Module D-06 Flashcards Preview

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Flashcards in Module D-06 Deck (56):
1

the 2 types of photoreceptors

Rods and cones

2

Describe the parts of the photoreceptor cells

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

3

Where is the visual pigment for phototransduction found?

In the outer segment

4

What are rod spherules and cone pedicles?

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

5

Which cells do the photoreceptors synapse onto?

bipolar cells and horizontal cells

6

Which neurotransmitter is released by the photoreceptors?

Glutamate

7

Under what conditions is glutamate released from the photoreceptors?

Under Dark but reduced in light

8

What conditions do rods let us see?

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

9

What conditions do cones let us see?

less sensitive to light
work best at higher light intensities

10

In which of the photoreceptors is signal amplification developed?

rods

11

which of the photoreceptors has higher temporal resolution?

Cones

12

how many types of Cones and rods are there?

3 types of cones
1 type of rods

13

In which of the photoreceptors is convergence higher?

Rods

14

Fovea only contains ________

Cones

15

Why is there no central vision in dim light conditions?

because the fovea only contains cones no rods

16

Describe the process of Disk Shedding

- 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.

17

What happens to the discarded discs ?

Phagocytosed by the retinal pigment epithelium (RPE) cells

18

What is the visual pigment of rods?

Rhodopsin, found on the disk membrane

19

What are the two components of Rhodopsin?

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).

20

What is Retinal derived from?

Vitamin A which is synthesized from Beta carotene in our food

21

Describe structure of Opsin molecule

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

22

Describe the structure of Retinal

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

23

What differs Photoreceptors from other sensory receptors?

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

24

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

Transducin

25

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

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

Describe the Phototransduction Process

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

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

400-700 nm

28

What are the 3 cone types?

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

29

What is the maximum sensitivity of S cones?

430 nm

30

What is the maximum sensitivity of M cones?

530 nm

31

What is the maximum sensitivity of L cones?

560 nm

32

What is Retinitis Pigmentosa?

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

33

What are the symptoms of Retinitis Pigmentosa

- starts with night blindness, then loss of peripheral vision (tunnel vision) then blindness

34

What causes photoreceptor degeneration in retinitis pigmentosa?

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

35

What is Nyctalopia?

Night Blindness

36

What causes night blindness?

Vitamin A deficiency

37

Which is the most common type of color blindness?

Red -green color blindness (X linked recessive)

38

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

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

39

What are the 5 cell types in the retina?

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

40

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

Outer plexiform layer

41

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

Inner plexiform layer

42

In what layer of the retina are Horizontal cells found?

Inner plexiform layer

43

What type of cell are the horizontal cells?

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

44

In what layer of Amacrine cells found?

the inner plexiform layer

45

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

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

46

where do the Retinal ganglion cells synapse ?

lateral geniculate nucleus (LGN) of the diencephalon

47

What are the 2 types of retinal bipolar cells?

1) ON bipolar cells- depolarized when light is on
2)OFF bipolar cells- depol. when light is off

48

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

ionotropic

49

Effect of glutamate on OFF bipolar cells

an excitatory transmitter, depolarized during
darkness

50

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

metabotropic type

51

Effect of glutamate on ON bipolar cells

an inhibitory transmitter, depolarized during
light

52

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

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

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

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

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

reduces frequency of APs (APs over time)

55

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

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

56

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

Increases frequency of APs