PHGY 210 Lecture Notes - Membrane Potential, Visual Phototransduction, Transducin

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31 Jan 2013
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The photoreceptors convert (i. e. transduce) light energy into changes in membrane potential. Rods outnumber cones in the human retina by 20 to 1. In the photoreceptor, light stimulation of the photopigment activates g-proteins, which in turn activate an effector enzyme that changes the cytoplasmic concentration of a second messenger molecule and causes a membrane ion channel to close and alter membrane potential. In complete darkness, the membrane potential of the rod outer segment is about -30 mv. This depolarization is caused by the steady influx of na+. The movement of positive charge across the membrane occurring in the dark is called dark current. Sodium channels (gated) are stimulated to open by an intracellular second messenger called cyclic guanosine monophosphate (cgmp). cgmp is continually produced in the photoreceptor by guanylyl cyclase keeping the na+ channels open. Light reduces cgmp, na+ channels to close and the membrane potential becomes more negative. The receptor is opsin which has seven transmembrane alpha helices.

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