Vision Retina 1 Flashcards
Draw the electromagnetic spectrum.
slide 2
What is the wavelength range that is visible to humans?
750nm - 350nm
describe the info transmission pathway from the eyes to the brain
- retina
- bipolar cells
- ganglion cells (ipRGCs)
- LGN
- visual cortex
describe the info transmission pathway from the eyes to the brain that drives the circadian rhythm
- retina
- bipolar cells
- ganglion cells (ipRGCs)
- SCN (suprachiasmatic nucleus)
- PVN (paraventricular nucleus) - retinohypothalamic tract
- IML (intermediolateral nucleus)
- SCG (superior cervical ganglion)
- pineal gland - release melatonin
Describe the info transmission pathway from the eyes to the brain that regulates the pupillary reflex.
- retina
- bipolar cells
- ipRGCs
- IGL (intergeniculate leaflet)
- OLN (olivary pretectal nucleus)
- EW nucleus
- motor neurons in ciliary ganglion (CG) –> control iris muscle
draw the pathways from the eyes to the brain
slide 3
What is rhodopsin
light-sensitive receptor protein involved in visual phototransduction
What are the different types of eyes? Describe them.
- compound eyes: composed of numerous identically repeated visual units, the ommatidia
- camera type eyes: lens, retina, photoreceptor cells, afferent neurons (SINGLE UNIT)
classify the following as having compound or camera type eyes: snail, ark clam, scallop, fly, vertebrate, cephalopod
- compound eyes: ark clam, fly
- camera type eyes: snail, scallop, vertebrate, cephalopod
What are the 2 parts of rhodopsin? Define them.
- opsin = protein with 7 trans-membrane alpha helices
- retinal = chromophore (absorbs light); vitamin A derivative
What is a chromophore? example?
- protein that absorbs light
- retinal
TRUE or FALSE: retinal is a vitamin B derivative
FALSE: vitamin A
TRUE or FALSE: in vertebrates, 11-cis-retinal is attached to the 7th transmembrane domain
TRUE
how is activated rhodopsin form? What is it called once activated? What does it start as? What results after?
- 11-cis-retinal converted by LIGHT to ALL-TRANS RETINAL
- conformation change in the OPSIN to produce activated rhodopsin –> METARHODOPSIN II
- G protein cascade results
label the structures of the vertebrate eye
slide 7
function of the ciliary body
contraction –> lens fattens –> focus near objects
function of fovea
photoreceptors most densely packed here
where are there no photoreceptors in the eyeball?
optic disk (where optic nerve is attached)
arrange the order from front to back, structurally and in order of info transmission: photoreceptors, ganglion cells, bipolar cells
- structural: ganglion cells –> bipolar cells –> photoreceptors
- info: photoreceptors –> bipolar cells –> ganglion cells
what kind of connections do horizontal and amacrine cells make in the retina?
lateral
between which cell layers are horizontal cells located in the retina? amacrine?
- horizontal cell = between photoreceptors and bipolar cells
- amacrine cell = between bipolar cells and ganglion cells
which scientist first visualized the human rod and cone?
ramon y cajal
draw a rod and a cone and label the structures:
- disc
- outer segment
- inner segment
- synaptic ending
- nucleus
- axon
- mitochondria
- connecting cilium
slide 11
Which structure of the photoreceptor is rhodopsin found in?
disc
define the structural difference between rods and cones
- rods: pigment rhodopsin (black dots) is embedded in membranes arranged in the form of disks, which are NOT continuous with the outer membrane of the cell
- cones: the pigment molecules are on infolded membranes that are CONTINUOUS with the surface membrane
hint: CONes = CONtinuous
TRUE or FALSE: light depolarizes vertebrate photoreceptors
FALSE: hyperpolarizes
describe the mechanism by which light hyperpolarizes photoreceptors (draw)
- light is absorbed, rhodopsin is activated
- the G protein TRANSDUCIN is stimulated; GTP exchanged from GDP on alpha subunit
- alpha-subunit separates and activates cGMP phosphodiesterase; breakdown cGMP to 5’-GMP
- as cGMP decreases, cGM detaches from cation channels, channels CLOSE; less Na+ enters the cell and the cell HYPERPOLARIZES
slide 14
What is meant by “G protein coupled processes are amplifiers”, in terms of photoreceptors and light?
G protein processes make photoreceptors extremely SENSITIVE TO LIGHT
What are the 2 main steps of amplification in photoreceptors?
- each transducin can activate at least 12-14 molecules of cGMP-phosphodiesterase
- each cGMP phosphodiesterase hydrolyzes 100,000s of cGMP
What is the end result of the amplification in photoreceptors in response to light?
single activated rhodopsin closes 100s or ion channels, preventing influx of one million Na+ ions
Single photon leads to a ______ pA (reduction/increase?) in current.
