dark adaptation Flashcards
what is dark adaptation
how your eyes adapt to the dark to see in dimmer lighting conditions
describe the structure of photoreceptors
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
what are the types of photopsin in cones
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
what does each photopigment contain
each photopigment contains - a light absorbing portion - (chromophore) - retinal
and a protein called opsin
what happens to retinal in phototransduction
- 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
describe photoreceptors in dark conditions
- in dark conditions - photoreceptors can be described as on - but because it is releasing a inhibitory neurotransmitter glutatmate
describe the state of photoreceptors on light conditions
relatively hyperpolarised
stops the release of glutatmate impulse
what is bleaching
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
describe phototransduction in the dark
- 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
describe phototransduction in the light
- 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
what is bleaching a result of
- it takes a long time for retinal and opsin to be turned back into their orginal configuration
describe the function of rods and its opsin
rods give black and white vision/ night vision and sense contrast and brightness motion - their opsin is rhodopsin
what is the function of cones and its opsin
cones give fine resolution and colour vision - they have three opsins absorbing different wavelengths to provide a visible spectrum
what do discs contain
- discs contain the photopigmenrs and are found in the photoreceptor outer segments
what is phototransduction
phototransduction is the conversion of light to a neural impluse
what happens when retinal absorbs a photon
retinal absorbs a photon , causing a cascade resulting in hyperpolarisation of the cell and cessation of glutamate release (inhibitory neurotransmitter)
what is bleaching
bleaching means that rhodposin is unable to operate under bright conditions
define dark adaptation
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
what are the three types of vision
photopic - cones dominate , bright light
scotopic - rods dominate low light
mesopic - overlap between the two - both cones and rods- twilight/ transitional
describe how the different photoreceptors operate in different lighting conditions
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
how is luminance defined and measured
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
describe range of luminance
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
what is the process of dark adaptation
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
what is visual threshold
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
what happens to the eye in light
pupil is constricted to focus light onto the cones
rhodopsin = bleached
what happens to the pupil in the dark
pupillary dilation - maximises light and allows it to hit peripheral rods
regeneration of 11 cis retinal bound with its opsin
how is dark adaptation tested
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
describe the dark adaptation curve
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
what factors affect dark adpation
duration and intensity of pre- exposure
size/location of test spot
length of exposure of test light
retinal positon
stimulus wavelength
pupil size
age
what is the limiting factor of dark adpatation
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
what is night blindess called
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
what are the three types
photopic - daytime - cones ]
mesoscopic - twilight/ transitional - cones and rods
scotopic - night vision - rods only
what does visual threshold refer to
the minimum luminance of a test spot that can be percieved (produce a visual sensation) is the visual threshold
what does dark adaptation involve
dark adaptation involves pupillary dilation and regeneration of 11 cis retinal
what do the two discreete area of recovery on the classic dark adaption curve show
- they show a cone recovery phase and a rod recovery phase seperated by the rod cone break
when is max cone sensitvity and max rod sensitivity reached
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
