the final learning objective for the topic of vision is to briefly describe the process of light transduction light transduction is simply the process by which light energy is converted into electrical signals in the retina of the eye now when I say briefly I really do mean briefly try not to get too bogged down in all the detail here sometimes I need to mention that detail so that you understand the whole process so as I said light transduction is simply the process of taking light and turning it into a neural signal in the eye the photo receptors are the site of light transduction now more specifically the transduction of light into a receptor potential occurs in the outer segment of both our rods and our cones sticking out of the plasma membrane in the outer segment of these photo receptors are integral proteins called photo pigments the first step in the transduction of light is the absorption of Light by a photop pigment a photopigment is an integral protein that changes shape when it absorbs light in rods the specific photo pigment is called ropson in cones there are three different types of photop pigments for each of our blue green and red cones all photo pigments associated with vision contain two parts a glycoprotein known as opsin and a derivative of vitamin A called retinol retinol is the light absorbing part of all photopigments but we have have four different types of opsins we have rod opsins in rods and then three different types of opom in the three different colored cones with small variations in the sequence of amino acids in these opom what allows the different opom to absorb different wavelengths of light and therefore differentiate between different colors now having said all of that I will recap this information for you as as well as the information coming up and I will highlight the really important parts that you need to know but as I mentioned sometimes to understand the whole picture you need to hear a bit about this background detail first so starting with our photo pigments which remember are integral proteins that stick out of the plasma membrane of the outer segment of our photo receptor so this is the outer receptor of our rod and in Darkness the retinal portion of the photo pigment has a bent shape when it's in its bent shape it's called CIS retinol and this CIS retinol fits really nicely into the opson portion of that photo pigment when CIS retinol absorbs a photon of light it straightens out to a shape called trans retinol and this CIS to trans conversion is what we call isomerization after retinol undergoes that change in shape several chemical reactions occur and it's these chemical reactions which we'll talk about shortly that lead to the production of a receptor potential after about a minute of being in this straight form trans retinol completely separates from the opsin and this is a process we call bleaching of the pigment the reason for this is when trans retinal separates from opsin the remaining opsin appears colorless in the third step and a enzyme called retinol isomerases converts trans retinol in its straight structure back to CIS retinol which is its bence form and in the fourth step that Cy retinol then binds again with the opsin reforming a functional photo pigment so this part of the cycle is called the Regeneration of the photo pigment so I promise I would recap all of that for you so in the dark the retinol in a photo pigment is in a Cy retinol or a bent form when the retinol absorbs light the CIS retinol changes shape to trans retinol which is straight trans retinol then separates from opsin which is what we call bleaching of the photopigment an enzyme called retinol isomerize converts trans retinol back to CIS retinol or from from the straight version to the bent version then the Cy retinol canbin again with the opsin because it fits really nicely when it's bent inside the opsin and it becomes a functioning photo pigment once more so this last process is called the Regeneration of the photo pigment so that was the process of light actually being absorbed by the outer segment of a photo receptor here we're going to take a look at that process in a little more detail as well as the next steps so how that receptor potential then develops into an action potential that travels along the optic nerve to the brain so we've established in the dark that retinol is in its Cy retinol or its bent form in this form sodium flows into the outer segment of a photo receptor through liand gated sodium channels the liand or the chemical that causes these channels to open is called cyclic GMP the influx of sodium partially depolarize the photo receptor and this partial depolarization causes opening of voltage gated calcium channels which means calcium moves into the photo receptor the in flux of calcium into the photo receptor like in our other neurons causes the release of a neurotransmitter at the synaptic terminal the neurotransmitter in rods which is also believed to be the same in cones is called glutamate and here glutamate is an inhibitory neurotransmitter meaning that it triggers an inhibitory post synaptic potential or hyperpolarization in the bipolar cell hyperpolarization of the bipolar cell means that no neurotransmitter is relieved uh released sorry at the synaptic Cliff between our bipolar cell and our gangan cell and this means that our