NEUR 3200 Midterm: NEUR-3200 Exam 3- SA- Module 14-21
13 Feb 2017
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Q: describe the differences in membrane polarity between photoreceptors in the light and in darkness, and the molecular mechanisms underlying those differences, including the effect of photon absorption (note that this is not a question about light adaptation). High levels of guanylyl cyclase, which synthesizes cyclic guanosine monophosphate (cgmp), therefore high cgmp levels. cgmp opens cgmp-gated ion channels, causing depolarization via the dark current" of na+ influx. This na+-mediated depolarization opens voltage-gated ca2+ channels, and increased [ca2+] leads to release of glutamate, which signals to bipolar cells. Light reduces signals from photoreceptor to bipolar cell. Photons are detected by rhodopsin, located in disk membranes in photoreceptors. Rhodopsin is composed of retinal (derived from vitamin a) and opsin (a g-protien coupled receptor that tunes the light absorption to a specific wavelength). There"s one type of opsin in rods, one of three different types of opsin in cones. When retinal absorbs a photon, it is converted from cis to trans configuration.
