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Neuroscience > Module D-06 > Flashcards

Module D-06 Flashcards

1
Q

the 2 types of photoreceptors

A

Rods and cones

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2
Q

Describe the parts of the photoreceptor cells

A

1) outer segments are oriented towards the retinal pigment epithelium (RPE)
2) inner segments towards the interior of the eye ball

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3
Q

Where is the visual pigment for phototransduction found?

A

In the outer segment

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4
Q

What are rod spherules and cone pedicles?

A

the inner segments form synapses transmitting the visual information on to the subsequent cells of the retina

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5
Q

Which cells do the photoreceptors synapse onto?

A

bipolar cells and horizontal cells

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6
Q

Which neurotransmitter is released by the photoreceptors?

A

Glutamate

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7
Q

Under what conditions is glutamate released from the photoreceptors?

A

Under Dark but reduced in light

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8
Q

What conditions do rods let us see?

A

highly sensitive to light and enable us to see under low intensity light conditions

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9
Q

What conditions do cones let us see?

A

less sensitive to light

work best at higher light intensities

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10
Q

In which of the photoreceptors is signal amplification developed?

A

rods

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11
Q

which of the photoreceptors has higher temporal resolution?

A

Cones

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12
Q

how many types of Cones and rods are there?

A

3 types of cones

1 type of rods

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13
Q

In which of the photoreceptors is convergence higher?

A

Rods

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14
Q

Fovea only contains ________

A

Cones

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15
Q

Why is there no central vision in dim light conditions?

A

because the fovea only contains cones no rods

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16
Q

Describe the process of Disk Shedding

A
  • stacks of disks containing visual pigment molecules in the outer segments of
    photoreceptors are constantly renewed.
  • New disks are added at the base of the
    outer segment, while old disks are displaced up the outer segment and are
    pinched off at the tips.
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17
Q

What happens to the discarded discs ?

A

Phagocytosed by the retinal pigment epithelium (RPE) cells

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18
Q

What is the visual pigment of rods?

A

Rhodopsin, found on the disk membrane

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19
Q

What are the two components of Rhodopsin?

A

1) Opsin, a protein which is synthesized
in the photoreceptor(cones have different
types of opsins).
2) Retinal, a chromophore, is the light absorbing compound or the visual pigment (of both rods on cones).

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20
Q

What is Retinal derived from?

A

Vitamin A which is synthesized from Beta carotene in our food

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21
Q

Describe structure of Opsin molecule

A

A seven membrane spanning domains.
Its amino terminal is located in the disk interior,
its carboxy terminal in the cytoplasm of the
photoreceptor

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22
Q

Describe the structure of Retinal

A

covalently attached to one of the amino acids of the seventh membrane spanning region of the protein

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23
Q

What differs Photoreceptors from other sensory receptors?

A

depolarized during darkness, i.e. in the absence of their adequate “stimulus”

24
Q

What is the name of the G protein involved in phototransduction?

A

Transducin

25
Q

Describe the state of the photoreceptor during darkness , ie, the dark current?

A

1)the visual pigment, which is coupled to a G protein is in its inactive state
2)the G protein does not activate the enzyme cGMP phosphodiesterase.
3) As a consequence, there is plenty of cytoplasmic cGMP (cyclic guanosine 3’5’ monophosphate) available, which keeps the cGMP gated channels in the photoreceptor membrane open.
4) This allows a continuous inward current of sodium ions,
5)photoreceptors stay depolarized in the dark.
release their neurotransmitter (glutamate)

26
Q

Describe the Phototransduction Process

A

1) Light is absorbed by the visual pigment
2) causes a conformational change of the retinal molecule from its inactive 11-cis isomer to its active all-trans isomer.
3) Visual pigment, the enzyme cGMP phosphodiesterase is activated via the G
protein.
4) The enzyme activity causes a breakdown of
cytoplasmic second messenger molecule cGMP, metabolizing it to 5’ GMP.
5) Drop in cGMP concentration in the cytoplasm causes closing of cGMP gated channels
6)Photoreceptors are hyperpolarized
7) Reduce (or terminate) the release of their neurotransmitter

27
Q

What is the range of the visible part of the light spectrum?

A

400-700 nm

28
Q

What are the 3 cone types?

