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lect fin fall 2020

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ackman678
2020-12-14 10:18:11 -05:00
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vision1.md
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@@ -239,7 +239,7 @@ Note:
---
## What does light do?
## What does light energy do?
* In the dark, the resting potential of the photoreceptor is -40 mV
* Light shining onto outer segment leads to the **hyperpolarization** of the photoreceptor and reduction of neurotransmitter released
@@ -247,25 +247,9 @@ Note:
Note:
*reason for this backwards arrangement of hyperpolarization to depolarization is not currently known*
---
## cGMP gated cation channels are key
In the dark channels open due to cGMP binding. Na^+^ and Ca^2+^ rushes in and cell is depolarized
<figure><img src="figs/Neuroscience5e-Fig-11.08-0_5a9e700.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 11.8</figcaption></figure>
Note:
cyclic nucleotide gated cation channel
the nucleotide cyclic guanosine monophosphate
*these cGMP gated channels are permeable to both Na+ and Ca2+ actually*
*reason why depolarization is the baseline state in the dark is not known*
Greater energy effiency during rest maybe (less Na+/K+ ATPase activity)?
---
@@ -278,18 +262,39 @@ the nucleotide cyclic guanosine monophosphate
Note:
---
## cGMP gated cation channels are key
In the dark channels open due to cGMP binding. Na⁺ and Ca²⁺ rushes in and cell is depolarized
<figure><img src="figs/Neuroscience5e-Fig-11.08-0_5a9e700.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 11.8</figcaption></figure>
Note:
cyclic nucleotide gated cation channels in the outer membrane segment of the photoreceptors
the nucleotide cyclic guanosine monophosphate
*these cGMP gated channels are permeable to both Na+ and Ca2+ actually*
Balanced by K+ selective channels in the inner segment of the photoreceptor cell
Light transduction results in a **decrease in cGMP levels** thus closing the cGMP cation channels. K+ efflux becomes dominant and hyperpolarization ensues. Then less Ca2+ dependent transmitter release at synapse with bipolar cells
---
## In the light
* A photon of light is absorbed by photopigment (retinal or retinaldehyde, an aldehyde of Vitamin A) that is coupled to a protein in the outer segment called opsin. Absorption causes a change in conformation of retinal (photon absorbtion breaks a carbon double bond and switching from cis to trans configuration) that in turn changes the conformation of opsin
* This leads to the disassociation of trimeric G-proteins (special α subunit called transducin) from the receptor
* Transducin activates a cGMP phosphodiesterase which degrades cGMP to GMP. Channel opening probability decreases, cell gets hyperpolarized
* The opsin then can activate the trimeric G-protein **transducin**
* Transducin in turn activates a cGMP phosphodiesterase. The phosphodiesterase then hydrolyzes cGMP to GMP. Channel opening probability decreases, cell gets hyperpolarized
Note:
---
--
## Phototransduction in rod photoreceptors
@@ -372,10 +377,10 @@ Tremendous amplification. Single photon hitting rhodopsin is estimated to activa
---
## Need to inactivate opsin signaling after a light flash
## After photon absorbtion, opsin signaling is inactivated and cis-retinal gets regenerated
* Rhodopsin kinase/arrestin– activated rhodopsin is phosphorylated by rhodopsin kinase, permitting the protein arrestin to bind to rhodopsin. **Prevents further activation of transducin**, thus ending the phototransduction cascade
* All-trans retinal gets shed, transported to pigment epithelium cells, changed to cis-retinol and then reincorporated into opsin
* All-trans retinal gets shed, transported to pigment epithelium cells, changed to cis-retinal and then reincorporated into opsin
Note:
@@ -448,7 +453,7 @@ Why the cone shape? Shape of cone preferentially accepts light directed straight
Note:
---
--
## Range of luminance values over which the visual system operates
@@ -464,7 +469,7 @@ Note:
---
--
## More factoids
@@ -531,7 +536,7 @@ Cone response over in about 200 ms (with an overshoot of inward current), wherea
Note:
---
--
## Distribution of rods and cones in the human retina
@@ -730,6 +735,9 @@ Off response
Note:
luminance increment in receptive field center vs luminance decrements in receptive field center
Contrast. luminance change, increments or decrements, carried by separate channels to brain by increased spike rate
<!--
## On- and off-center retinal ganglion cell responses to stimulation