 Hello and welcome to Physiology Open. The spectrum of electromagnetic radiation which is visible to us, we call it as light. The visible light spectrum ranges in the wavelength from 400 to 700 nanometers. Now light is also depicted as a particle that is photon which is the minimum unit of light. Phototransduction is the process by which energy of a photon is converted to electrical energy by the photoreceptors that is rods and cones. Now there are certain questions which we should think about before we proceed. Like a photon from which part of the spectrum is converted to electrical energy and how many photons from each end of the spectrum are we talking about? Then what is perceived as light intensity and what is perceived as the quality of light or colour? Well basically there are two things. Light wavelength which describes the spectrum of electromagnetic radiation and is responsible for colour and light intensity or brightness at a particular wavelength which depends on the number of photons. So how do photoreceptors that is rods and cones respond to these different aspects of light? Rods and cones structurally have three functional segments. An outer segment which is responsible for phototransduction then an inner segment which houses the nucleus and hence the biosynthetic machinery and finally there is the synaptic terminal which makes contact with other cells of the retina and releases neurotransmitters. Let's zoom this outer segment. This outer segment which is responsible for photodransduction consists of stacked membranous disks which have photopigments on their membrane. The stacking of membranous disks basically increases the surface area tremendously and that's why they can have a large number of photopigments on them. Secondly this outer segment also has ligand gated sodium channels which are dependent on the presence of CGMP. The concentration of CGMP is dependent on two things. One is synthesis from GTP by the enzyme guanyl cyclase and the other is degradation into 5-GMP by the enzyme phosphodiesterase. Now normally in dark or in absence of photons the concentration of CGMP is high since the enzyme guanyl cyclase is continuously active. This keeps the sodium channels open causing entry of sodium ions and the cell remains in depolarized state in dark. And just like any other neuron depolarization causes release of the neurotransmitters from the synaptic terminal. So basically in dark this neurotransmitters are continuously being released just since the channels are open. In presence of light as photons strike the photopigment the enzyme phosphodiesterase is activated decreasing the concentration of CGMP in the cells. Let's go bit into the micro details of how this happens. The photopigment is known as rhodopsin in rods and photopsin in cones. Fundamentally they function similarly except for some differences. So let's see how rhodopsin works then we'll see how photopsin differs from rhodopsin. Rhodopsin is a combination of G protein coupled receptor that is opsin and its ligand which is an aldehyde derivative of vitamin A that is retinal. In dark this retinal is in 11 cis retinal form and it remains bound to its T protein coupled receptors that is opsin. When the photon strikes this rhodopsin photon is absorbed by 11 cis retinal which then gets converted into all trans retinal. Now with the conversion of 11 cis retinal to 11 trans retinal it detaches from GPCR opsin and due to this opsin also changes conformation and there is activation of trimeric G proteins known as transducin. The G-alpha part of transducin activates the enzyme phosphodiesterase which breaks down CGMP to 5-GMP. Thus reducing the concentration of CGMP inside the cell this leads to closure of CGMP dependent sodium channels. Thus decreasing sodium entry into cells and causing hyperpolarization this decreases the neurotransmitter release. So in light the cell is hyperpolarized and the release of neurotransmitters is less. In the meantime this all trans retinal gets converted to 11 cis retinal by the enzyme isomerase and goes and binds with opsin again ready for the next stimulus. Now this is the basics of photo transduction in the photoreceptors but some questions remain here. One, do rods and cones respond to photons from the entire visual spectrum equally or differently? Secondly, how do they respond to different intensities of light? Now see rods and cones have different forms of opsin. The binding of retinal to different forms of opsin actually tubes this retinal molecule to a particular region of the spectrum. Tuning means just by the virtue of binding to different forms of opsin they respond to photons from different wavelengths of the spectrum. So this diagram is showing the response of the rods and different types of cones to photons from different ranges of the spectrum. X axis shows the wavelength to which they respond while the Y axis shows their relative responses. So basically photoreceptors respond to a wide range of wavelength but their relative responses vary across the range. Let's focus on rods. See that they respond maximally to photons from a particular wavelength here at the peak of this curve. Only one photon may be sufficient to excite the receptor but at other wavelengths of this curve see their relative responses are less. So more photons may be required for this particular wavelength. Similar is the case with the different types of cones. The different types of cones respond to photons from different wavelengths you see and this signal from the cones is processed by further visual pathways and it is interpreted as colours. In actuality in real world there are no colours as such. It is we who construct these wavelengths as colours. Secondly rods and cones work maximally under different conditions of illumination. Rods have a low threshold to light that is they are very sensitive while cones have a higher threshold. This happens because in rods there is so much amplification of the signal that excitation by a single photon can cause degradation of as many as 1000 CGMP molecules causing closure of approximately 200 sodium channels and hence hyperpolarization of the receptors. Cones on the other hand require at least 100 photons since amplification of the signal is not much in cones. Thus a single photon cannot cause closure of the channels hence cones are not responsible for dark vision. Also rods are slow to respond so basically it's a slow but amplified activation in rods. So due to this slow response they kind of sum up the responses from a large number of photons. So this makes them suitable for dark or scatopic vision. But also note that because of this fusion of the responses they will become ineffective for detecting flicker. Since for detecting flicker it is required that they should detect each light signal separately rather than sum the response. But cones on the other hand are quick to respond so they can't sum up the responses and that is why they are good at detecting the flicker. This we also call as cones having a better temporal resolution that is they are better at detecting quick successive light signals separately. Now given the rate of response of the photoreceptors that is even a single photon in rods causes closure of 200 ion channels you can imagine how fast all the channels will be closed with only few photons. That means the photoreceptors will not be able to detect light over and above those number of photons since no more channels are there which can be closed. So since rods respond even to a single photon they will saturate fast and are not useful any further. So that's why they are responsible only for scatopic vision while cones start their response only with a higher intensity of light. So they have a higher threshold right? Nonetheless even with that they will saturate very fast. That means our photoreceptors will saturate only within a narrow band of light intensity. But does this happen in reality? See in actuality we can appreciate light intensity is ranging from absolute dark to as bright as broad day sunlight. Now this is possible due to a phenomena known as light adaptation. Basically the retina doesn't encode absolute light intensity. Now focus here little bit. So it's not that one photon causes closure of 200 ion channels and two photon causes closure of 400 ion channels and so on. Instead as light intensity increases this closure of sodium channels becomes less and less. So maybe like two photons will cause closure of 100 more channels and maybe one more photon if it comes it will cause closure of additional 50 channels only. Point is that the relationship is not linear. This happens because these sodium channels also allow entry of calcium ions. When the channels close in response to light calcium entry also decreases. This leads to decrease in intracellular calcium levels. Now normally calcium has many roles inside the photoreceptors. It inhibits the activity of guanyl cyclase. So with decrease in calcium what will happen? Background guanyl cyclase activity will increase causing increase in CGMP. Also the affinity of channels to CGMP increases. So channels are kept open with a lower CGMP concentration. So basically this decrease in intracellular calcium is offsetting the action of the photon. So as light intensity increases the magnitude of amplification decreases due to decrease in intracellular calcium ions. This allows us to appreciate wide ranges of light intensity. This is known as wide dynamic range. In short we don't appreciate absolute light intensities but relative light intensities. Now this adaptation mechanism is more effective for cones. I hope now you will appreciate rods and cones not only as a phototransduction apparatus but also as something which helps us in detecting features of light ranging from colours, varied light intensities and also flicker. Thanks for watching the video. If you liked it do share and like the video and subscribe to the channel Physiology Open. Thank you.