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PHGY 311 > Quiz 3 - Neural Circuits > Flashcards

Quiz 3 - Neural Circuits Flashcards

1
Q

In what ways is the eye similar as a camera? (Which structures correspond to what?)

A
  • Inversion of image by the lens
  • Focus = contraction of lens by ciliary muscle
  • Film = photo-receptors in retina
  • Aperture/diaphragm = iris
  • USB port = optic nerve
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2
Q

What corresponds to the pixels of a camera in the eye?

A

Photoreceptors :
Cones = day light and color vision
Rods = star light vision (more present in outter sheet of the retina)

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3
Q

What is the resolution of photoreceptors?

A

100 mega-pixels

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4
Q

What is the sensor of the eye and its parts?

A

Rhodopsin = Opsin + Retinal
Opsin = transducer (7 trans-membrane protein, G-protein coupled receptor)
Retinal = the sensor (11-cis at rest, all-trans when striked by a photon, Opsins ligand)

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5
Q

What is the structure of a Rod?

A

The photon enters by the synaptic terminaln (outside terminal)
Inner segment (with the sooma, mitochondrias, etc.)
Outer segment (Discs containing Rh + transduction machinery)
*Photons must pass through entirety of retina before reaching discs

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6
Q

What is the structure/ the components of a disc?

A
  • They all share the same cytoplasm (same rod cell)
  • Each have their own lumen and their own plasma membrane
  • At surface, Rhodopsin
    + other proteins involved in transduction cascade
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7
Q

What is Retinal composed of?

A

Retinal = Vitamin A

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8
Q

Does Rhodopsin have equal sensitivity to all the photomagnetic spectrum?

A

No, highest absorption = green-yellowish
lowest absorption = red
Blue = 0.5

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9
Q

What is the Quantum Efficiency?

A

The fraction of absorbed photons that cause thodopsin to activate (isomerize retinal)
For rhodopsin = 2/3 (very sensitive photo pigments)
Photographic film = 1/10

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10
Q

What are the different sources of loss of photons when entering the eye?

A
  1. 2.5% reflected at cornea
  2. 9% scattered in anterior compartment
  3. Some photons pass right through the retina without being absorbed in any discs
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11
Q

Why does the eye saccades?

A

Stimulate different photoreceptors every 0.1s bc photoreceptors are bleached by light so use new ones until they regenerate (stabilizes our view)

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12
Q

Explain the 5 steps of the transduction cascade.

A
  1. Photon is absorbed → Rhodpsin activates (11-cis to all-trans)
  2. Rhodopsin binds to G protein transducer
  3. Binding causes exchange of GDP → GTP
  4. GTP-bound transducin’s alpha subunit freed → goes on to bind cG phosphodiesterase (PDE)
  5. Turned on cG phosphodiesterase (PDE) chews up cGMP → GMP
  6. Concentration of cGMP required to bind ion channels goes down so channels close
  7. Photoreceptor is hyperpolarized (K+ still goes out, but Na+/Ca+ can’t come in)
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13
Q

How long does Rhodopsin stay activated when it changes conformation?

A

100 ms
So shutter time of our camera is 100ms

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14
Q

How does the relative amount of disc proteins influence transduction?

A

100% Rhodospins
12.5% tranducins G-proteins
2% phosphodiesterases

So transducin and PDE are always saturated even when small amount of Rh are activated

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15
Q

How many cGMPs are hydrolysed for every Rhodopsin activated (1 photon absorbed)?

A

1 Rh* = 700 G proteins/transducins alpha = 1400 cGMP hydrolysed

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16
Q

What is the equation to calculate % CG channels open depending on #CH hydrolysed?

A

% CG channels open = [1 - CG hydrolysed/1x10^5]^3

To find the amount that were closed due to photon, do 1-% CG channels open (to find current induced)

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17
Q

What direction is the photocurrent?

A

Outward (K+ goes out) → Hyperpolarizes the cell

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18
Q

How does increasing light intensity influence the photocurrent?

A

Increase in photocurrent with increasing of light, but saturates when all cG gated channels are closed

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18
Q

What does the IV curve of cGMP channels look like?
Why is it important?

A

Flat until approx 0mV, increases exponentially
As cell depolarizes, pores get plugged so the curve is flat → the current is only proportional to the number of photons striking
photoreceptor, independent of Vm (< -20 mV)

*mixed channel to reverses around 0

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19
Q

What is the single CG channel current?

A

3fA or 3x10^-15A

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20
Q

How many cG channels is there in a Rod?

A

1.14x10^4

21
Q

Can rods see single photon response?

A

Yes, little depolarisation with 1 single photon

22
Q

How many contacts do Rods make with Bipolar cells?

A

Each Bipolar Cell contacts with ~ 10 Rods

22
Q

How to BC contact with photoreceptors

A

Synaptic vesicles attach to the synaptic ribbon (in the photoreceptor) and released into the synaptic cleft
When the photoreceptor hyperpolarizes, stops releasing glutamate

23
Q

How do OFF BC function?

A

Its ionotropic Kainate receptors receive glutamate from the photoreceptors (ribbon) in the dark →
allows channels to stay open → inward going current (depolarizing current)

When light strikes, the stop receiving glutamate → Kainate channels close → hyper polarization bc outward current takes over
“Copy Photoreceptors”

**Positive current = outwards = hyperpolarizing

24
Q

How do ON BC function?

