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Vision: Photoreception

by Thad Wilson, PhD
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    00:00 Light and Photoreceptor Arrangement The reason why this is an important concept is because it seems a bit countering intuitive.

    00:11 Light is travelling down towards the pigment epithelium.

    00:18 It’s interesting that travels through these cells to get to the photoreceptor layer and the pigment epithelium.

    00:27 The pigment epithelium is going to have chromophores in it, very similar to melanin, which is in your skin to absorb light.

    00:37 Chromophores again are particles that absorb light.

    00:43 Photoreceptors are located just superficial to the pigment epithelium.

    00:50 And these will be what are going to respond to those photons of light.

    00:56 Some of these here are rods and some of them are cones.

    01:00 But both of them will respond to various forms of light.

    01:04 But the interesting the light is travelling through all these layers of nerves to get to these particular point.

    01:11 Looking at photoreceptors in more detail.

    01:14 You can see the rod structure and the cone structure.

    01:18 There’s a couple of different segments that we need to highlight here.

    01:22 You have the synaptic terminals where are the points that which the photoreceptor will interphase with the bipolar cell.

    01:30 You have the inner segments.

    01:32 The inner segments have this stuff of the cell.

    01:36 The cytoplasm has some the mitochondria, has a nucleus.

    01:40 And then you have the outer segments.

    01:42 And this is what has these specialized photoreceptor response elements.

    01:48 In the rod, they look like little discs.

    01:52 In the cone, they look like evaginations.

    01:54 And so you can see how they named rods and cones base upon how they look in terms of the histology.

    02:01 How they work? So the photoreceptor part in the rod is going to be those discs.

    02:10 Those discs have a molecule called rhodopsin in them.

    02:14 Rhodopsin is a protein hooked to a retinal molecule.

    02:19 So the ups it is the protein and the row is the retinal component.

    02:25 Its starts out in this 11-cis retinal configuration bound to the opsin molecule.

    02:33 If light is present this 11-cis retinal molecule conformationally change as to an all-trans retinal.

    02:44 When these all-trans conversion happens, it causes a further conversion to the rhodopsin molecule and forms this metarhodopsin II.

    02:56 These activates a G protein coupled receptor. And this G protein is transducin.

    03:04 Transducin activates an enzyme called phosphodiesterase.

    03:09 And if you remember what a phosphodiesterase does, it changes either cyclic AMP or cyclic GMP back to its original form.

    03:19 In this case 5'-GMP.

    03:22 Why this becomes important? Is it decreases cyclic GMP within the cell? Cyclic GMP has an important function. In that, it is linked to sodium channels.

    03:38 If you have high levels of cyclic GMP around, you’ll have these sodium channels open to a greater degree.

    03:45 If you decrease cyclic GMP, you close these sodium channels.

    03:50 The closure of the sodium channels hyperpolarized the photoreceptor membrane.

    03:57 Why this is important? The hyperpolarization allows for a greater amount of on versus off component of the photoreceptor.

    04:10 So if light is present, you get hyperpolarization.

    04:15 If light is absent, you get depolarization.

    04:20 That’s a little bit different than some receptors that we have talk about does far in physiology.

    04:26 So remember, Light - Hyperpolarizes Dark - Depolarizes


    About the Lecture

    The lecture Vision: Photoreception by Thad Wilson, PhD is from the course Neurophysiology.


    Included Quiz Questions

    1. All-trans retinal
    2. 11-cis retinal
    3. Transducin
    4. 3’ GMP retinal
    1. Outer segment
    2. Synaptic terminal
    3. Inner segments
    4. Pigment epithelium
    5. Basal cells
    1. G protein
    2. 11-cis retinal
    3. All-trans retinal
    4. Meta-rhodopsin II
    5. Phosphodiesterase
    1. Increase in 5'-GMP
    2. Sodium influx
    3. Increase in cGMP
    4. Dark
    5. Inactive phosphodiesterase

    Author of lecture Vision: Photoreception

     Thad Wilson, PhD

    Thad Wilson, PhD


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