dark adaptation

Question 1

what is dark adaptation

Answer 1

how your eyes adapt to the dark to see in dimmer lighting conditions

Question 2

describe the structure of photoreceptors

Answer 2

the outer segment contains membranous discs - invaginations of the cell membrane which contain photopigments - colour pigments which absorb light

• rhodopsin (rods)

photoopsin (cones)

each photopigment contains

light absorbing portion (chromophore)- i.e. - activated by light - retinal

and a protein called opsin

Question 3

what are the types of photopsin in cones

Answer 3

3 different types of photopsin in cones

corresponding to different wavelenghs of light

opn1nw - green

opn1lw - red

opn1sw- blue

rods - rhodopsin

therefore 4 different types of photoreceptor

Question 4

what does each photopigment contain

Answer 4

each photopigment contains - a light absorbing portion - (chromophore) - retinal

and a protein called opsin

Question 5

what happens to retinal in phototransduction

Answer 5

• retinal is the light absorbing portion (chromophere) - in a photoreceptor
• • a photon of light hits retinal meaning that 11 cis retinal isomerises to all - trans retinal

Question 6

describe photoreceptors in dark conditions

Answer 6

• in dark conditions - photoreceptors can be described as on - but because it is releasing a inhibitory neurotransmitter glutatmate

Question 7

describe the state of photoreceptors on light conditions

Answer 7

relatively hyperpolarised

stops the release of glutatmate impulse

Question 8

what is bleaching

Answer 8

light causes disscoiation of 11 cis retinal with the opsin and conversion to 11 all trans retinal

11 trans retinal transported to the rpe to be converted back into 11 cis retinal then transported to the outer segement

until 11 cis retinal is bound again to its opsin - rhodopsin cannot detect more light

this bleaching means rhodopsin struggles to operate in bright conditions

Question 9

describe phototransduction in the dark

Answer 9

• there= high levels of cGMP

these bind to sodium channels and keep them open - causing a influx of sodium- causing depolarisation- causes release of nt called glutamate - transmitter causes graded potentials in bipolar cells

Question 10

describe phototransduction in the light

Answer 10

• light = absorbed by the photopigment

hits 11 cis retinal turns into 11 trans retinal and dissociates from its opsin

• levels of cGMP decrease

sodium channels close - less sodium entering the cell

the cell hyperpolarises

calcium channels close

stops the release of glutamate

tells bipolar cells that their = light

Question 11

what is bleaching a result of

Answer 11

• it takes a long time for retinal and opsin to be turned back into their orginal configuration

Question 12

describe the function of rods and its opsin

Answer 12

rods give black and white vision/ night vision and sense contrast and brightness motion - their opsin is rhodopsin

Question 13

what is the function of cones and its opsin

Answer 13

cones give fine resolution and colour vision - they have three opsins absorbing different wavelengths to provide a visible spectrum

Question 14

what do discs contain

Answer 14

• discs contain the photopigmenrs and are found in the photoreceptor outer segments

Question 15

what is phototransduction

Answer 15

phototransduction is the conversion of light to a neural impluse

Question 16

what happens when retinal absorbs a photon

Answer 16

retinal absorbs a photon , causing a cascade resulting in hyperpolarisation of the cell and cessation of glutamate release (inhibitory neurotransmitter)

Question 17

what is bleaching

Answer 17

bleaching means that rhodposin is unable to operate under bright conditions

Question 18

define dark adaptation

Answer 18

the process including dilation of the pupil , increase in sensitivity of the retinal rods and regeneration of rhodopsin by which the eye adapts to conditions of reduced illumination

Question 19

what are the three types of vision

Answer 19

photopic - cones dominate , bright light

scotopic - rods dominate low light

mesopic - overlap between the two - both cones and rods- twilight/ transitional

Question 20

describe how the different photoreceptors operate in different lighting conditions

Answer 20

rods respond to low luminance light - dominate scotopic vision

cones respond to high luminance light and they dominate photopic vision

mescopic vision is the range where the two are both working together

Question 21

how is luminance defined and measured

Answer 21

luminance is the measurement of the brightnes of light emitted from a light source

illuminance is a measurment if how bright the reflection is of emitted light from an object

unit = candela/m2

Question 22

describe range of luminance

Answer 22

in photopic condtiions it goes up to 3. 000,000 cd/m2

in mesoscopic it goes up to 3 cd/m2

in scotopic it gors up to 0.0003cd/m2

Question 23

what is the process of dark adaptation

Answer 23

moving from light into dark conditions causes a period of poor vision

process wherby the eye becomes more sensitive to light in areas of low luminance is dark adpatation

Question 24

what is visual threshold

Answer 24

the sensitivity of the eye to light can be described using the visual threshold

the minimum luminance of a test spot that can be percieved (produce a visual sensation) - minimum amount of light we can see

in the dark the eye becomes progressivley more sensitive to light stimulation until the threshold reaches a minimum after approx 30 mins

Question 25

what happens to the eye in light

Answer 25

pupil is constricted to focus light onto the cones rhodopsin = bleached

Question 26

what happens to the pupil in the dark

Answer 26

pupillary dilation - maximises light and allows it to hit peripheral rods regeneration of 11 cis retinal bound with its opsin

Question 27

how is dark adaptation tested

Answer 27

light adaapt observer to bleach photopigment large flashing 420nm wavelength light against dark background (stimulates both rods and cones) observer decreases luminance of test light until it is barley percieved and continues to do so

Question 28

describe the dark adaptation curve

Answer 28

2 distinct regions of recovery seperated by rod - cone break 1st component resulting from increase in cone senstivity 2nd component resulting from increase in rod sensitivity minmium threshold reached after 30 mins under these conditions we say that the photopigments are fully regenerate visual threshold improves by 6 log units - 1, 000,000 times rods are adpating during the cone phase - however cones are more sensitive at this point due to rod bleaching cone plateau - 5-8 mins is the visual threshold for the cone system rod plateau- 30 mins rod threshold

Question 29

what factors affect dark adpation

Answer 29

duration and intensity of pre- exposure size/location of test spot length of exposure of test light retinal positon stimulus wavelength pupil size age

Question 30

what is the limiting factor of dark adpatation

Answer 30

the dark adpation and regeneration of rhodopsin are dependent on the local conc of 11 cis retinal - this is quicker in cones which is why the first phase is in cones limiting factor for recovery after a large bleach is rate of 11- cis retinal is delivered to opsin in bleached photoreceptors

Question 31

what is night blindess called

Answer 31

night blindness is referred to as nyctalopia retinis pigmentosa - rod cells gradually lose function resulting in progressive nyctalopia and loss of peripheral visual field vitamin a defficency - vit a is necessary for regeneration of 11 cis retinal

Question 32

what are the three types

Answer 32

photopic - daytime - cones ] mesoscopic - twilight/ transitional - cones and rods scotopic - night vision - rods only

Question 33

what does visual threshold refer to

Answer 33

the minimum luminance of a test spot that can be percieved (produce a visual sensation) is the visual threshold

Question 34

what does dark adaptation involve

Answer 34

dark adaptation involves pupillary dilation and regeneration of 11 cis retinal

Question 35

what do the two discreete area of recovery on the classic dark adaption curve show

Answer 35

- they show a cone recovery phase and a rod recovery phase seperated by the rod cone break

Question 36

when is max cone sensitvity and max rod sensitivity reached

Answer 36

max cone sensitivity is reached after 8 mins - followed by rod sensation until the rod plateau - minimum threshold after 30 mins at this the point is maximally dark adapted