Visual system

Question 1

What is the structure of the retina?

Answer 1

Laminated layers of cells with photoreceptors at the bottom

Question 2

What is the structure of the fly retina?

Answer 2

• Many different lenses with similar laminated retina

- Leads to a lower spatial resolution (pixilated)

Question 3

What can the larvae visual system detect?

Answer 3

Detect colour and light intenisty but not images

Question 4

What are the structural components of rods and cones?

Answer 4

• Cell body
• Projection
• Mass filled with pigments

Question 5

Describe the effect of light on a photoreceptor

Answer 5

Induces second messenger cascade

• Rhodopsin changes confirmation
• G protein activates and cleaves cGMP from Na channel
• channel closes and cell hyperpolarises

Question 6

What are the two types of bipolar cell?

Answer 6

Sign conserving (off) and sign inverting (on)

Question 7

How do bipolar cells transmit an off signal?

Answer 7

Ionotropic

Decreased glutamate transmission results in glutamate gated Na channels closing

Question 8

How do bipolar cells transmit an on signal?

Answer 8

Metabotropic

• Less glutamate means less activation of mGluR
• Then mGluR cannot maintain low cGMP levels allowing it to open cGMP dependent Na channels
• Cell depolarises

Question 9

What are the characteristics of receptor ganglion cells?

Answer 9

• Spiking

- Have receptive fields

Question 10

What spiking activity is seen in a RGC with an on-centre off-surround receptive field when:

a) no light
b) light at on-centre
c) light on entire receptive field
d) light at off-surround only

Answer 10

a) baseline firing
b) maximum firing
c) baseline firing
d) no firing

Question 11

What are the characteristic of horizontal cells?

Answer 11

• Connect to surrounding cones of a receptive field

- Receive excitatory inputs and emits inhibitory inputs

Question 12

What happens when the on-centre is iluminated?

Answer 12

• Centre cone is hyperpolarised and surrounding cones are depolarised
• Strong depolarisation from surrounding cells means that horizontal cells receive a strong input and so send strong inhibition
• This means even less neurotransmitter is released from the centre cone and so the biopolar cell becomes very depolarised

Question 13

What happens when the entire receptive field is illuminated?

Answer 13

• All of the cones are hyperpolarised so horizontal cells do not provide much inhibition
• Bipolar cell only moderately depolarised

Question 14

What happens when the off-surround of the receptive field is illuminated?

Answer 14

• Surround cones are hyperpolarised while centre cone is depolarised, therefore there is little input to the horizontal cell and little inhibitory input
• Depolarised centre does not depolarise bipolar cell

Question 15

What is the difference between an on centre and an off centre receptive field?

Answer 15

Off centre - off bipolar cell (sign conserving)

On centre - on bipolar cell (sign inverting)

Question 16

Why are the non-spiking inputs converted into spiking?

Answer 16

In order to travel long distances without decay

Question 17

What pathway does input to the retina take in the brain?

Answer 17

Via LGN to primary visual cortex

Question 18

What are the characteristics of LGN neurons?

Answer 18

Also have field properties with several different types of ganglion cell

Question 19

What are the characteritics of the primary visual cortex?

Answer 19

• Where initial visual processing occurs
• Left and right eye are processed by alternating Hubel and Weisels columns
• Other columns reflect processing channels

Question 20

What are the 3 different processing channels of the primary visual cortex? What do they process?

Answer 20

1. M channel (magnocellular) - motion
2. P-IB channel (Parvocellularinterblob) - shape
3. Blob channel (Parvo and koniocellular) - colour

Question 21

Describe the cells of the M channel?

Answer 21

• IVC alpha contains ‘simple’ cells that respond to slits of light and dark with on/off fields that are selective to orientation
• IVB cells are similar but can respond to both eyes

Question 22

Describe the cells of the P-IB channel

Answer 22

• Layer III has ON/OFF field, highly responsive to orientation and small fields for object shape
• Neighbouring cells respond to different orientations

Question 23

What are the cells of the blob channel responsive to?

Answer 23

Different wavelengths

Question 24

What are the mechanisms behind direction sensitivity?

Answer 24

• When moving in one direction the response of the first cells is delayed and so arrives at the same time as the rest causing a summation of depolarisation
• When going in reverse responses arrive at different times and so do not reach threshold

Question 25

What is the two streams hypothesis?

Answer 25

Primary visual cortex projects to:
parietal lobe - where
temporal lobe - what

Question 26

How can the neurotransmitter of a neuron be determined?

Answer 26

• Fix proteins in tissue so that they retain conformation
• Apply treatment which creates ‘holes’
• Wash with antibody for specific type of neurotransmitter
• Second wash of something that binds to this antibody and if fluorescent
• Observe under fluorescent microscope

Question 27

What neurotransmitter is used by horizontal cells?

Answer 27

GABA

Question 28

What neurotransmitters do amacrine cells use?

Answer 28

• GABA/Glutamine
• Neuropeptides
• Biogenic amines

Question 29

What is the function of amacrine cells?

Answer 29

Regulate what bipolar cells transmit to ganglion cells

Question 30

What is the effect of circadian rhythms on amacrine cells?

Answer 30

• Dopamine levels are high during the day and low at night
• When you inject dye into amacrine cells you see that they are more connected at higher levels of dopamine
• therefore modulate to suit light levels