ποΈ Sensory Biophysics: Phototransduction Cascade
Phototransduction is the G-protein mediated process by which retinal photoreceptors (rods and cones) absorb photons of light and convert them into electrical hyperpolarizing receptor potentials, governed by cyclic GMP (cGMP) hydrolysis.
1. Photoreceptor Architecture & The Dark State
Unlike standard sensory receptors that depolarize upon stimulation, vertebrate photoreceptors are depolarized in the dark (~ -40 mV) and hyperpolarize upon light absorption (~ -70 mV).
- The Dark Current: In darkness, high cytoplasmic levels of cyclic GMP (cGMP) keep cGMP-gated cation channels open in the outer segment. A continuous inward current of $Na^+$ and $Ca^{2+}$ depolarizes the cell.
- Continuous Transmitter Release: Depolarization keeps voltage-gated $Ca^{2+}$ channels open at synaptic terminals, driving tonic, continuous release of glutamate onto bipolar cells.
The Rhodopsin Phototransduction G-Protein Cascade
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Figure 3.1: Phototransduction enzymatic cascade: photon absorption → rhodopsin isomerizes → transducin activation → PDE6 hydrolyzes cGMP → cGMP channels close → hyperpolarization.
2. Rods vs Cones Functional Divergence
| Physiological Parameter | Rods (Scotopic Vision) | Cones (Photopic Vision) |
|---|---|---|
| Sensitivity | High (detects single photons; night vision) | Low (requires hundreds of photons; day vision) |
| Photopigment | Rhodopsin (absorbance peak ~498 nm) | 3 Opsins: S-cone (420 nm, Blue), M-cone (530 nm, Green), L-cone (560 nm, Red) |
| Spatial Resolution (Acuity) | Low (high retinal convergence: many rods to 1 ganglion cell) | High (1:1 foveal cone-to-bipolar-to-ganglion circuitry) |
| Temporal Resolution | Slow response kinetics, prolonged integration | Fast response kinetics, rapid dark adaptation recovery |