Φ | | #‡ | | Number of photoisomerizations per rod per flash |

*R* *(*t*) | | # | | Number of activated rhodopsins per rod at time *t* |

*G* *(*t*) | | # | | Number of free, activated G-proteins per rod at time *t* |

*E* *(*t*) | | # | | Number of activated PDE catalytic subunits per rod at time *t* |

*cG*(*t*) | | μM | | Concentration of free cGMP in the outer segment |

*F*(*t*) | | # | | Normalized circulating current at time *t* |

ν_{RP} | | s^{−1} | | Rate of production of E*s per R* |

τ_{R} | | s | | Time constant for first-order inactivation of R* catalytic activity |

τ_{E} | | s | | Time constant for first-order inactivation of G*–E* complex |

α(*t*) | | μM s^{−1} | | Rate of synthesis of cGMP by guanylyl cyclase |

α_{dark} | | μM s^{−1} | | Dark rate of cGMP synthesis |

β_{sub} | | s^{−1} | | Rate constant of cGMP hydrolysis by single PDE catalytic subunit |

β(*t*) | | s^{−1} | | Rate “constant” of PDE activity in outer segment |

β_{dark} | | s^{−1} | | Rate constant of cGMP hydrolysis in the dark |

*cG*_{dark} | | μM | | Resting cytoplasmic concentration of cGMP |

*n*_{H} | | # | | Hill coefficient of the cGMP-activated channels in situ |

*A* | | s^{−2} | | Amplification constant; equal to v_{RP}β_{sub}*n*_{H} |