NEUR 3200 Study Guide - Midterm Guide: Cyclic Adenosine Monophosphate, Ventral Anterior Nucleus, Parasympathetic Nervous System
7 Feb 2016
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Q1: 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) Photons detected by rhodopsin, located in disk membranes in photoreceptors. Opsins are gpcrs that bind retinal and tune the light absorption to a specific wavelength. One opsin in rods, one of three different opsins in cones. Photon absorbance by retinal converts it from cis- to trans- conformation. In darkness, photoreceptor is depolarized (-40 mv) In darkness, cgmp levels are high cgmp synthesized by guanylyl cyclase. Cgmp opens cgmp-gated na+ channels causing depolarization. Na+ ion movement termed the dark current". Light reduces signals from photoreceptor to bipolar cel. When retina absorbs a photon, it is converted from cis to trans configuration. Retinal no longer fits the opsin binding site and dissociates.