gangan cell is therefore not depolarized if the gangan cell isn't depolarized no action potential will travel along the optic nerve so in the dark when there is no light being absorbed by the photo pigment in the photo receptor no action potential is generated in our bipolar cell or the gangan cell or our optic nerve in light however the absorption of a photon of Light by that retinal causes a change in shape from Cy retinol to trans retinol or the bent retinol to the straight retinol this change activates enzymes that break down that cyclic GMP so now our Liang gated sodium channels will remain shut because the photo receptor is no longer being depolarized because we don't have any sodium coming into the photo receptor the voltage gated calcium channels also remain shut and no calcium enters the photo receptor because no calcium enters the photo receptor the neurotransmitter glutamate which remember is inhibitory to bipolar cells is not released because the inhibitory neurotransmitter is not released the bipolar uh cell will then become depolarized neurotransmitter at the synaptic of the bipolar cell is now released the neurotransmitter released at the syapse between our bipolar cell and gangan cell is also glutamate but for our gangan cells glutamate is actually excitatory so our gangan cell is now depolarized and we will have an action potential travel along our optic nerve so so recapping all of that information and this is what I'd like you to be able to do briefly walk me through an image like this so in our retina we have photo receptors within the outer segment of those photo receptors are integral proteins called photopigments photop pigments are made up of retinol and of opsin in the dark which is this gray side here the retinol takes a form of Cy retinol in this form we have cyclic GMP present which causes the opening of Liang gated sodium channels in the photo receptor sodium goes into that photo receptor changes our membrane potential and opens our voltage gated calcium channels on that photo receptor the influx of calcium causes a neurotransmitter to be released the sinapse between our photo receptor and our bipolar cell here we release glutamate which is inhibitory to our bipolar cells so because we've had hyperpolarization of our bipolar cells we won't have any sodium coming into the cell our voltage gated calcium channels remain shut and no neurotransmitter at this syapse between our bipolar cell and our gangan cell is released because we're not releasing a neurotransmitter our gangan cell won't be depolarized and will have no action potential running along our optic nerve in contrast in the light when light is absorbed by retinol it changes from CIS retinol to trans retinol this causes the brain breakdown of cyclic GMP by enzymes therefore our liand gated sodium channels stay shut no sodium goes into our photo receptor so we don't have a change in membrane potential and our our voltage gated calcium channels will also remain shut no calcium in this axon terminal will cause that neurotransmitter to not be released because it's in inhibitory we now have a depolarizing effect on our bipolar cell so our bipolar cell will be in a state of continuous depolarization unless we release this inhibitory neurotransmitter so without that inhibitory neurotransmitter our bipolar cell depolarizes causes the release of glutamate now this syapse between our bipolar cell and our gangan cell glutamate is causes depolarization in our gangan cell so we have depolarization of our gangan cell and an action potential running along along our optic nerve now lastly after an action potential is generated and travels along the optic nerve the optic nerve then exits the eye at the back of the eyeball and the optic nerve from each eye passes through a bit of a cross that we call the optic kaym the optic kaym forms across at the base of the bane just inferior to the hypothalamus here some of the axons cross over to the opposite side while other axons remain uncrossed after passing through this optic km We Now call those tracks our optic tracks so these nerves are now just called our optic tracks and these enter the the brain where most axons terminate in the thalamus they then syapse with neurons which project to the primary visual area in the occipital lobe and it's here where our visual perception Begins the axons that don't terminate in the thalamus in said syapse in the superior calculi which remember is in the midbrain and it controls things like our extrinsic eye muscles and the accommodation of our lens and the raction of our pupil which are involved in reflexes so once more summarizing that information and putting it into text for you so our optic nerve exits at the back of our eye and we have one coming from the back of each eyeball they then cross over at the base of the brain in the optic kaym some axons will cross over to the opposite side some will stay on the same side once those axons have crossed over or or gone through that cross but stayed on the same side we now call these axons the optic tract so these are the optic tracks most of those axons will then syapse in the thalamus and then the primary visual area those that don't pass through the thalamus or sinapse with a superior cicular eye and go to various eye muscles because these tracks are involved in reflexes that are related to vision