Phototransduction Flashcards
in the dark, what is the membrane potential of the retina?
-40mV (depolarized)
why is the retina depolarized in the dark?
sodium channels are kept open
what is the enzyme that produces cyclic guanosine monophosphate (cGMP)?
enzyme guanylate cyclase
where is cyclic guanosine monophosphate produced?
in the photoreceptor
what is the function of cyclic guanosine monophosphate?
keeps Na+ channels open (-40mV)
light travels through the retinal layers and is absorbed by the ____
pigment epithelium
when light is absorbed, it triggers…
absorption of electromagnetic radiation by the photopigment in membrane of stacked discs in photoreceptor’s outer segment
light causes ____ of the retina (photoreceptors)
hyperpolarization
the photopigment in the rods is called ____
rhodopsin (retinal bound to an opsin)
retinal is a derivative of ____
vitamin A
what is opsin?
seven membrane, alpha helices receptor protein
like G protein coupled receptors
how many opsins do cones have?
3
Young-Helmholtz trichromacy theory
states that the combination of relative excitation & inhibition of the red, blue, & green opsins in the cones accounts for our colour vision
light ____ retinal, which activates ____
bleaches (cis to trans)
opsin
activation of opsin stimulates ____ in the ____, which activates ____
- G protein: Transducin in disc membrane
- effector enzyme: phosphodiesterase (PDE)
PDE breaks down ____, which causes…
cGMP
Na+ channels close -> photoreceptor membrane potential hyperpolarizes to -70mV
hyperpolarization of the photoreceptor membrane causes…
amount of glutamate released to decrease
what happens to photoreceptor sensitivity to light when we go from a lit room to a dark room?
sensitivity to light increased 1,000,000x
when going from light to dark, the pupils ____, causing…
dilate
unbleached rhodopsin to regenerate in rods (trans to cis) via a process that requires Ca++
what is receptive field?
area of the retina that when stimulated by light, changes the cell’s membrane potential
what are the 2 receptive fields?
center & surround
center receptive field
photoreceptors that directly synapse with the bipolar cell
surround receptive field
surrounding photoreceptors that synapse on horizontal cells, which synapse with the bipolar cell
the receptive fields centers are ____istic
antagonistic:
- ONcenter/OFFsurround
- OFFcenter/ONsurround
when do ON bipolar cells depolarize?
in the light (in center receptive field)
How are ON bipolar cells are depolarized?
- less glutamate released by photoreceptors
- G-protein coupled glutamate receptors on bipolar cells cause depolarization of bipolar cells
when do OFF bipolar cells depolarize?
in the dark
How are OFF bipolar cells are depolarized?
- more glutamate released by photoreceptors
- glutamate-gated direct ion channels cause depolarization of the bipolar cell
a) when exposed to light, what happens to photoreceptors in the receptive field center of an ON-center bipolar cell?
b) What does the bipolar cell do?
a) hyperpolarize
b) depolarize
a) when exposed to light, what happens to photoreceptors in the receptive field surround of an ON-center bipolar cell?
b) What does the bipolar cell do?
a) hyperpolarize
b) hyperpolarize
a) In the dark, what happens to photoreceptors in the receptive field center of an ON-center bipolar cell?
b) What does the bipolar cell do?
a) depolarize
b) hyperpolarize
a) In the dark, what happens to photoreceptors in the receptive field surround of an ON-center bipolar cell?
b) What does the bipolar cell do?
a) depolarize
b) depolarize
a) In the light, what happens to photoreceptors in the receptive field center of an OFF-center bipolar cell?
b) What does the bipolar cell do?
a) hyperpolarize
b) hyperpolarize
a) In the light, what happens to photoreceptors in the receptive field surround of an OFF-center bipolar cell?
b) What does the bipolar cell do?
a) hyperpolarize
b) depolarize
a) In the dark, what happens to photoreceptors in the receptive field center of an OFF-center bipolar cell?
b) What does the bipolar cell do?
a) depolarize
b) depolarize
a) In the dark, what happens to photoreceptors in the receptive field surround of an OFF-center bipolar cell?
b) What does the bipolar cell do?
a) depolarize
b) hyperpolarize
ganglion cells have ____ receptive fields
antagonistic
ganglion cells in the retina are mainly responsive to…
differences in illumination within their receptive fields
name the 2 types of ganglion cells
M cells (magnocellular)
P cells (parvocellular)
M cells
- large receptive fields
- respond to object motion & low contrast stimuli
- aid in low resolution vision
P cells
- small receptive fields
- colour opponent cells (sensitive to wavelength): red-green, blue-yellow
- responsible for colour vision and discrimination of fine detail
layers 1,2 of the lateral geniculate nucleus contain ____ cells and receive synapses from ____
M cells
M ganglion cells
layers 3,4,5,6 of the lateral geniculate nucleus contain ____ cells and receive synapses from ____
P cells
P ganglion cells
layers 1,4,6 receive fibers from the ____ retina
contralateral
layers 2,3,5 receive fibers from the ____ retina
ipsilateral
LGB fibers from the ipsilateral retina end up in the primary visual cortex next to…
LGB fibers from contralateral retina
(alternating ocular dominance columns)
what happens to information from M cells that reaches the primary visual cortex?
streamed through V2 (areas 18,19) to the parietal cortex for analysis of motion
what happens to information from P cells that reaches the primary visual cortex?
- separates info about shape (ends in interblobs) and colour (ends in blobs)
- 2 channels are streamed through V2 to temporal cortex for shape & colour perception and visual memory
