L27 The Retina

Question 1

what makes up the structure of the photoreceptors?

Answer 1

outer segment

inner segment

Question 2

describe the outer segment of the photoreceptors

Answer 2

they are oriented towards the RPE

contain visual pigment for photoreceptor transduction

Question 3

describe the inner segment of the photoreceptors

Answer 3

directed toward the inside/center of eyeball

forms synapses which transmit the visual info on the retinal cells (bipolar/horizontal cells)

Question 4

what NT is released from retinal photoreceptors?

Answer 4

glutamate

Question 5

is glutamate released in the light or dark?

Answer 5

dark - light reduces the release of glutamate

Question 6

what is disk shedding?

Answer 6

the outer segment renews disks from the bottom up

the top disks are shed and phagocytozed by RPE cells

Question 7

what is the main function of rods? describe the characteristics

Answer 7

to enable vision in the dark ->
cellular amplification mechanism is well developed which increases light sensitivity
temporal summation is poor (distinguish between 2 flashes of light)
convergence is high
only 1 type

Question 8

what are the characteristics of cones

Answer 8

amplification is low = less sensitive to light
works better in bright conditions
3 different types of cones that allow color vision
convergence is low = increasing spatial resolution (visual acuity) showing better vision in the light

Question 9

are rods and cones evenly distributed?

Answer 9

NO

NO rods in the FOVEA = no central vision in the dark

Question 10

what is rhodopsin

Answer 10

the visual pigment of rods (2 components)

Question 11

what are the 2 components of rhodopsin?

Answer 11

opsin

retinal

Question 12

what is opsin

Answer 12

protein synthesized in the photoreceptor

have 7 membrane spanning domains

Question 13

what is retinal

Answer 13

light absorbing compound derived from vitamin A and is the chromopore of the visual pigment

covalently attached to the 7th domain of opsin

Question 14

what is the mechanism that occurs in the dark

Answer 14

visual pigment inactive– G-protein inactive – cGMP phosphodiesterase inactive – intracellular cGMP CAN gate the cGMP-gated Na+ channel to OPEN = depolarization — glutamate released

Question 15

when do photoreceptors depolarize

Answer 15

in the dark

Question 16

when do photoreceptors hyperpolarize?

Answer 16

in the light

Question 17

what is the mechanism that occurs in the light

Answer 17

visual pigment active – G-protein (transducin) active – cGMP phosphodiesterase – metabolizes intracellular cGMP – close Na+ channel - hyperpolarize == stopping glutamate!

Question 18

describe blue cones

Answer 18

aka S cones

have max sensitivity to short waves 430 nm = BLUE light

Question 19

describe green cones

Answer 19

aka M cones

max sensitivity to medium waves 530 nm = GREEN light

Question 20

describe red cones

Answer 20

aka L-cones

max sensitivity to long waves 560 nm = RED light

Question 21

where is rhodopsin park of

Answer 21

disk membrane of photoreceptors

Question 22

what is the first step in the phototransduction process in the absorbtion of light

Answer 22

absorption of light causes a conformational change of the retinal molecules from inactive 11-cis isomer –> active all-trans isomer

Question 23

describe relative stimulation

Answer 23

relative hyperpolarization of the different cones will determine the color perception of the person

there is a mix of stimulation though regardless of the color, there is at least some stimulation of ALL THREE

Question 24

what is the visible range of light

Answer 24

400-700nm

Question 25

what is monochromatic light

Answer 25

light with only 1 wavelength - very rare occurs in different bands of rainbow artificial light produced by a laser

Question 26

what are the 5 major cells in the retina

Answer 26

``` retinal photoreceptors bipolar cells horizontal cels amacrine cells retainal ganglion cells ```

Question 27

describe retinal photoreceptors

Answer 27

rods and cones are hte input cells of the retina depolarized during dark hyperpolarized during light

Question 28

describe bipolar cells

Answer 28

retinal photoreceptors synapse on these | in the outer plexiform layers which transmits their info to the inner plexiform layer on the retinal ganglion cells

Question 29

describe horizontal cells

Answer 29

crucial for the "indirect" wiring pathway and are responsible for "lateral" inhibition

Question 30

describe amacrine cell

Answer 30

work like the horizontal cells only they are found in the inner plexiform layer

Question 31

describe retinal ganglion cells

Answer 31

output cells of the retina | transmit info to the LGN via the optic nerve, chiasm and tracts

Question 32

why are on bipolar cells called that

Answer 32

because they are depolarized in the light is ON

Question 33

why are off bipolar cells called that

Answer 33

because they are depolarized in the dark when the light is OFF

Question 34

what does the depolarization or hyperpolarization of the bipolar cells depend on

Answer 34

the synapse between the photoreceptor and the bipolar cells

Question 35

describe off bipolar cells

Answer 35

have ionotropic glutamate receptos which is stimulated by the release of glutamate = depolarize the cell

Question 36

describe on bipolar cells

Answer 36

metabotropic glutamate receptors which have an inhibitory effect when stimulated by the release of glutamate form photoreceptors

Question 37

describe horizontal cells

Answer 37

responsible for the lateral inhibitory effect needed to depolarize an ON bipolar cells in the event that light has not been places directly in its receptive field but in the surrounding area

Question 38

what is retinitis pigmentosa

Answer 38

genetic disease in which rods preferentially degenerate night blindness = earliest symptoms peripheral vision tunnel vision total blindness accumulation of pigment seen in exam photoreceptor degeneration is not cleaned up by phagocytes of the RPE

Question 39

what is night blindness

Answer 39

nyctalopia effects vitamin A deficient people as the retinal is derived from it a very necessary part of opsin/retinal combinatin that forms the photoreceptor pigment

Question 40

what is color blindness

Answer 40

lack of a particular type of cone most common is x-linked red-green color blindness prevalent in males protanopia - loss of red cone deuteranopia - loss of the green cone

Question 41

define scotoma?

Answer 41

area of lost or depressed vision within the visual field, surrounded by an area of less depressed or normal vision (pathologic blind spot)

Question 42

define temporal resolution

Answer 42

the ability to distinguish subsequent stimuli from each other (temporal: relating to time)

Question 43

define spatial resolution

Answer 43

the ability to distinguish adjacent stimuli from each other (spatial: relating to space)