L14 - Fixing Faulty Neural Circuits

Question 1

What are channelrhodopsin?

Answer 1

Unselective channel that transmits Na and K

When it opens the membrane depolarises

Question 2

What are channelrhodopsins stimulated by?

Answer 2

Blue light

Question 3

How was channelrhodopsins used in the mouse brain?

Answer 3

Expressed in the motor cortex

If brain stimulated by blue light the mouse moves a lot quicker and in opposite direction

Question 4

What are halorhodopsins?

Answer 4

When it opens the membrane hyperpolarises
Can also be used in correction of the circularity
- E.g. used in epilepsy

Question 5

What are halorhodopsins stimulated by?

Answer 5

Yellow light

Question 6

What can be used instead of channelrhodopsins?

Answer 6

Small organic compounds

Question 7

What is the benefit of the use of small organic compounds?

Answer 7

More specific

Contains a double bond and a part that binds an ion channel

Question 8

What happens if light is used on small organic compounds?

Answer 8

The configuration changes
From straight chain (trans) to bent (cis)
- In trans configuration – molecule blocks the channel
- In cis configuration – molecule does not block the channel

Question 9

How are GABA receptors photocontrolled?

Answer 9

Ligand for the receptor
In one configuration – ligand will bind and activate channel
In another configuration – ligand too far away from the binding site
GABA receptors are therefore activated in light

Question 10

What other receptors can be photocontrolled?

Answer 10

Acetylcholine receptors
Potassium channels
Glutamate receptors
Either hyperpolarised or depolarised in response to light

Question 11

What is Retinitis pigmentosa?

Answer 11

Occurs due to retinal degeneration
- Photoreceptors die
Visual field gradually becomes smaller and smaller

Question 12

How could an artificial retina work?

Answer 12

Could use

• Light activated electrodes
  
  • Don’t last very long
  • Ganglion cells are the cells exposed to the electrodes
• Channel/halorodopsin

Question 13

When is the retina most need to be stimulated?

Answer 13

For retinitis pigmentosa

Question 14

When is the visual cortex most needed to be stimulated?

Answer 14

When the optic nerve didn’t develop or is destroyed

Question 15

Why is it important to activate as early as possible in the visual pathway?

Answer 15

Retina performs complex computations
Different ganglion cells have different functions and project to different brain areas
Stimulation of retinal ganglion cells with simple stimuli is useless
- E.g. motion sensitive RGC is active only when there is motion

Question 16

What are the two ways ganglion cells can be artificially activated?

Answer 16

Electric stimulation
Channelrhodopsin and halorhodopsin
Only problem is that these stimulate ganglion cells not photoreceptors or bipolar cells

Question 17

What are the two classes of ganglion cells?

Answer 17

Magnocellular
Parvocellular
These can respond to light in different colours

Question 18

Direction selectivity is evident where?

Answer 18

In the retina
Responds to movements in some directions but not the others
Typically shown in ON/OFF cells

Question 19

What is the hypercomplex in the visual cortex?

Answer 19

Respond only in a certain orientation

If the bar is slightly tilted after leaving the receptive field a response is once again seen

Question 20

What is the solution to the activation of only ganglion cells by electrodes?

Answer 20

Use light to stimulate deeper layers - bipolar cells or even photoreceptors

Question 21

In a cell with retinal degeneration with only small parts of the photoreceptor left (still formed synapses with bipolar cells) what was expressed in photoreceptors?

Answer 21

Halorhodopsin

Used yellow light to activate halorhodopsin not photoreceptors

Question 22

In the cell with retinal degeneration how was it tested that the ganglion cells where intact?

Answer 22

ON cells spike with light
OFF cells spike when no light
Some function of the photoreceptors is restored

Question 23

In the cell with retinal degeneration how was it tested that the centre surround organisation of ganglion cells was intact?

Answer 23

Further increasing the size of light spot decreases the response of the ganglion cell

Question 24

In the cell with retinal degeneration how was it tested direction selectivity was intact?

Answer 24

Used moving spot of light

Showed null and preferred direction

Question 25

Why is injection of small organic molecule in retinitis pigmentosa retina not as successful?

Answer 25

No specificity

Will activate mainly ganglion cells