Lecture 15 Flashcards
What is the laminar organization of the brain from the outer side to the inner side?
Ganglion cell layer Inner plexiform layer Inner nuclear layer Outer Plexiform layer Outer nuclear layer Layer of photoreceptor outer segments Pigmented epithelium
What must light focused on the retina be converted into?
Neural activity
What must light pass through before it reaches the photoreceptors?
Ganglion cells
Bipolar cells
Where is the light absorbed once it has passed through the retina?
Pigmented epithelium
What increases visual acuity? i.e. clarity of image
Preventing light from bouncing back into the retina
What does the pigmented epithelium do?
Absorbed light to decrease resolution. This increases reflection of light (Cat’s eyes at night)
What spread light to different cells and are important in processing?
Amacrine and horizontal cells
What does the retina do as a displaced part of the CNS?
It has lots of processing involved before moving to the CNS
Some animals have reflective layers underneath photoreceptors but what do they do?
Reflects the light to increase the sensitivity to low light levels BUT THIS COMPROMISES ACUITY
What are photoreceptors?
Sensory transducers, rods and cones
What are horizontal cells?
Input from and output to photoreceptors, output to bipolar cells
What are bipolar cells?
Connect photoreceptors to ganglion cells
What are amacrine cells?
Input from bipolar cells, influence ganglion cells, bipolar cells and other amacrine cells
What are ganglion cells?
output from retina
What are the main structural features of rods?
Outer membranes have discs
- membraneous free-floating discs are where sensory transduction occurs
- contain light-sensitive photopigments that absorb light
What are the main structural features of cones?
Foldings of membranes where photons are absorbed
What is the Duplicity theory?
Can’t have high sensitivity and high resolution in single receptor
Thus separate systems for monochrome and color
What processes monochrome light?
Sensitive to light
In dim-light conditions, sight is in black-and-white
Only using rods
What processes color?
Has high resolution
Uses cones
What are main features of rods?
Greater number of discs
Higher photopigment concentration
1000 times more sensitive to light than cones
Enable vision in low light (scotopic) conditions
i.e. night time
Low visual acuity/resolution
~ 92 million rods in each human retina
What are main features of cones?
Fewer discs Used during daylight (photopic) conditions Enable color vision Lower sensitivity High visual acuity/resolution ~ 5 million cones in human retina
Which part of the eye contains most of the cones?
The fovea
- contains most of the 5 million cones
- no rods (highest resolution)
What forms central retina ?
Low convergence + high resolution
What forms peripheral retina?
high convergence + low resolution
Where does convergence occur?
Convergence of photoreceptor on retinal ganglion cell
How does low convergence happen?
One cone feeds one ganglion cell
- visual cortex will know that one ganglion cell is related to one photoreceptor
- easy to detect which cone is used
How does high convergence happen?
Multiple bipolar cells providing input to one ganglion cell
- light is being received by tiny bit of retina
- can’t tell which rod was used
What is the absorbance spectrum of human rod photopigments?
Rhodopsin
- peak at 500nm (green)
What is the absorbance spectrum of human cone photopigment?
Three varieties of opsins - S,M,L
- peaks: S - 420nm (blue)
M - 530nm (light green)
L - 560nm (yellow)
What is the absorbance spectrum of human retinal ganglion photopigment?
Melanopsin
- can sense whether we are in dark or light condition (controls circadian rhythm)
- peak at 475nm (light blue)
How is phototransduction controlled in the dark?
Rods are depolarized
Due to a Na reflux (but a bit of Ca involved)
Known as dark current
- keeps membrane less negative (-30mV)
- voltage-gated Ca channels open
- Ca come and activate vesicle release of neurotransmitter
Maintained by cGMP
- cGMP binds to Na channels to keep it open and let more Na in
How is phototransduction controlled in the light?
cGMP levels are decreased - phosphodiesterase breaks down cGMP - dark current gone Na channels close Na influx prevented Rods are hyperpolarized (-60mV) Voltage-gated Ca channels close Neurotransmitter isn't released Membrane does come back a bit during light condition - adaptation
What is activated by light?
Rhodopsin
- composed of retinal and opsin
What does rhodopsin stimulate?
G-protein, transducin, to become transducin GTP
- changes conformation of opsin
- activates rhodopsin
- acts like GPCR to initiate enzyme cascade
What subunits does transducin have?
alpha, beta and gamma
What does the alpha subunit of transducin activate?
Enzyme phosphodiesterase (PDE)
What does PDE do?
PDE reduces cGMP levels, closing Na channels
How is cGMP produced in the dark?
Enzyme guanylyl cyclase
What is saturation?
When rods cannot process bright light as they become saturated
cGMP levels are so low that no additional hyperpolarization can occur
Cones are not saturated as easily, so are used in bright light
What is light adaptation?
Photoreceptors initially hyperpolarize greatly
Photoreceptors gradually depolarize with continued bright light
What does light adaptation need?
calcium
How does light adaptation happen in the dark?
Ca normally enters cells and block guanylyl cyclase
This reduces cGMP production, so closes some ion channels
- guanylyl cyclase normally produces cGMP from GTP
How does light adaptation happen in the light?
Channels are shut so Ca cannot enter cells
Block on guanylyl cyclase is released
More cGMP produced = more channels open
Is feedback loop constantly regulated?
yes
What are the two types of bipolar cells?
ON cells
- hyperpolarized in the dark, ON in light
OFF cells
- depolarized in the dark, OFF in light
What are the signal from bipolar cells transferred to?
Retinal ganglion cells
What do retinal ganglion cells do?
The only cells to fire action potentials in the retina
How is light involved in circadian rhythm?
Daily light - cycle - entrains circadian rhythms
Changes in day length and shift work can cause mood alterations and cognitive deficits
Aberrant light cycles also increase depression, impair LTP in hippocampus and impair leaving
What appeared in the news in relation to vision?
How stem cells may cure blindness
- ESC: Cells in retinal pigment epithelia. These are embedded underneath the rods and cones