A

1) S (short wavelength sensitive) cones- “blue”
2) M (medium wavelength sensitive) cones-
“green” cones
3)L (long wavelength sensitive) cones- red

29
Q

What is the maximum sensitivity of S cones?

A

430 nm

30
Q

What is the maximum sensitivity of M cones?

A

530 nm

31
Q

What is the maximum sensitivity of L cones?

A

560 nm

32
Q

What is Retinitis Pigmentosa?

A 
genetically determined (autosomal dominant,
autosomal recessive, and X linked recessive) degenerative diseases, in which rods preferentially degenerate
33
Q

What are the symptoms of Retinitis Pigmentosa

A
  • starts with night blindness, then loss of peripheral vision (tunnel vision) then blindness
34
Q

What causes photoreceptor degeneration in retinitis pigmentosa?

A

Accumulation of pigment
Reduced phagocytosis by RPE cells during
the process of disk shedding

35
Q

What is Nyctalopia?

A

Night Blindness

36
Q

What causes night blindness?

A

Vitamin A deficiency

37
Q

Which is the most common type of color blindness?

A

Red -green color blindness (X linked recessive)

38
Q

What are the two types of red-green color-blindness?

A

Protanopia: L cone (“red cone”) absent, 1.3% of males
Deuteranopia: M cone (“green cone”) absent, 1.2% of males

39
Q

What are the 5 cell types in the retina?

A

1) Photoreceptor
2) Horizontal cell
3) Bipolar cell
4) Amacrine cell
5) Ganglion cell

40
Q

In what layer of the retina do photoreceptors synapse with bipolar cells?

A

Outer plexiform layer

41
Q

In what layer of the retina do Bipolar cells synapse with retinal ganglion cells?

A

Inner plexiform layer

42
Q

In what layer of the retina are Horizontal cells found?

A

Inner plexiform layer

43
Q

What type of cell are the horizontal cells?

A

inhibitory interneurons involved in the center / surround organization of the receptive fields
of bipolar cells

44
Q

In what layer of Amacrine cells found?

A

the inner plexiform layer

45
Q

What type of potentials are produced in the cell types of the retins

A

Only graded potentials , no APs, except retinal ganglion cells which move out of the retina and produce APs

46
Q

where do the Retinal ganglion cells synapse ?

A

lateral geniculate nucleus (LGN) of the diencephalon

47
Q

What are the 2 types of retinal bipolar cells?

A

1) ON bipolar cells- depolarized when light is on

2) OFF bipolar cells- depol. when light is off

48
Q

Glutamate receptors on the postsynaptic membrane of OFF bipolar cells are _______
receptors

A

ionotropic

49
Q

Effect of glutamate on OFF bipolar cells

A

an excitatory transmitter, depolarized during

darkness

50
Q

Glutamate receptors on the postsynaptic membrane of ON bipolar cells are ____________

A

metabotropic type

51
Q

Effect of glutamate on ON bipolar cells

A

an inhibitory transmitter, depolarized during

light

52
Q

Describe how photoreceptors and horizontal cells participate in opposite effect of a stimulus in the area surrounding a receptive field

A

When shining light into the surround area of the receptive field of the ON center bipolar cell the photoreceptors in
the periphery form excitatory (sign conserving) synapses on adjacent horizontal cells. These horizontal cells in turn form inhibitory (sign converting) synapses in the indirect wiring pathway

53
Q

Describe signal transduction on an ON center ganglion cell when light is on its receptive field

A

Photoreceptor hyperpolarizes during light ON => ON center bipolar cell is depolarized => excitatory (sign conserving) synapse with ON center ganglion cell => generate AP, increases AP frequency

54
Q

Describe the effect on an ON center ganglion cell when light is on its Periphery

A

reduces frequency of APs (APs over time)

55
Q

Describe signal transduction on an OFF center ganglion cell when light is on its receptive field

A

Photoreceptor hyperpolarizes during light ON => OFF center bipolar cell is depolarized => excitatory (sign conserving) synapse with OFF center ganglion cell => decreases AP frequency

56
Q

Describe the effect on an OFF center ganglion cell when light is on its Periphery

A

Increases frequency of APs

Neuroscience (32 decks)

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