L27 The Retina Flashcards
what makes up the structure of the photoreceptors?
outer segment
inner segment
describe the outer segment of the photoreceptors
they are oriented towards the RPE
contain visual pigment for photoreceptor transduction
describe the inner segment of the photoreceptors
directed toward the inside/center of eyeball
forms synapses which transmit the visual info on the retinal cells (bipolar/horizontal cells)
what NT is released from retinal photoreceptors?
glutamate
is glutamate released in the light or dark?
dark - light reduces the release of glutamate
what is disk shedding?
the outer segment renews disks from the bottom up
the top disks are shed and phagocytozed by RPE cells
what is the main function of rods? describe the characteristics
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
what are the characteristics of cones
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
are rods and cones evenly distributed?
NO
NO rods in the FOVEA = no central vision in the dark
what is rhodopsin
the visual pigment of rods (2 components)
what are the 2 components of rhodopsin?
opsin
retinal
what is opsin
protein synthesized in the photoreceptor
have 7 membrane spanning domains
what is retinal
light absorbing compound derived from vitamin A and is the chromopore of the visual pigment
covalently attached to the 7th domain of opsin
what is the mechanism that occurs in the dark
visual pigment inactive– G-protein inactive – cGMP phosphodiesterase inactive – intracellular cGMP CAN gate the cGMP-gated Na+ channel to OPEN = depolarization — glutamate released
when do photoreceptors depolarize
in the dark
when do photoreceptors hyperpolarize?
in the light
what is the mechanism that occurs in the light
visual pigment active – G-protein (transducin) active – cGMP phosphodiesterase – metabolizes intracellular cGMP – close Na+ channel - hyperpolarize == stopping glutamate!
describe blue cones
aka S cones
have max sensitivity to short waves 430 nm = BLUE light
describe green cones
aka M cones
max sensitivity to medium waves 530 nm = GREEN light
describe red cones
aka L-cones
max sensitivity to long waves 560 nm = RED light
where is rhodopsin park of
disk membrane of photoreceptors
what is the first step in the phototransduction process in the absorbtion of light
absorption of light causes a conformational change of the retinal molecules from inactive 11-cis isomer –> active all-trans isomer
describe relative stimulation
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
what is the visible range of light
400-700nm
what is monochromatic light
light with only 1 wavelength - very rare
occurs in different bands of rainbow
artificial light produced by a laser
what are the 5 major cells in the retina
retinal photoreceptors bipolar cells horizontal cels amacrine cells retainal ganglion cells
describe retinal photoreceptors
rods and cones
are hte input cells of the retina
depolarized during dark
hyperpolarized during light
describe bipolar cells
retinal photoreceptors synapse on these
in the outer plexiform layers which transmits their info to the inner plexiform layer on the retinal ganglion cells
describe horizontal cells
crucial for the “indirect” wiring pathway and are responsible for “lateral” inhibition
describe amacrine cell
work like the horizontal cells only they are found in the inner plexiform layer
describe retinal ganglion cells
output cells of the retina
transmit info to the LGN via the optic nerve, chiasm and tracts
why are on bipolar cells called that
because they are depolarized in the light is ON
why are off bipolar cells called that
because they are depolarized in the dark when the light is OFF
what does the depolarization or hyperpolarization of the bipolar cells depend on
the synapse between the photoreceptor and the bipolar cells
describe off bipolar cells
have ionotropic glutamate receptos which is stimulated by the release of glutamate = depolarize the cell
describe on bipolar cells
metabotropic glutamate receptors which have an inhibitory effect when stimulated by the release of glutamate form photoreceptors
describe horizontal cells
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
what is retinitis pigmentosa
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
what is night blindness
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
what is color blindness
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
define scotoma?
area of lost or depressed vision within the visual field, surrounded by an area of less depressed or normal vision (pathologic blind spot)
define temporal resolution
the ability to distinguish subsequent stimuli from each other (temporal: relating to time)
define spatial resolution
the ability to distinguish adjacent stimuli from each other (spatial: relating to space)
