Overview
This lecture explains phototransduction—the process by which light is converted into electrical signals in the retina—focusing on the roles of rods and cones, key molecules, and the visual cycle.
Phototransduction Process
- Phototransduction converts light energy absorbed by photosensitive pigments in rods and cones into electrical changes, enabling vision.
- Phototransduction occurs in the discs of the outer segments of rods and cones.
- The main steps include photochemical changes in visual pigments, generation of electrical signals, and neurotransmitter release.
Key Molecules and Steps
- Rhodopsin (a G-protein coupled receptor) is composed of the protein opsin and 11-cis-retinal (a vitamin A derivative) in rods.
- Light causes 11-cis-retinal to isomerize to all-trans-retinal, leading to changes in rhodopsin structure and its activation.
- Activated rhodopsin (metarhodopsin II) separates from all-trans-retinal, a process called "rhodopsin bleaching."
- Activated rhodopsin activates transducin (a G-protein) by exchanging GDP for GTP.
- Activated transducin activates phosphodiesterase (PDE) by removing its gamma subunits.
- Phosphodiesterase converts cyclic GMP (cGMP) to GMP, reducing cGMP levels.
Sodium and Calcium Channel Regulation
- In darkness, high cGMP levels keep sodium channels open, allowing sodium influx and depolarizing the cell (“dark current”).
- In light, cGMP decreases, sodium channels close, causing hyperpolarization and reduced neurotransmitter (glutamate) release.
- Inner segment sodium pumps maintain negativity inside the cell.
- Calcium also enters through cGMP-gated channels, and its intracellular concentration influences neurotransmitter release; less calcium enters in light.
Signal Amplification and Adaptation
- One activated rhodopsin can activate about 800 transducin molecules, amplifying the signal.
- Each PDE molecule can hydrolyze multiple cGMP molecules, leading to the closure of hundreds of sodium channels per photon.
- Light adaptation involves the reduction of amplification at higher illumination levels.
- Guanylate cyclase and other proteins help restore cGMP levels as calcium drops, ending the phototransduction cascade.
- Arrestin inactivates activated rhodopsin to stop the process.
Visual Cycle and Regeneration
- Rhodopsin bleaching: Light-induced conversion of 11-cis-retinal to all-trans-retinal.
- Rhodopsin regeneration: All-trans-retinal is converted back to 11-cis-retinal by retinal isomerase in the retinal pigment epithelium.
- The visual cycle maintains a balance between bleaching and regeneration.
Key Terms & Definitions
- Phototransduction — Conversion of light into electrical signals in the retina.
- Rhodopsin — Light-sensitive pigment in rods, made of opsin and 11-cis-retinal.
- Transducin — A G-protein that transmits activation from rhodopsin to PDE.
- Phosphodiesterase (PDE) — Enzyme that hydrolyzes cGMP, leading to channel closure.
- cGMP — Cyclic guanosine monophosphate, keeps sodium channels open in darkness.
- Dark current — Depolarizing current due to sodium influx in the dark.
- Arrestin — Protein that stops activated rhodopsin from continuing the cascade.
- Visual cycle — Repeated process of bleaching and regeneration of visual pigment.
Action Items / Next Steps
- Review the phototransduction cascade and each protein's role.
- Study the differences between the dark and light responses in rods and cones.
- Prepare notes on photoreceptor adaptation and the visual cycle for revision.