4 - The Visual System

Question 1

What 3 types of stimuli does the visual system recognise?

Answer 1

Food, Predator, Mates.

Question 2

Where does processing start in the eye?

Answer 2

The retina

Question 3

What is the function of the retina?

Answer 3

Image Acquisition.

Question 4

What structure pre-processes the visual information?

Answer 4

The lateral geniculate nucleus (LGN) while the main part happens in the visual cortex.

Question 5

What are the 2 main visual pathways in the cortex?

Answer 5

• Ventral - object feature stream “what” (inferior temporal)
• Dorsal - Spatial location stream “where” (posterior parietal)

Question 6

What regulates how much light enters the eye?

Answer 6

The Pupil

Question 7

What focuses images onto the fovea?

Answer 7

The Lens

Question 8

What is the fovea?

Answer 8

The part of the eye with the highest visual acuity and contains all the cones

Question 9

How does light travel through the retina?

Answer 9

Through the muller cells, which are more transparent than other parts of the retina.

Question 10

How many layer are in the retina and what are they?

Answer 10

5 layers
- 3 layers of neurons
- 2 layers of synapses (inner and outer plexiform layers)

Question 11

What are the feedforward neurones in the retina?

Answer 11

Photoreceptors, Bipolar cells, Ganglion cells

Question 12

What are the feedback neurones in the retina?

Answer 12

Horizontal cells and amacrine cells

Question 13

Rods

Answer 13

Active in dim light

Question 14

Cones

Answer 14

Active in bright light

Question 15

Where does phototransduction happen in the retina?

Answer 15

In the outer segment

Question 16

What is the key molecule responsible for phototransduction?

Answer 16

Cyclic GMP which activates specific ion channels in the cell.

Question 17

What is cGMP’s affect in the cell?

Answer 17

Raises the membrane potential by activating open channels. (Depolarisation)

Question 18

What converts cGMP to GMP?

Answer 18

Phosphodiesterase.

Question 19

What activates phosphodiesterase?

Answer 19

Light changing the conformation of rhodopsin which triggers a g-protein cascade.

Question 20

What molecule does photoreceptors release?

Answer 20

Glutamate.

Question 21

When is less glutamate released?

Answer 21

In the light during hyperpolarisation.

Question 22

How are the neurones below photoreceptors hyperpolarised?

Answer 22

The cells that synapse with the photoreceptors receive less glutamate.

Question 23

On cells…

Answer 23

Depolarise when the light intensity increases.

Question 24

Off cells…

Answer 24

Hyperpolarises when the light intensity increases.

Question 25

What is the mechanism of on cell activation?

Answer 25

- They use mGluR instead of AMPAR and cGMP is not required for ion channels to close. - TRPM1 is expressed in on cells and not off cells

Question 26

What molecules are required for light and glutamate responses in On cells?

Answer 26

Nycalopin and Proteglycan

Question 27

Where do the off cells project in the inner plexiform layer?

Answer 27

To the layer closest to the bipolar cells.

Question 28

Where do the on cells project in the outer plexiform layer?

Answer 28

Into the part that is closer to the ganglion cell layer.

Question 29

When do retinal ganglion cells fire action potentials?

Answer 29

When specific areas of the retina are illuminated.

Question 30

What is the receptive field?

Answer 30

An area in the retina where when illuminated activates a visual neurone.

Question 31

What is centre-surround organisation of the receptive field?

Answer 31

What many visual neurons have, illumination of the centre and the surroundings leads to responses in opposite polarities.

Question 32

What is the mechanism of centre surround organisation?

Answer 32

- Many photoreceptors converge to one bipolar cell. - Some activate directly which is centre. - Others activate via horizontal cells which is surround causing depolarisation.

Question 33

What other cells have centre surround organisation?

Answer 33

Ganglion cells.

Question 34

In Off centre ganglion cells..

Answer 34

Action potentials stop spiking when the light is central.

Question 35

In On centre ganglion cells...

Answer 35

Action potentials spike when the light is in the centre and decreases when light is in the surround.

Question 36

What do on and off centre ganglion cells in diffuse light?

Answer 36

No response.

Question 37

What are the 2 classes of ganglion cells?

Answer 37

Parvocellular and magnocellular.

Question 38

Characteristics of parvocellular cells

Answer 38

They have a smaller dendritic tree and a smaller receptive field.