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vision1.md
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## Today’s learning goals
* Be able to identify the different parts of the eye and their functions
* Identify the major parts of eye and retinal anatomy and their functions
* Understand the main proteins involved in the signal transduction pathway that leads to changes in neurotransmitter release by photoreceptors in response to light
* Learn the neural pathway that takes information from photoreceptors to the brain
* Understand the concept of the receptive field
@@ -25,8 +25,7 @@ Note:
Note:
---
--
## Anatomy of the human eye video
@@ -82,6 +81,10 @@ Contraction of ciliary muscle as a ring around the lens causes zonule fibers to
Pupil has circular muscles that contract when pupil closes, and radial bands of muscles that contract when pupil dilates.
* Efferent pathway controlling the iris and ciliary muscle are via the Edinger-Westphal nucleus --> ciliary ganglion (parasympathetic, cranial nerve III, oculomotor nerve) --> ciliary muscle
* more on this in vision2
---
## Myopia & Hyperopia
@@ -138,10 +141,10 @@ genetic disorder, diabetes, surgery, long term steroid use, UV light
## The retina
* The retina, despite its peripheral location, is part of the CNS
* Contains neural circuitry that converts light energy into action potentials that travel out of the eye within the optic nerve into the brain
* The retina is part of the central nervous system!
* Contains neural circuitry that converts photon energy into action potentials that travel out of the eye within the optic nerve into the brain
* Is a layered structure, relatively simple for a CNS structure
* Surrounded on one side by pigmented epithelium which contains melanin that helps reduce backscattering of light. Also plays a role in maintenance of photoreceptors
* Surrounded on one side by pigmented epithelium which contains melanin that helps reduce backscattering of light. Also plays a key role in maintenance of photoreceptors
* 5 types of neurons in the retina: photoreceptors, bipolar cells, retinal ganglion cells, horizontal cells, and amacrine cells
* A direct 3 neuron chain is the basic unit of transmission. Photoreceptor to bipolar cell to ganglion cell
@@ -153,7 +156,9 @@ neural tube—> CNS (and retina)
<figure><img src="figs/Neural_crest_6d22927.png" height="100px"><figcaption>Public domain [commons.wikimedia](https://en.wikipedia.org/wiki/Neural_crest#/media/File:Neural_crest.svg)</figcaption></figure>
spiral ganglion neurons in cochlea are also from neural tube/CNS
* olfactory bulb mitral neurons (primary afferent neurons of olfactory system) are from neural tube (telencephalon)
* retinal ganglion neurons in retina (first order afferent neurons of visual system) are from neural tube (diencephalon)
* spiral ganglion neurons in cochlea (first order afferent neurons of auditory system) are also from neural tube
---
@@ -162,28 +167,14 @@ spiral ganglion neurons in cochlea are also from neural tube/CNS
Light travels through the retina to hit the photoreceptors in the photoreceptor layer
<div><img src="figs/Neuroscience5e-Fig-11.05-1R_85d3586.jpg" height="300px"><figcaption>Neuroscience 5e Fig. 11.5</figcaption></div>
<div><img src="figs/Neuroscience5e-Fig-11.05-2R_31b8655.jpg" height="300px"><figcaption>Neuroscience 5e Fig. 11.5</figcaption></div>
Note:
[from: http://www.huffingtonpost.com/2015/03/18/human-retina-backwards_n_6885858.html](http://www.huffingtonpost.com/2015/03/18/human-retina-backwards_n_6885858.html)
>researchers at Technion–Israel Institute of Technology in Haifa built a computer model of a human retina and then compared how light behaves in the model with the way it behaves in the retinas of guinea pigs.
>The comparison showed that when light travels through cell layers before reaching the rods and cones (photoreceptors), it's actually being sorted into red, green, and blue light
>What's doing the sorting? Tiny structures known as Muller glia cells, according to the researchers.
However >"We should also remember that several animal classes do not have a 'backward-pointing' eye, and also have Muller cells,"
>study was presented at a meeting of the American Physical Society on March 5, 2015 in San Antonio, Texas.
[from: http://hubel.med.harvard.edu/book/b8.htm](http://hubel.med.harvard.edu/book/b8.htm)
>Because the rods and cones are at the back of the retina, the incoming light has to go through the other two layers in order to stimulate them. We do not fully understand why the retina develops in this curious backward fashion.
>One possible reason is the location behind the receptors of a row of cells containing a black pigment, melanin (also found in skin)
number of rods and cones vary across the retina. In the center where vision is best (fovea) there are only cones. This area is about 0.5mm in diameter.
@@ -195,6 +186,17 @@ number of rods and cones vary across the retina. In the center where vision is b
high degree of convergence, together with more direct path in and near fovea (one cone—>one bipolar—>one ganglion cell) can explain the 125:1 ratio of receptors to optic nerve fibers without having really bad vision.