What does this mean
0.5 pA reduction; very sensitive to light bc of GPCR cascade
Draw the current resulting from single photos, flases of increasing intensity, and steady illumination.
slide16
TRUE or FALSE: vertebrate photoreceptors are active in the light
FALSE: active in the dark
TRUE or FALSE: cyclic nucleotide gated channels normally OPEN in the dark
TRUE
Describe the steps in a dark current of a photoreceptor
- cGMP-gated Na+ channels OPEN IN THE DARK
- inflow of Na+ (dark current)
- membrane potential depolarized -30 mV
- GLUTAMATE released at synaptic terminals INHIBITS bipolar cells
Describe the steps in the light (current) of a photoreceptor.
- under LIGHT< isomerization of retinal activates enzyme that breaks down cGMP
- cGMP-gated Na+ channels CLOSE IN THE LIGHT
- inflow of Na+ SLOWS
- membrane potential HYPERPOLARIZED
- GLUTAMATE release turned OFF, which EXCITES bipolar cells
TRUE or FALSE: photoreceptors produce APs in their axons
FALSE: not AP; produce GRADED POTENTIALS because axons are short
by how much can a single photon hyperpolarize membrane potential
1mV
Draw a diagram of the membrane potential of the photoreceptor in dim, dark, and bright.
(hint: graded potentials)
slide 17
TRUE or FALSE: light depolarizes vertebrate photoreceptors and hyperpolarizes invertebrate photoreceptors
FALSE: hyperpolarize vertebrate, depolarize invertebrate
Draw the membrane potential in response to light in invertebrate photoreceptors vs vertebrates.
slide 18
Draw photoreceptors in insects vs vertebrates. Compare with ancestral neuroepithelial cell.
slide 19
Compound eyes are composed of an array of _______________.
ommatidia
What are the cells that make up an ommatidium? How are they arranged? How many in drosophila?
- retinular cells
- arranged in circle
- 8 in drosophila (per ommatidium)
What forms the lens of an ommatidium?
cornea overlying a crystalline cone
What do the microvilli on the rhabdomere of an ommatidium conatin?
rhodopsin, G proteins, cation channels
Draw the structure of ommatidia; label.
slide 20
Describe the steps of transduction in rhabdomeric photoreceptors. Draw.
- activated rhodopsin activates a G protein (NOT TRANSDUCIN); GTP exchanged for GDP on an a-subunit
- a-subunit separates and activates PLC
- PLC catalyzes: PIP2 –> IP3 and DAG
- causes the opening of transient receptor potential (TRP) channel
- Na+ (and Ca2+) enter cell; DEPOLARIZATION; scaffolding protein speeds up the process
slide 21
What is pigment bleaching?
when retinal is converted to all-trans isomer, it separates from the opsin
What is pigment regeneration?
opsin is transported to pigment epithelium where it is converted BACK TO 11-cis retinal, then transported back to photoreceptors
How long does pigment regeneration take?
30+ minutes
draw a diagram to show chromophore bleaching and recylcing. Label the rate limiting step
RLS = all-trans retinal –> 11-cis retinal + OPSIN (dark adaptation)
slide 23
What is another name for pigment regeneration?
dark adaptation
What is the key ion in light adaptation?
Ca2+
How do intracellular levels of Ca2+ affect light adaptation? light vs dark
- in dark, Ca2+ continuously flowing into cell though open cation channels, but removed by pumps
- in light, cation channels closed, LESS Ca2+ enter, but still removed by pumps
summary: DECREASE in intracellular Ca2+ leads to light adaptation
TRUE or FALSE: metarhodopsin II needs to be activated for light adaptation
FALSE: inhibited
fall in intracellular calcium leads to 4 processes that contribute to light adaptation. what are these 4 processes?
- speeds metarhodopsin II shutoff via RECOVERIN
- lowers gain of PDE activation
- activates cyclase
- increase channel affinity for cGMP via CALMODULIN
i.e. prevent hyperpolarization
Why must metarhodopsin II be shut off in light adaptation?
if not stopped, it will continue to activate transducins, leading to hyperpolarization and closing of channels
recoverin in the dark vs in the light
- dark: Ca2+ bound recoverin inhibits PHOSPHORYLATION that shuts off metarhodopsin II
- light: recoverin can’t inhibit phosphorylation step; metarhodopsin shut off at greater rate; cation channels open
note: Ca2+ binds in the dark because lots of Ca2+ influx in the dark
TRUE or FALSE: phosphorylation activates metarhodopsin II
FALSE: inactivates
TRUE or FALSE: without that phosphorylation site, cation channels stay open for longer because metarhodopsin II is not being inactivated
FALSE: channels stay CLOSED for longer bc metarhodopsin II activates transducin, leading to hyperpolarization
How does cyclase affect the transduction mechansim in a photoreceptor? What process does it play a role in?
converts 5’-GMP back to cGMP; light adaptation
