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Lecture 15

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PHGY 1030
Luming Wang

PHOTOTRANSDUCTION The photoreceptors convert (i.e. transduce) light energy into changes in membrane potential. Rods outnumber cones in the human retina by 20 to 1. Phototransduction in Rods 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 RESPONSE TO DARKNESS  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. IN RESPONSE TO LIGHT +  Light reduces cGMP, Na channels to close and the membrane potential becomes more negative. Photoreceptors hyperpolarize in response to light.  The response is initiated by the absorption of electromagnetic radiation by photopigment in the membrane of the stacked disks in the rod outer segments. o This pigment is called rhodopsin which is a receptor protein with a pre-bound agonist. The receptor is opsin which has seven transmembrane alpha helices. The pre-bound agonist is called retinal (derivative of vitamin A). o The absorption of light causes a change in conformation of retinal so that it activates opsin (see Fig. 9.18). This process is called bleaching because it changes the wavelengths absorbed by the rhodopsin (photopigment changes from purple to yellow). o Bleaching of rhodopsin stimulates G-protein transducin in the disk membrane which activates the effector enzyme phosphodiesterase (PDE). PDE breaks down the cGMP present in the rod at dark. The reduction in cGMP causes the Na channels to close and the membrane to hyperpolarize. Phototransduction in Cones In bright sunlight, cGMP levels in rods fall to the point where the response to light becomes saturated making vision during the day dependent on cones whose photopigments require more energy to become bleached. The process
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