Bio312-02 Group Homework Assignment - Vision/Color Vision Due: Nov 15, 2021 via Canvas Assignment submission of Word or RTF format document Participating Group Members (list) Noah Van Asten Michaela Kramolis Andreea Bouruc This assignment relies on understanding the absorption spectra of the photoreceptor cells found in the normal, human retina, as described in your lecture materials and also in Saladin Fig. 16.42 (p. 607). It also makes use of the description of processing that occurs in the retinal neurons of the photoreceptor, bipolar, and ganglion cell layers, which is described most fully in the lecture slides on vision. One point is allotted to answers for each of the following questions, unless it is noted otherwise. 1. Clearly distinguish between the rods and cones in terms of the intensity of light that is required in order to activate the rod population versus the cone population. - Rods are very sensitive to light; therefore, they are most important for vision in the dark or dim lighting. So, when the time of day is dark, the rods are activated for us to see properly. Cones, on the other hand, are less sensitive to light; therefore, they are most important for vision in daylight or artificial light areas. So, when the time of day is light, the cones are activated for us to see properly. 2. Provide a reason why it may require a different intensity of light to activate these two populations (rods/cones), and relate this to photoreception. - Rods have a concentration of a transmembrane protein, Rhodopsin, which gets activated by light. Because rods have lots of rhodopsin, they are highly sensitive to light; therefore, it only takes one photon of light to activate a rod (high % of photons of light make it through to strike photopigments). Therefore, rods allow for us to see even in dim light conditions and does not require a high intensity of light to activate. As for cones, they also have a concentration of transmembrane protein, Opsin, which gets activated by light. However, because cones are less sensitive to light, they tend to filter out the incoming light, so little amounts of photons make it through to strike photopigments. Thus, a more intense form of light, i.e daylight, is required for the photopigments from the cones to be able to interpret the incoming light and send the electrical signal, allowing us to see in the daylight at very optimal/detailed degrees. 3. Describe why it is that, even though rods do have a best wavelength (associated with the peak of the rod’s absorption spectrum), humans don’t perceive colors of light when the rods are active. (2 points) - Rods are not particularly effective at color discrimination; thus, we are left with shades of gray to perceive. This is due to rhodopsin not being able to differentiate between different wavelengths of light, only allowing for black and white vision (gray shades). Additionally, rods are most concentrated in the periphery of the retina. As a result, at night, our peripheral vision is often better than our focal point vision, in which most cones containing color discrimination reside. Because cones are responsible for color vision, and they are sensitive to the dark, the cones are not the primary photoreceptor working. Therefore, we are not gaining color information from inactivated cones, leaving us dependent on the rods that have no color discrimination. 4. Graph out the response of the rods and the three cone populations (S,M,L) in response to light at low intensity (imagine moonlight conditions) versus high intensity (outdoors on a sunny day). Use a barchart with bar height representing the % of maximum response for each of the four populations (rods and three cone types). Hand-drawn graphs are fine. (2 points)5. Based on your responses in #4, provide your analysis for why you can easily detect color in the sunlight condition but not in the moonlight condition, despite the fact that all four photoreceptor types have a preferred wavelength (and therefore, color) of light. - Under normal dark light conditions (moonlight), the rods are the only one of the four photoreceptors that are responding. Therefore, you are not receiving any interpretations from any of the three cone photoreceptors associated with much of the color spectrum we can visibly interpret. Thus, there is no color discrimination occurring, leaving us to only interpret shades of gray. In bright light conditions (sunlight), the rods then drop out because they have been bleached out. Thus, the rod photopigments are in a recovery phase. In this instance, the other three cone photoreceptors are active and allow for the interpretation of the color spectrum we can visibly see (a color discrimination), provided there is a high enough light intensity. Additionally, in sunlight conditions, there is not a single photoreceptor designated to a specific color. In these instances, the cone photoreceptors (S,M,L), all work together (compare) and piece bits of color-processing information to eventually interpret one solid color. The type of photoreceptor(s) that contribute to this are dependent on the strength of the wavelength (i.e. 650 nm, M and L photoreceptors are both interpreted). 6. Finally, the retinal pathway starts with a receptor cell that hyperpolarizes in response to light, and somehow this produces an output in the axons of the optic nerve that code for stimulus intensity using action potential frequency, with greater action potential frequency in the axons of “on-type” ganglion cells corresponding to higher light intensity (described in some detail in your lecture slides).Describe the conversion of this signal from [3 points total] A. a rod or cone through the bipolar cell, - In the rods and cones, a generator potential is created in response to the level of darkness (i.e. more dark = greater depolarization). This creates a graded (local) potential that is carried a short distance with minor decrement. The increase in darkness, a greater overall depolarization, allows for a greater increase in the release of glutamine across the synaptic terminals onto the bipolar cells that associate with it. On the contrary, less darkness, an overall hyperpolarization, allows for less glutamine to be released. Glutamine causes inhibition of the bipolar cells through a G protein-linked receptor. Therefore, light causes the removal of this inhibition of bipolar cells, allowing the bipolar cells to continue its electrical communication