<!--
[from: http://www.huffingtonpost.com/2015/03/18/human-retina-backwards_n_6885858.html](http://www.huffingtonpost.com/2015/03/18/human-retina-backwards_n_6885858.html)
>researchers at Technion–Israel Institute of Technology in Haifa built a computer model of a human retina and then compared how light behaves in the model with the way it behaves in the retinas of guinea pigs.
>The comparison showed that when light travels through cell layers before reaching the rods and cones (photoreceptors), it's actually being sorted into red, green, and blue light
>What's doing the sorting? Tiny structures known as Muller glia cells, according to the researchers.
>"We should also remember that several animal classes do not have a 'backward-pointing' eye, and also have Muller cells,"
>study was presented at a meeting of the American Physical Society on March 5, 2015 in San Antonio, Texas.
-->
---
## Layers of the retina
@@ -202,6 +204,8 @@ high degree of convergence, together with more direct path in and near fovea (on
* Three main cell body layers (photoreceptor cell bodies, inner nuclear layer, and ganglion cell layer)
* Two main synaptic transmission layers (outer plexiform and inner plexiform)
<div><img src="figs/Neuroscience5e-Fig-11.05-2R_31b8655.jpg" height="300px"><figcaption>Neuroscience 5e Fig. 11.5</figcaption></div>
<div><img src="figs/image5_b3120c1.png" height="300px"><figcaption>[H. Kolb Webvision, med.utah.edu](http://webvision.med.utah.edu/book/part-i-foundations/gross-anatomy-of-the-ey/)</figcaption></div>
<div><img src="figs/Neuroscience5e-Ch11-Opener_4aa788d.jpg" height="300px"><figcaption>Neuroscience 5e Ch. 11</figcaption></div>
@@ -217,7 +221,7 @@ Note:
* Photoreceptors do not exhibit action potentials– light causes a graded change in membrane potential that changes the rate at which neurotransmitter is released
* Within the retina projections are rather short– do not need action potentials
* Light absorption leads to hyperpolarization of the photoreceptor. This leads to less release of neurotransmitter to the post-synaptic cell
* Light absorption leads to **hyperpolarization** of the photoreceptor. This leads to less release of neurotransmitter to the post-synaptic cell
Note:
@@ -247,9 +251,9 @@ Note:
---
## cGMP gated Na⁺ channels are key
## cGMP gated cation channels are key
In the dark channels open due to cGMP binding. Na⁺ rushes in and cell is depolarized
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>
@@ -267,7 +271,7 @@ the nucleotide cyclic guanosine monophosphate
## In the dark
* cGMP gated Na⁺ channels in outer segment are open allowing ions to flow inside the cell. This leads to a resting potential of -40 mV or so
* cGMP gated cation channels in outer segment are open allowing ions to flow inside the cell. This leads to a resting potential of -40 mV or so
* The probability of these channels being open is regulated by the levels of cGMP
* In the dark, high levels of cGMP keep the channels open
@@ -279,7 +283,7 @@ Note:
## 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 that in turn changes the conformation of opsin
* 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
@@ -295,18 +299,21 @@ Note:
Note:
Vertebrates typically have four cone opsins (LWS, SWS1, SWS2, and Rh2)
Four types of cone opsins in vertebrates (LWS, SWS1, SWS2, and Rh2)
[from: https://en.wikipedia.org/wiki/Opsin](https://en.wikipedia.org/wiki/Opsin)
name | abbr | type | bandwidth | color | human gene
--- | --- | --- | --- | --- | ---
long-wave sensitive | LWS | cone | 500–570 nm | green, yellow, red | OPN1LW "red" / OPN1MW “green"
short-wave sensitive 1 | SWS1 | cone | 355–445 nm | ultraviolet, violet OPN1SW "blue"
short-wave sensitive 2 | SWS2 | cone | 400–470 nm | violet, blue (extinct in therian mammals)
rhodopsin-like 2 | Rh2 | cone | 480–530 nm | green (extinct in mammals)
rhodopsin-like 1 (vertebrate rhodopsin) | Rh1 | rod | ~500 nm | blue-green OPN2 = Rho = human rhodopsin
short-wave sensitive 1 | SWS1 | cone | 355–445 nm | ultraviolet, violet | OPN1SW "blue"
short-wave sensitive 2 | SWS2 | cone | 400–470 nm | violet, blue (extinct in therian mammals) |
rhodopsin-like 2 | Rh2 | cone | 480–530 nm | green (extinct in mammals) |
rhodopsin-like 1 (vertebrate rhodopsin) | Rh1 | rod | ~500 nm | blue-green | OPN2 = Rho = human rhodopsin
Melanopsin OPN4
: circadian rhythms, pupillary reflex, and color correction in high-brightness situations
: circadian rhythms, pupillary reflex, and color correction in high-brightness situations
: expressed in a small fraction of retinal ganglion neurons distributed across retina
therian mammals
: giving birth to live young
@@ -368,7 +375,7 @@ Tremendous amplification. Single photon hitting rhodopsin is estimated to activa
## Need to inactivate opsin signaling after a light flash
* 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 retinol gets shed, transported to pigment epithelium cells, changed to cis-retinol and reincorporated into opsin
* All-trans retinal gets shed, transported to pigment epithelium cells, changed to cis-retinol and then reincorporated into opsin
Note:
@@ -492,20 +499,16 @@ Cone response over in about 200 ms (with an overshoot of inward current), wherea
*15-30 rod to bipolar cell convergence, reduces spatial resolution of rod system but increases light detection*
<!--
[from https://en.wikipedia.org/wiki/Adaptation_(eye)](https://en.wikipedia.org/wiki/Adaptation_(eye))
>The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly 1,000,000,000 apart.
