Vision: Photoreception

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