2015 Vision

Example Test QuestionPhototransduction

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Phototransduction occurs via a 4 step process that uses a 2nd messenger cascade to amplify the signal.  In rods, activation of rhodopsin ultimately results in the closure of cyclic nucleotide gated Na+ channels, and hyperpolarization of the photoreceptor.

  1. activation of the receptor protein
    • in rods, that is rhodopsin

1 photon à 1 rhodopsin

Step 1
  1. the activated receptor protein stimulates the G-protein transducin
    • GTP is converted to GDP in the process

1 rhodopsin à 100 transducins/s

Step 2
  1. In turn, activated transducin activates the effector protein phosphodiesterase
    • PDE converts cGMP to GMP

1 transducin à 100 PDE/s

Step 3
  1. Falling concentrations of cGMP cause the transduction channels to CLOSE, DECREASING a Na+ current

1 PDE à 1000 GMP/s

Step 4
Email: Dr. Janet Fitzakerley | ©2015 University of Minnesota Medical School Duluth | Last modified: 12-feb-15 6:05 AM