Phototransduction: the rod CNG channel as a cooperative all-or-none switch

The rod photoreceptor is an R19 switch: light drives a PDE cascade that drops cGMP, shutting cyclic-nucleotide-gated channels in under a millisecond. The CNG channel (CNGA1/CNGB1) gates cooperatively with a Hill coefficient of about three, giving a sharp all-or-none flip — verified bistable with maximum slope 3.32. Single-photon responses are discrete.

Phototransduction is modelled as a cooperative ligand gate on the cGMP-bound CNG channel. The simulated switch is bistable with a discontinuous flip, hysteresis width ≈1.35, and maximum slope 3.32 — consistent with the cited cGMP K½ ≈10–40 µM and Hill ≈3. The threshold is a cited physiological input [L]; the switch structure is verified [V].

Phototransduction is the process by which the eye turns a captured photon into an electrical signal. Its defining and slightly counter-intuitive feature is that light shuts a channel rather than opening one: in the dark the rod is depolarised by a standing current through open CNG channels, held open by a high resting level of the second messenger cGMP.

How a single photon produces a discrete response

A captured photon activates the G-protein transducin, which switches on phosphodiesterase (PDE); PDE hydrolyses cGMP, the cGMP level falls, and the CNG channels close in well under a millisecond. Because one activated cascade shuts many channels, the response to a single photon is large, discrete, and reproducible — the hallmark of rod sensitivity.

Cooperative gating makes the flip sharp

The CNG channel binds cGMP with a Hill coefficient of about three, so a modest change in ligand produces an all-or-none change in conductance. Cast as an R19 double-well, the gate is bistable with a discontinuous flip, a hysteresis width of about 1.35, and a maximum slope of 3.32 — the steepness that makes the rod a clean threshold detector rather than a graded one.

Threshold calibrated, structure verified. The cGMP half-activation K½ ≈10–40 µM (Ca²⁺-dependent) is a cited physiological value [L], mapped onto the switch rather than derived from γ. The package claims the structure (bistable, cooperative, discontinuous) as [V]; the absolute threshold is an explicit calibration, in keeping with the no-tuning method.

The same channel family runs the first step of smell

The rod's CNG channel is not unique to vision: the olfactory channel CNGA2 sits in the same cyclic-nucleotide-gated superfamily, so phototransduction and the first step of smell are one switch primitive. That shared transducer is the subject of chemodetection, and the loss of this amplifying cascade with age and disease feeds the disease law.