A

Glutamate released from photoreceptor ribbon binds to metabotropic receptor (MGLUR6) → activates G-protein → closes non-selective cation channel TRPM1 → no inward current

When light strikes → No glutamate → No inhibition of TRPM1 → opening of cation channels → inward current → depolarization

“Inverts photoreceptors”

25
Q

Why does OFF BC have a baseline current and not ON BC?

A

In the dark, Ribbon synapse has a resting rate of release
OFF: In the dark, the cell is depolarized by glutamate binding from photoreceptors → inward current
ON: 0 current in the dark bc TRPM1 are inhibited so no current

26
Q

How do inputs combine each other in Bipolar cells?

A

Excitatory and inhibitory potentials sum linearly

27
Q

What is the equation for current in a receptive field?

A

I(x) = Ipeak * e^(-(x)^2/2(R/4)^2)

28
Q

Do ganglion cells respond better to well-aimed small spots or to diffuse light?

A

well-aimed small spots in the excitatory regions

29
Q

How are Ganglion cells organized ?

A

In a Moosaic fashion with amacrine cells overlapping
ON are on 1 layer and OFF are on another layer

30
Q

Describe the receptive field of a RGC

A

Photoreceptor current = center
Suround = amacrine cells

31
Q

What is the Outer plexiform layer and the Inner plexiform layer?

A

Outer plexiform layer = where photoreceptors bind to BC

Inner plexiform layer = where BC bind to RGC and amacrine cells

*Axons of RGC join in the optic nerve

32
Q

What are starburst amacrine cells?

A

*Has distinct dendritic morphology
Can get input from wide area to detect motion
Synapses onto direction selective RGCs

33
Q

What do amacrine cells release?

A

release GABA when excited

34
Q

What is Channel Rhodopsin and Normal Rhodopsin?

A
  • Discovered in blue-green algae that could sense light
  • 7-Transmembrane protein (instead of being G-protein, they’re ion channel)
  • Binds all-trans retinal → 13-cis when activated → open pore
  • light sensitive ion channel → spiking in response to light
35
Q

What is the Quantum efficiency of Channel Rhodopsin?

A

Q.E = 0.5
*Also has wavelength preferences

36
Q

How can pore be mutated for different uses?

A

Can be mutated to let different ions pass (identify bc have different E rev)

37
Q

Where do the ON-RGC and OFF-RGC synapse with the PR and amacrine cells?

A

In the Inner Plexiform Layer
ON-RGC synapse in the inner IPL

OFF-RGC synapse in the outer IPL

38
Q

What are the 4 main directions of the direction selective RGC?

A

For mvt:
- Toward the ears (~10˚)
- Above the head (~110˚)
- Towards the nose (~190-200˚)
- Towards belly (~270˚)
*Not all exactly at this angle but sum up to this prefered direction

39
Q

What is the shape of Starburst amacrine cell?
How is its inhibition described?

A

nearly perfect circle-like dendritic arbor

Gives assymetric inhibition to Direction Selective RGC
Release of acetylcholine from Starburst cell is not symmetric everywhere

40
Q

Without starburst amacrine cells, can we still have direction selectivity from RGCs?

A

No, destroying starbursts detroys direction selectivity

41
Q

How can we find the magnitude of the response? normalized

A

(x̄, ȳ) = (sum(x)/sum (|x|), sum(y)/sum(|y|))

response = sqrt(x̄^2 + ȳ^2)

θ = tan-1(ȳ/x̄)

42
Q

Explain how an OFF directive RGC would differ from an ON directive BC

A

Surround is ON and center is OFF so the prefered direction would be the one that hits the starburst amacrine cells first

43
Q

What is the Quantum Efficiency of Channel Rhodopsin?

A

0.5

*Also has wavelength preference

44
Q

How can we know a channel is a mixed cation channel?

A

Its I-V curve reverses around 0

45
Q

What is E rev of a Cl channel?

A

-80 mV (reversal point on I-V)

46
Q

What is important to consider when we are expressing Channel Rhodopsin in a cell?

A

resting Rin changes because we are adding channels

1/Rin new = 1/Rin + 1/R of channel Rhodopsin = 1/Rin + G (single channel)*number channels added

47
Q

What is the conductance of a single channel Rhodopsin?

A

G = 100 fS

48
Q

How can we calculate the Steady-State depolarization following the addition of ChR to a membrane?

A
  1. Calculate new resistance
  2. Depending on I-V curve of ChR, find current of the channel at resting Vm
  3. V = IR gives you change in Vm
  4. Consider if hyper- or depolarizing
49
Q

Related to Channel Rhodopsin, what do we need to do if we are given a calculated whole-cell photocurrent and Vm with the I-V curve of the ChR?

A

Calculate I = G(Vm - Rev)
- I being the current induced by the change in membrane resistance du to the addition of ChR
- G being the total conductance of added channels = single channel conductance#channels

50
Q

When we talk about whole-cell current, what do we talk about?

A

It is current induced by a change in cell properties (ex: Rin)

PHGY 311 (3 decks)

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