4. Vision - Retina Flashcards
cells in the eye which carry information about colour, shape, movement, etc
classic retinal ganglion cells (RGCs)
classical retinal ganglion cells project to the:
lateral geniculate nucleus (LGN), then the primary visual cortex
intrinsically photosensitive retinal ganglion cells (ipRGCs) project to the:
suprachiasmatic nucleus (SCN) and the paraventricular nucleus (PVN) to form the retinohypothalamic tract
OR
the intergeniculate leaflet (IGL) and olivary pretectal nucleus (OPN)
the suprachiasmatic nucleus (SCN) and paraventricular nucleus (PVN) are involved in:
circadian rhythms (stimulate melatonin release from the pineal gland)
the olivary pretectal nucleus projects through the _____ to activate motor neurons in the ____ that controls muscle of the ____
Edinger-Westphal nucleus (EW), ciliary ganglion, iris
a light-sensitive receptor protein involved in visual phototransduction
rhodopsin
what are the two major types of eyes
compound eyes and camera type eyes
rhodopsin is a photopigment composed of two parts:
retinal and opsin
a protein with seven transmembrane alpha-helices (GPCR) and is part of the rhodopsin photopigment
opsin
a chromophore which absorbs light, is derived from vitamin A, and is part of the rhodopsin photopigment
retinal
in vertebrates, _____ is attached to the seventh transmembrane domain of the opsin protein
11-cis retinal
11-cis retinal is converted, by light, to:
all-trans retinal
the conversion of 11-cis retinal to all-trans retinal causes a conformation change in the opsin to produce:
activated rhodopsin (results in a G protein cascade)
list the eight major parts of the vertebrate eye
- lens
- iris
- pupil
- cornea
- ciliary body
- retina
- fovea
- optic nerve
in the retina, light has to pass through the ganglion cells to activate:
the photoreceptors
list the five major cell types found in the retina
- photoreceptors (rods and cones)
- bipolar cells
- retinal ganglion cells (RGCs)
- horizontal cells
- amacrine cells
do photoreceptors produce action potentials?
no, the axons are too short (just not worth it)
which are larger and more abundant: rods or cones?
rods
in rods, the pigment rhodopsin is embedded in:
membranes arranged in the form of disks (not continuous with the outer membrane of the cell)
in cones, the pigment rhodopsin is embedded in:
infolded membranes that are continuous with the surface membrane
true or false: light depolarizes vertebrate photoreceptors
false, light hyperpolarizes photoreceptors
list the four steps involved in hyperpolarization of photoreceptors
1) light is absorbed and rhodopsin becomes activated
2) the G protein transducin is stimulated (GTP exchanged for GDP on the alpha subunit)
3) the alpha subunit separates and activates cGMP phosphodiesterase, which catalyzes the breakdown of cGMP to 5’-GMP
4) as concentration of cGMP decreases, cGMP detaches from cation channels, which causes them to close (less Na+ enters the cell and the cell hyperpolarizes)
each transducin can activate at least 12-14 molecules of:
cGMP phosphodiesterase
each cGMP phosphodiesterase hydrolizes 100 000s of:
cGMPs
how many ion channels are closed by a single activated rhodopsin?
~300
closing a single Na+ ion channel prevents the influx of:
one million Na+ ions
does light depolarize or hyperpolarize invertebrate photoreceptors?
depolarizes them
compound eyes are composed of an array of:
ommatidia
contains a ‘cornea’ overlying a crystalline cone that forms a lens and retinular cells
ommatidia
the retinular cell in an ommatidia are arranged in a:
circle
rhabdomeric receptor cells
retinular cells
in retinular cells, microvilli on the rhabdomere contain the:
rhodopsin, associated G proteins, and cation channels
what do rhabdomeric photoreceptors look like?
a comb
list the five steps of signal transduction in rhabdomeric receptors
1) activated rhodopsin activates a G protein (GDP exchanged for a GTP on the alpha subunit)
2) the alpha subunit separates and activates phospholipase C (PLC)
3) this catalyzes the formation of IP3 and DAG from PIP2
4) this causes the opening of transient receptor potential (TRP) non-specific cation channels
5) Na+ (and Ca++) enter the cell and produce a depolarization
when retinal is converted to all-trans isomer, it separates from the opsin
pigment bleaching
transported to pigment epithelium where it enzymatically converted back to 11-cis retinal, then transported back to photoreceptors
pigment regeneration
how long does pigment regeneration take?
