Module D-06 Flashcards
the 2 types of photoreceptors
Rods and cones
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
Where is the visual pigment for phototransduction found?
In the outer segment
What are rod spherules and cone pedicles?
the inner segments form synapses transmitting the visual information on to the subsequent cells of the retina
Which cells do the photoreceptors synapse onto?
bipolar cells and horizontal cells
Which neurotransmitter is released by the photoreceptors?
Glutamate
Under what conditions is glutamate released from the photoreceptors?
Under Dark but reduced in light
What conditions do rods let us see?
highly sensitive to light and enable us to see under low intensity light conditions
What conditions do cones let us see?
less sensitive to light
work best at higher light intensities
In which of the photoreceptors is signal amplification developed?
rods
which of the photoreceptors has higher temporal resolution?
Cones
how many types of Cones and rods are there?
3 types of cones
1 type of rods
In which of the photoreceptors is convergence higher?
Rods
Fovea only contains ________
Cones
Why is there no central vision in dim light conditions?
because the fovea only contains cones no rods
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.
What happens to the discarded discs ?
Phagocytosed by the retinal pigment epithelium (RPE) cells
What is the visual pigment of rods?
Rhodopsin, found on the disk membrane
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).
What is Retinal derived from?
Vitamin A which is synthesized from Beta carotene in our food
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
Describe the structure of Retinal
covalently attached to one of the amino acids of the seventh membrane spanning region of the protein
What differs Photoreceptors from other sensory receptors?
depolarized during darkness, i.e. in the absence of their adequate “stimulus”
What is the name of the G protein involved in phototransduction?
Transducin
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)
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
What is the range of the visible part of the light spectrum?
400-700 nm
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
What is the maximum sensitivity of S cones?
430 nm
What is the maximum sensitivity of M cones?
530 nm
What is the maximum sensitivity of L cones?
560 nm
What is Retinitis Pigmentosa?
genetically determined (autosomal dominant, autosomal recessive, and X linked recessive) degenerative diseases, in which rods preferentially degenerate
What are the symptoms of Retinitis Pigmentosa
- starts with night blindness, then loss of peripheral vision (tunnel vision) then blindness
What causes photoreceptor degeneration in retinitis pigmentosa?
Accumulation of pigment
Reduced phagocytosis by RPE cells during
the process of disk shedding
What is Nyctalopia?
Night Blindness
What causes night blindness?
Vitamin A deficiency
Which is the most common type of color blindness?
Red -green color blindness (X linked recessive)
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
What are the 5 cell types in the retina?
1) Photoreceptor
2) Horizontal cell
3) Bipolar cell
4) Amacrine cell
5) Ganglion cell
In what layer of the retina do photoreceptors synapse with bipolar cells?
Outer plexiform layer
In what layer of the retina do Bipolar cells synapse with retinal ganglion cells?
Inner plexiform layer
In what layer of the retina are Horizontal cells found?
Inner plexiform layer
What type of cell are the horizontal cells?
inhibitory interneurons involved in the center / surround organization of the receptive fields
of bipolar cells
In what layer of Amacrine cells found?
the inner plexiform layer
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
where do the Retinal ganglion cells synapse ?
lateral geniculate nucleus (LGN) of the diencephalon
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
Glutamate receptors on the postsynaptic membrane of OFF bipolar cells are _______
receptors
ionotropic
Effect of glutamate on OFF bipolar cells
an excitatory transmitter, depolarized during
darkness
Glutamate receptors on the postsynaptic membrane of ON bipolar cells are ____________
metabotropic type
Effect of glutamate on ON bipolar cells
an inhibitory transmitter, depolarized during
light
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
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
Describe the effect on an ON center ganglion cell when light is on its Periphery
reduces frequency of APs (APs over time)
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
Describe the effect on an OFF center ganglion cell when light is on its Periphery
Increases frequency of APs