> in any given moment of time, the eye can only sense a contrast ratio of one thousand.
>the eye adapts its definition of what is black.
> takes approximately 20–30 minutes to fully adapt from bright sunlight to complete darkness and become ten thousand to one million times more sensitive than at full daylight
>takes approximately five minutes for the eye to adapt to bright sunlight from darkness
>Dark adaptation is far quicker and deeper in young people than the elderly
-->
---
@@ -788,13 +791,13 @@ Note:
---
## ON and OFF RGCs
## ON and OFF bipolar cells
<div style="font-size:0.7em;">
<div></div>
* Have dendrites that arborize in separate strata of the inner plexiform layer, forming selective synapses with different types of bipolar cells. ON in sublamina A and OFF in sublamina B
* Synapse with bipolar cells. Bipolar cells do not use action potentials, but use graded potentials to release transmitter
* RGC dendrites terminate in separate strata of the inner plexiform layer, forming selective synapses with different types of bipolar cells
* Bipolar cells do not use action potentials, but use graded (passive/electrotonic) potentials to release neurotransmitter
* There are two types of bipolar cells– ON center and OFF center. OFF center uses AMPA receptors (ionotropic) that cause the cell to depolarize in response to glutamate released by photoreceptors. ON center use metabotropic glutamate receptors that lead to the closing of Na⁺ channels and hyperpolarize the cell
</div>
@@ -804,6 +807,8 @@ Note:
off center bipolars: AMPA receptors (sign conserving)
on center bipolars: mGluR6 (sign inverting)
RGC dendrites: ON in sublamina A and OFF in sublamina B
<!--
## On and Off center RGCs
@@ -813,7 +818,7 @@ on center bipolars: mGluR6 (sign inverting)
---
## Circuitry responsible for generating receptive field center responses
## Circuit that shapes the RGC response to light hitting the receptive field **center** includes two types of bipolar cells
<div style="font-size:0.7em;">
<div></div>
@@ -834,12 +839,14 @@ Note:
<div><img src="figs/Neuroscience5e-Fig-11.18-1R_copy_a353fc8.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 11.18</figcaption></div>
<div><img src="figs/Neuroscience5e-Fig-11.18-2R_copy_7bfe450.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 11.18</figcaption></div> <!-- .element: class="fragment fade-in" data-fragment-index="1"-->
<div><img src="figs/Neuroscience5e-Fig-11.18-2R_copy_7bfe450.jpg" height="400px"><figcaption>Neuroscience 5e Fig. 11.18; y-axis is voltage</figcaption></div> <!-- .element: class="fragment fade-in" data-fragment-index="1"-->
Note:
Explain distinction of graded potential vs. action potentials
graded potential vs. action potentials
yaxis in plots is membrane potential
middle panels are membrane potential/graded potential. Bottom is spikes/APs.
@@ -887,26 +894,6 @@ Note:
plus sign: sign conserving synapse
minus sign: sign inverting synapse
<!--
Circuitry that generates the antagonistic surrounds of retinal ganglion cell receptive fields
<div><img src="figs/image17_600d500.png" height="100px"><figcaption></figcaption></div>
A bunch of photoreceptors, but all the 1-1-1 circuits are overlapping giving series of slight shifted center-surround receptive fields.
Circuitry that generates the antagonistic surrounds of retinal ganglion cell receptive fields
<div><img src="figs/image18_7256558.png" height="100px"><figcaption></figcaption></div>
The Hermann grid illusion
<div><img src="figs/image19_5e99664.png" height="400px"><figcaption></figcaption></div>
Explanation of the Hermann grid
<div><img src="figs/image20_b017121.png" height="400px"><figcaption></figcaption></div>
-->
---
## Summary