Retina and Vision Processing

Question 1

What are the layers of the retina from most deep to most superfical?

In what direction does electrical impulse flow?

Answer 1

PHotoBAG

Photoreceptors
Horizontal cells
Bipolar cells
Amacrine cells
Ganglion cells

From photoreceptors up to ganglion cells

Question 2

There are 2 types of photoreceptors, cones and rods. Compare each based on the following factors:

• better in light/dark
• B+W/colour vision
• acuity
• position in the retina

Answer 2

Rods

• peripheries of retina
• B+W vision
• many rods signals go to one ganglion -> better in dark but lower acuity (sharpness)

Cones

• centre of retina
• colour vision
• few cones to one ganglion cell -> better in bright light with higher acuity

\Cones = Centre = Colour (all Cs)//

Question 3

In the presence of light does a photoreceptor cell depolarise/hyperpolarise and emit a electric signal.

Explain how this happens

Answer 3

HYPERPOLARISE to emit an electric signal

In presence of light cGMP Na+ channels close whilst K+ efflux continues = hyperpolarisation

Question 4

What is rhodospin and where is it found?

Answer 4

Visual pigment molecules found in membrane folds in the outer segment of rods

Question 5

Explain the pathway that takes 11-cis-retinal to hyperpolarisation of photoreceptor (and transmission to bipolar cell)

Answer 5

In the presence of light 11-cis-retinal -> all-trans-retinal

All-trans-retinal activates transducin -> reduced cGMP

Reduced cGMP -> closure of Na+ channels

Closure of Na+ channels -> hyperpolarise photoreceptor

Question 6

How can we see different colours?

Answer 6

Different colours have different wavelengths (red = longer)

Different wavelengths activate different cones

Question 7

Where in the occipital lobe are visual signals processed?

Answer 7

Layer 4C in the visual cortex

Question 8

Explain the ocular dominance columns found in layer 4C?

Answer 8

Before layer 4C in the visual cortex neurons can receive signals from both eyes

Once in visual cortex neurons only receive signals from ONE eye - columns form where one side has neurons receiving signals from one eye and visa versa

Question 9

Explain why it is so important kids have eye problems corrected during their development.

What can this lead to if not corrected?

Answer 9

During development brain recieves signals from both eyes and creates axons to help transmit these signals

If one eye is weaker than the other they will create less axons in layer 4C
The more dominant eye will then create more axons and eventually take up all avaliable space in 4C with axons for itself

These can lead to amblyopia or cortical blindness (no problem with eye itself but blind due to poor axon formation)

Question 10

Once visual signals have travelled through the optic chiasm along the optic tract where do they go before they reach the visual cortex?

Answer 10

LGN - lateral geniculate nucleus