Visual System

Question 1

What 3 different types of stimuli is the visual system tuned to recognise? (and give an example of how)

Answer 1

1. Food
2. Predator
3. Mate
   Sensory neuron has large dendritic trees

Question 2

What is an information bottleneck?

Answer 2

Cant process more than a certain amount of information
- Means the retina needs to decide what is worth sending to the brain

Question 3

How is the visual system organised?

Answer 3

1. Retina containing photoreceptors etc.
2. Optic nerve carries visual info from the retina to the brain
3. Axons of retinal ganglion cells synapse the LGN of the thalamus
4. LGN acts as a relay station transmitting visual info to the primary visual cortex

Question 4

How are the left and right visual field correlated to the brain?

Answer 4

Left visual field processed on right side of the brain
Right visual field processed on left side of brain

Question 5

Once information enters the visual cortex what two main paths can it go down?

Answer 5

1. Dorsal stream: ‘where’ pathway responsible for locating object and its spatial position e.g. is a moving object moving towards or away from us + its size
2. Ventral stream: ‘what’ pathway responsible for coding for object identity

Question 5

How is light focussed?

Answer 5

• Pupil regulate the amount of light that falls on retina
• Lens focuses image on fovea (largest number of photoreceptors and tightly packed with lots of cones for high visual acuity)
• Rest of retina has smaller acuity and primarily contains rods
• When something happens , the part of visual stimuli that is interesting to us is moved to be in line with the fovea so we can see it the most clearly

Question 6

How does light hit photoreceptors when the retina isn’t transparent?

Answer 6

• Light goes through Muller cells
• Much more transparent
• Acts as a ‘light guide’ this allows a certain number of photoreceptors to be activated

Question 7

Describe the layout of the Retina

Answer 7

the ganglion cell layer, closest to the vitreous humor, outputs info from retina to brain

inner plexiform layer - synapses, between ganglion (gabaergic) and amacrine/bipolar cells

inner nuclear layer - amacrine cells, horizontal cells involved in modulating info and bipolar cell bodies

outer plexiform layer - closer to back of the eye - synapses, between bipolar and photoreceptor cells (Glutaminergic)

outer nuclear layer - cell bodies of the photoreceptors
photoreceptor outer segments

pigmented epithelium - absorbs light the photoreceptors don’t to prevent reflection back to the retina, increasing visual acuity

*signal transduced at photoreceptors (GP), bipolar cells (GP) send signal onto ganglion cells (AP)

Question 8

How do photoreceptors respond to light?

Answer 8

Hyperpolarisation (decrease in membrae potential)

Question 9

Compare rods and cones

Answer 9

Rods -
High sensitivity
More disks, with higher concentration of photopigments in each disks, so 1000x more sensitive than cones (so at night - only rods used, so no colour seen)
Low resolution - many receptors to one bipolar neuron
Cones -
High resolution/visual acuity (and colour)
Fewer disks and lower photopigment concentrations
Not as easily saturated (less sensitive) so better at continuous response to stimuli

Question 10

why in the peripheral retina are there many bipolar neurons synapsing eith one ganglion cell?

Answer 10

allows for high sensitivity (light levels are lower) but low acuity as it is unclear exactly where the light came from, seeing as there is multiple inputs

Question 11

what makes photoreceptors so unusual (and how does it work)?

Answer 11

• depolarised at rest and hyperpolarised when activated
• ligand gated ion channel on the inside, ligand being cGMP
• when it’s dark cGMP keeps these channels open allowing influx of cations (mostly Na+), keeping the cell depolarised (open K+ channels allow K+ to leave so as not to depolarise too far)
• when its light cGMP levels drop, channels close, hyperpolarise the cell

Question 12

What causes cGMP levels to drop?

Answer 12

• Photons change the retinal in the GPCR from cis to trans exchanging the GPCR’s GDP for GTP
• Alpha subunit activates phosphodiesterase, an enzyme that converts cGMP to GMP
• Light causes low cGMP levels and the LGICs close

Question 13

How does hyperpolarisation lead to constant glutamate release?

Answer 13

• Hyperpolarisation spreads to synapse and the synapse for photoreceptor releases glutamate, sustains constant release
• So when you flash a light, downstream cells receive less glutamate

Question 14

What are ON and OFF Bipolar cells?

Answer 14

Some bipolar cells hyperpolarise while others depolarise in response to light
OFF= hyperpolarise in response to light (excitatory ionotropic receptor)
ON= depolarise in response to light (inhibitory metabotropic GPCR)

Question 15

how does light adaption work, specifically the role of calcium?

Answer 15

initial light = high hyperpolarisation
if it continues, photoreceptors gradually depolarise in order to make room for response if light intensity increases

the LGICs allow in calcium as it is a cation
calcium inhibits guanylyl cyclase, the enzyme making cGMP, so in the dark calcium comes in and prevents too much cGMP from overly depolarising the cell

in the light, when LGICs are closing, no calcium allows guanylyl cyclase to go crazy, opening those channels and allowing depolarisation even in the presence of light which initially causes hyperpolarisation

Question 16

What receptors are used in ON and OFF Bipolar cells?

Answer 16

OFF - AMPA receptors joined with glutamate receptor, when glutamate binds the channel opens and neuron is depolarised signalling a DECREASE in light intensity (light OFF)

ON - Metabotropic Glutamate Receptors generating signalling cascade

Question 17

What is TRPM1?

Answer 17

• Expressed in ON Bipolar cells
• Linked to the regulation of membrane potential

Question 18

What is Nyctalopin?

Answer 18

• Expressed in ON cells but not OFF cells
• Proteoglycan required for light and glutamate responses in ON cells

Question 19

What happens when there is KO experiments done with Nyctalopin and TRPM1?

Answer 19

• No current which shows TRPM1 is responsible for ion channel activation
  No ON cell activity

Question 20

So what is the purpose of ON Bipolar cells?

Answer 20

The respond to increases in light, so initially they are depolarised with light but they keep releasing glutamate until they are hyperpolarised
- This info is then sent to ganglion and amacrine cells

Question 21

Where do ON and OFF cells send signals to?

Answer 21

OFF cells stratify in the **Inner Plexiform layer **
**ON **cells stratify in the outer plexiform layer (closer to photoreceptor cells)

Question 22

What are the two types of receptive field for the retina?

Answer 22

**On-centre/off-surround: **
- Ganglion cells are excited by the stimulation of the centre (hyperpolarisation)
- Ganglion cells are inhibited by stimulation of the surround (depolarisaton)

Off-centre/ on-surround
- Ganglion cells are inhibited by stimulation of the centre (depolarisation)
- Ganglion cells are excited by the stimulation of the surround (hyperpolarisation)

Question 23

How do the centre-surround arrangements created?

Answer 23

1. Convergence of input from bipolar cells- each bp cells receives input from multiple cones/rods
2. Lateral inhibition by horizontal cells- activation of one photoreceptor can inhibit the neighbour to enable higher contrast between light and dark regions (spatial resolution)
3. Feedback from amacrine cells- Provide feedback inhibition to bipolar cells to help spatial properties of receptive field
4. Convergence on to ganglion cells for centre-on/off fields formed through integration of excitatory and inhibitory inputs from bipolar cells and horizontal cells

Question 24

How do you test receptive fields experimentally?

Answer 24

Using ring lights on the eyes that target just the centre of the retina or just the surrounding

Question 25

What are ganglion cells designed to do?

Answer 25

GANGLION CELLS ARE DESIGNED TO RESPOND TO DIFFERENCES IN ILLUMINATION THAT OCCUR WITHIN THE RECEPTIVE FIELD