SA for absorption
- V sensitive (night vision)

Question 10

Describe the structure of a cone cell

Answer 10

Invaginations of surface membrane of outer segment of the cell
Less dense than in rod cell –> smaller SA, need a high light intensity
–> colour vision (daytime)

Question 11

Describe the regional organisation of the retina and the properties of each area

Answer 11

• Blind spot = optic nerve leaves eye, no photoreceptors
• Increasing proportion of cones closer to fovea
• Fovea = all cones
  Peripheral Retina - high sensitivity to light, low acuity
• Rods (lots of photopigment)
• Many rods connect to one ganglion cell
  Macula/Central Retine
  Fovea - low sensitivity, high acuity
• cones (less photopigment)
• 1 cone : 1 ganglion cell
• lateral displacement of cells other than photoreceptors

Question 12

What is the definition of focusing in terms of light hitting the retina?

Answer 12

Moving eye structures to focus the image on the fovea

Question 13

What are the 2 components of rhodopsin?

What is the role of each component?

Answer 13

Retinal (chromophore)

Opsin (GPCR)

Question 14

Describe phototransduction in rods in the dark

Answer 14

• cyclic GMP gated cation channel = OPEN
• Na+ influx (dark current) –> depolarisation
• so Em = -30mV
• Depolarisation –> glutamate release onto the synapse with a bipolar cell

Question 15

Describe phototransduction in rods in the light

Answer 15

Retinal absorbs light
- hydrogen ion flips at 11-cis position(11-cis –> all trans)
Opsin activation
GTP binds to Gprotein (transducin)
G protein activates PDE
PDE breaks down cyclicGMP –> GMP
- cyclic GMP gated cation channel now closed
–> hyperpolarisation of rod
–> decrease glutamate release onto bipolar cell

Question 16

What are the 3 cell types in the direct pathway of transmission through the retina

Answer 16

Photoreceptor –> Bipolar cell –> Ganglion cell

Question 17

Direct Pathway
- Photoreceptors always …………. in the light
- Response of the other cells is dependent on their …….. and their different ………
- There are two classes of bipolar cells
ON (…………… in light)
and
OFF (………………. in light)

Answer 17

Photoreceptors are always hyperpolarised in the light
Response of other cells is dependent on their type and their different receptors
There are two classes of bipolar cells
ON - depolarised in light
OFF - hyperpolarised in light

Question 18

Describe the electrical position of the following cells in the following situations:

• ON bipolar cell in the light
• ON bipolar cell in the dark
• OFF bipolar cell in the light
• OFF bipolar cell in the dark

Answer 18

• ON bipolar cell in the light = decreased glutamate –> depolarised
• ON bipolar cell in the dark = increased glutamate –> hyperpolarised
• OFF bipolar cell in the light = decrease glutamate –> hyper polarised
• OFF bipolar cell in the dark = increased glutamate –> depolarised

Question 19

Describe the indirect pathway of transmission through the retina

Answer 19

Each bipolar or ganglion cell has a receptive field (region of the retina that influence that cell)
Centre = direct connections
Surround = affect Horizontal/Amacrine cells
Light in receptive field surround
- opposite electrical responses to B/G cells
- because of the influence of Horizontal/Amacrine cells
- important for contrast at image borders

Question 20

OFF-centre ganglion cell output:

- where can you see the greatest response?

Answer 20

Dark centre and light surround

Question 21

What can the visual field be divided up into?

Answer 21

• left and right visual hemifield
• nasal and temporal
• right/left optic nerve
• left/right optic tract
• optic chiasm

Question 22

Look at visual field defects

Answer 22

…

Question 23

Explain the 3 responses of OFF ganglion cell to light in receptive field

Answer 23

a) centre = light --> hyperpolarised
surround = light --> depolarised 
= overall hyperpolarised 
b) centre = dark --> depolarised 
surround = light --> depolarised 
= greatest response 
c) centre = dark --> depolarised 
surround = dark --> hyper polarised 
= overall depolarised, but less so