Rhodopsin Amplification Cascade

Light Stimulus

Background light rate 0 photons/s

PDE catalytic gain (β) 18

Rhodopsin lifetime τ_R 0.30 s

Cascade State

Active R* (rhodopsin)0.00

Active PDE, E*0.0

cGMP level100%

Photocurrent (channels open)35.0 pA

How it works

A rod photoreceptor's outer segment is a stack of membrane discs studded with rhodopsin. One absorbed photon isomerises rhodopsin to its active form R*, which catalytically activates hundreds of transducin/PDE complexes before it decays — the first stage of a huge biochemical amplification.

dR*/dt  = -R*/τR                         (+1 per photon absorbed)
dE*/dt  = kRE·R* - E*/τE                 (active PDE, driven by R*)
dcGMP/dt = α - (β0 + β·E*)·cGMP          (synthesis vs. E*-driven hydrolysis)
I/Idark = (cGMP / cGMP_dark)^n,  n ≈ 3   (Hill: channel cooperativity)

Flash one photon — a single R* event, the classic "single-photon response" that first demonstrated rod photoreceptors can count individual photons.
Background light rate — a Poisson stream of photon absorptions, showing how steady light drives the cascade into a new adapted steady state.
PDE gain β — how fast each active PDE molecule hydrolyses cGMP; higher gain means a bigger current drop per photon (higher single-photon amplification).
τR — how long rhodopsin stays catalytically active before it is shut off by phosphorylation/arrestin; longer τR means each photon triggers more downstream PDE activation.

Closing cGMP-gated channels stops Na⁺/Ca²⁺ influx and hyperpolarises the cell — this is how a single photon becomes a nerve signal. Real rods amplify roughly 10⁵–10⁶-fold between one photon and the resulting current change.

Photon Active PDE (E*) Open channel Closed channel

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