Chapter 9 - Vision Flashcards
What is in the optical system of the eye?
Cornea and lens
What does the optical system do?
Projects physical image of the world onto the retina
What does the retina do with the optical image?
It converts it into a neural image
What constitutes the retina?
3 interconnected layers and 5 classes of neurons
What are the 3 stages of vertical information flow?
1) Transduction of the image by photoreceptors (physical energy (photons) to electrochemical energy)
2) photoreceptors synapse on bipolar cells
3) bipolar cells synapse on ganglion cells
What is the lateral information flow?
At each stage of the vertical transmission there are specialized laterally connecting neurons called horizontal cells and amacrine cells.
These cells modify forward transmission across the synaptic layers ( largely by inhibition)
When are rods and cones depolarized?
When they are in darkness
In the rod, what maintains sodium and potassium concentrations?
Na/K pump
What happens to channels in outer segment when in the light?
Cell hyperpolarizes
What is the role of the visual pigment molecules?
Absorb electromagnetic energy and divert the energy into a biological process
What could you compare the rhodopsin to?
Is like a G-protein receptor
What could you compare retinal?
To a ligand (it’s a chromophore)
What are the steps of phototransduction?
1) Light stimulation of rhodopsin leads to activation of a G-protein, transducin
2) Activated G-protein activates cGMP phosphodiesterase (PDE)
3) cGMP binds Na+ channels opening them
4) PDE hydrolyzes cGMP, reducing its concentration
5) this leads to closure of sodium channels
Explain dark reaction
1) cGMP is high in cytoplasm
2) inward current of Na+ through cGMP-Gated channels which depolarized cell. This is the dark current which is usually ~ -50pA (in current) keeping cell’s Vm at ~ -40mV
3) depolarization causes transmitter release (glutamate) at terminal region
Explain light reaction
1) Light activates rhodopsin
2) Transducin, the g-protein is active
3) phosphodiesterase (PDE), the effector enzyme, is activated
4) PDE activity reduces the cGMP level
5) cGMP-gated N’a+ channels close- no Na+ current (dark current)
6) Cell hyperpolarizes to ~ -70mV, transmitter release stops
1) OFF bipolars are … by …/… by light
2) ON bipolars are … by…/… by light
1) depolarized — glutamate—hyperpolarized
2) hyperpolarized—glutamate—-depolarized
What are the two ways of thinking about the receptive fields’ visual system?
1- the set of photoreceptors to which the cell is connected
2- the set of the visual field in which various visual stimuli can affect the discharge rate of the cell (slide 39)
What are the two pathways of the receptive field? What are the components of each?
A) vertical pathway: - photoreceptors
- bipolar cell
- ganglion
Forms the center of receptor field (blue)
B) horizontal pathway: - photoreceptors
- horizontal cells
- bipolar cells
- ganglion cell
Forms the inhibitory surround of receptive field
What are the different types of ganglion cells?
M: large receptive fields, not wavelength selective, respond well to large objects and movement, terminate in M (magnocellular) layers of LGN
P: small receptive fields, selective to particular wavelengths of light (color), concerned with analysis of fine detail and color, terminate in P (parvocellular) layers of LGN
What is scotopic vision?
Vision under dim-light (dark)
What is photopic vision?
Vision under well lit conditions
Name the parallel pathways
Magnocellular M(motion)
Parvocellular - P(shape, detail)
Koniocellular - non-M-non-P (color)
For every point of the visual field we have at least 10 ganglion cells. What are they?
- On-M
- Off-M
- On-P (2)
- Off-P (2)
- non-M-non-P (4)
What does LGN stand for?
Lateral geniculate nucleus