30+ minutes (dark adaptation)
in light, cation channels are closed, thus less Ca++ enters the cell, but it is still removed by:
pumps
a fall in intracellular Ca++ leads to:
light adaptation
Ca++ bound recoverin inhibits phosphorylation step that shuts off:
meta-rhodopsin II
in light, phosphorylation leads to inactivation of:
meta-rhodopsin II (cation channels open)
without the phosphorylation site, cation channels stay closed for longer because:
meta-rhodopsin II is no longer inactivated
what are the four major effects that a drop in intracellular Ca++ has on photoreceptors?
- speeds meta-rhodopsin II shutoff
- lowers gain of PDE activation
- activates cyclase
- increases channel affinity for cGMP
a fall in Ca++ activates ____ leading to an increase in the concentration of ____
guanylate cyclase, cGMP
rods are specialized for:
scotopic vision (dim light)
cones are specialized for:
phototopic vision (bright light) and colour vision
true or false: rods and cones have equal distribution throughout the retina
false, cones are concentrated at the fovea, and rods are located solely in the peripheral parts of the retina
which has more convergance between cells: rods or cones?
rods
as convergence between cells increases, acuity _____ and sensitivity _____
decreases, increases
what is the average convergence of rod cells?
120 photoreceptors to 1 retinal ganglion cell
what is the average convergence of cone cells?
6 photoreceptors to 1 retinal ganglion cell (becomes a 1:1 ratio in the fovea)
what are the three types of cones found in placental mammals?
- S-cones (blue)
- M-cones (green)
- L-cones (red)
what is the peak absorbance of the S-cone?
420nm
what is the peak absorbance of the M-cone?
534nm
what is the peak absorbance of the L-cone?
564nm (yellowish-green colour)
which cones are the most sensitive cones?
L-cones
occurs in people who are missing their L-cones
protanopia
occurs in people who are missing their M-cones
deuteranopia
occurs in people who are missing their S-cones
tritanopia
protanopia, deuteranopia, and tritanopia are all forms of:
dichromacy
occurs in people who are missing both their M and L-cones
S-cone monochromacy
occurs in people who are missing all of their cones
rod monochromacy
what are the most common forms of colourblindness?
protanopia and deuteranopia
photoreceptors end with:
ribbon synapses
what type of neurotransmitters do photoreceptors release?
glutamate (excitatory or inhibitory)
what type of neurotransmitters do bipolar cells release?
glutamate (excitatory)
what type of neurotransmitters do horizontal cells release?
GABA (inhibitory)
what type of neurotransmitters do amacrine cells release?
glycine (inhibitory)
have electrical synapses with one another and some bipolar cells
horizontal cells
some amacrine cells have electrical synapses with:
bipolar cells
go review slides 265 and 266
typing all of that looks like hell
what type of receptive field do retinal ganglion cells have?
centre-surround receptive fields
describe an “ON” or D-bipolar cell
- light falling on a single rod causes it to become hyperpolarized
- glutamate stops being released
- the D-bipolar cell becomes depolarized through loss of inhibition
describe an “OFF” surround receptive field
- light falling on the surround area prevents glutamate from being released by photoreceptors
- horizontal cells become hyperpolarized and don’t release GABA (inhibitory neurotransmitter) onto the axon of the centre photoreceptor
- the photoreceptor connected to the D-bipolar cell becomes depolarized
- the photoreceptor once again releases glutamate and hyperpolarizes the bipolar cell
describe an “OFF” or H-bipolar cell
- light falling on a single photoreceptor causes it to be hyperpolarized
- glutamate stops being released and the H-bipolar cell becomes hyperpolarized
describe an “ON” surround receptive field
- light falling on the surround area prevents glutamate from being released by photoreceptors
- horizontal cells become hyperpolarized and don’t release GABA (inhibitory neurotransmitter) onto the axon of the centre photoreceptor
- the photoreceptor connected to the H-bipolar cell becomes depolarized
- the photoreceptor once again releases glutamate and depolarizes the bipolar cell
many amacrine cells are important for:
scotopic vision
