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Neuroscience > Lecture 15 > Flashcards

Lecture 15 Flashcards

1
Q

What is the laminar organization of the brain from the outer side to the inner side?

A 
Ganglion cell layer
Inner plexiform layer
Inner nuclear layer
Outer Plexiform layer
Outer nuclear layer
Layer of photoreceptor outer segments
Pigmented epithelium
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2
Q

What must light focused on the retina be converted into?

A

Neural activity

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3
Q

What must light pass through before it reaches the photoreceptors?

A

Ganglion cells

Bipolar cells

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4
Q

Where is the light absorbed once it has passed through the retina?

A

Pigmented epithelium

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5
Q

What increases visual acuity? i.e. clarity of image

A

Preventing light from bouncing back into the retina

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6
Q

What does the pigmented epithelium do?

A

Absorbed light to decrease resolution. This increases reflection of light (Cat’s eyes at night)

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7
Q

What spread light to different cells and are important in processing?

A

Amacrine and horizontal cells

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8
Q

What does the retina do as a displaced part of the CNS?

A

It has lots of processing involved before moving to the CNS

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9
Q

Some animals have reflective layers underneath photoreceptors but what do they do?

A

Reflects the light to increase the sensitivity to low light levels BUT THIS COMPROMISES ACUITY

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10
Q

What are photoreceptors?

A

Sensory transducers, rods and cones

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11
Q

What are horizontal cells?

A

Input from and output to photoreceptors, output to bipolar cells

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12
Q

What are bipolar cells?

A

Connect photoreceptors to ganglion cells

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13
Q

What are amacrine cells?

A

Input from bipolar cells, influence ganglion cells, bipolar cells and other amacrine cells

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14
Q

What are ganglion cells?

A

output from retina

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15
Q

What are the main structural features of rods?

A

Outer membranes have discs

  • membraneous free-floating discs are where sensory transduction occurs
  • contain light-sensitive photopigments that absorb light
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16
Q

What are the main structural features of cones?

A

Foldings of membranes where photons are absorbed

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17
Q

What is the Duplicity theory?

A

Can’t have high sensitivity and high resolution in single receptor
Thus separate systems for monochrome and color

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18
Q

What processes monochrome light?

A

Sensitive to light
In dim-light conditions, sight is in black-and-white
Only using rods

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19
Q

What processes color?

A

Has high resolution

Uses cones

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20
Q

What are main features of rods?

A

Greater number of discs
Higher photopigment concentration
1000 times more sensitive to light than cones
Enable vision in low light (scotopic) conditions
i.e. night time
Low visual acuity/resolution
~ 92 million rods in each human retina

21
Q

What are main features of cones?

A 
Fewer discs
Used during daylight (photopic) conditions
Enable color vision
Lower sensitivity 
High visual acuity/resolution
~ 5 million cones in human retina
22
Q

Which part of the eye contains most of the cones?

A

The fovea

  • contains most of the 5 million cones
  • no rods (highest resolution)
23
Q

What forms central retina ?

A

Low convergence + high resolution

24
Q

What forms peripheral retina?

A

high convergence + low resolution

25
Q

Where does convergence occur?

A

Convergence of photoreceptor on retinal ganglion cell

26
Q

How does low convergence happen?

A

One cone feeds one ganglion cell

  • visual cortex will know that one ganglion cell is related to one photoreceptor
  • easy to detect which cone is used
27
Q

How does high convergence happen?

A

Multiple bipolar cells providing input to one ganglion cell

  • light is being received by tiny bit of retina
  • can’t tell which rod was used
28
Q

What is the absorbance spectrum of human rod photopigments?

A

Rhodopsin

- peak at 500nm (green)

29
Q

What is the absorbance spectrum of human cone photopigment?

A

Three varieties of opsins - S,M,L
- peaks: S - 420nm (blue)
M - 530nm (light green)
L - 560nm (yellow)

30
Q

What is the absorbance spectrum of human retinal ganglion photopigment?

A

Melanopsin

  • can sense whether we are in dark or light condition (controls circadian rhythm)
  • peak at 475nm (light blue)
31
Q

How is phototransduction controlled in the dark?

A

Rods are depolarized
Due to a Na reflux (but a bit of Ca involved)
Known as dark current
- keeps membrane less negative (-30mV)
- voltage-gated Ca channels open
- Ca come and activate vesicle release of neurotransmitter
Maintained by cGMP
- cGMP binds to Na channels to keep it open and let more Na in

32
Q

How is phototransduction controlled in the light?

A 
cGMP levels are decreased
 - phosphodiesterase breaks down cGMP
 - dark current gone
Na channels close
Na influx prevented
Rods are hyperpolarized (-60mV)
Voltage-gated Ca channels close
Neurotransmitter isn't released
Membrane does come back a bit during light condition - adaptation
33
Q

What is activated by light?

A

Rhodopsin

- composed of retinal and opsin

34
Q

What does rhodopsin stimulate?

A

G-protein, transducin, to become transducin GTP

  • changes conformation of opsin
  • activates rhodopsin
  • acts like GPCR to initiate enzyme cascade
35
Q

What subunits does transducin have?

A

alpha, beta and gamma

36
Q

What does the alpha subunit of transducin activate?

A

Enzyme phosphodiesterase (PDE)

37
Q

What does PDE do?

A

PDE reduces cGMP levels, closing Na channels

38
Q

How is cGMP produced in the dark?

A

Enzyme guanylyl cyclase

39
Q

What is saturation?

A

When rods cannot process bright light as they become saturated
cGMP levels are so low that no additional hyperpolarization can occur
Cones are not saturated as easily, so are used in bright light

40
Q

What is light adaptation?

A

Photoreceptors initially hyperpolarize greatly

Photoreceptors gradually depolarize with continued bright light

41
Q

What does light adaptation need?

A

calcium

42
Q

How does light adaptation happen in the dark?

A

Ca normally enters cells and block guanylyl cyclase
This reduces cGMP production, so closes some ion channels
- guanylyl cyclase normally produces cGMP from GTP

43
Q

How does light adaptation happen in the light?

A

Channels are shut so Ca cannot enter cells
Block on guanylyl cyclase is released
More cGMP produced = more channels open

44
Q

Is feedback loop constantly regulated?

A

yes

45
Q

What are the two types of bipolar cells?

A

ON cells
- hyperpolarized in the dark, ON in light
OFF cells
- depolarized in the dark, OFF in light

46
Q

What are the signal from bipolar cells transferred to?

A

Retinal ganglion cells

47
Q

What do retinal ganglion cells do?

A

The only cells to fire action potentials in the retina

48
Q

How is light involved in circadian rhythm?

A

Daily light - cycle - entrains circadian rhythms
Changes in day length and shift work can cause mood alterations and cognitive deficits
Aberrant light cycles also increase depression, impair LTP in hippocampus and impair leaving

49
Q

What appeared in the news in relation to vision?

A

How stem cells may cure blindness

- ESC: Cells in retinal pigment epithelia. These are embedded underneath the rods and cones

Neuroscience (28 decks)

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