Phototransduction Process

Overview

This lecture explains phototransduction—the process by which light is converted into electrical signals in retinal photoreceptors (rods and cones)—including the key molecular steps and important differences from other sensory receptors.

Phototransduction Definition & Location

• Phototransduction is the conversion of light energy into electrical changes, leading to vision.
• Occurs in the discs of the outer segment of rods and cones in the retina.

Key Molecules & Steps in Phototransduction

• Rhodopsin: Light-sensitive pigment in rods, made of opsin (protein) and 11-cis retinal (from vitamin A).
• Light causes 11-cis retinal to isomerize to all-trans retinal, activating rhodopsin.
• Activated rhodopsin (metarhodopsin II) separates from all-trans retinal—called rhodopsin bleaching.
• Metarhodopsin II activates transducin (a G-protein) by exchanging GDP for GTP.
• Activated transducin activates phosphodiesterase (PDE) by removing its inhibitory gamma subunits.
• PDE converts cyclic GMP (cGMP) into GMP, reducing cGMP concentration.
• cGMP is required to keep sodium channels open; when cGMP decreases, sodium channels close.

Electrical Changes in the Photoreceptor

• In the dark, high cGMP keeps sodium channels open, allowing sodium (and some calcium) to enter—cell is depolarized.
• In light, sodium channels close, less sodium enters, cell becomes more negative (hyperpolarized).
• Sodium is continually pumped out at the inner segment, generating the "dark current."
• Depolarization (in dark) leads to continuous neurotransmitter (glutamate) release; hyperpolarization (in light) reduces neurotransmitter release.

Signal Amplification and Adaptation

• One activated rhodopsin can activate about 800 transducin molecules, leading to closure of ~200 sodium ion channels.
• The amplification rate adjusts with illumination level (light adaptation).
• Decreased internal calcium during light activates guanylate cyclase, increasing cGMP and reopening sodium channels (negative feedback).
• Arrestin protein can inactivate activated rhodopsin to stop phototransduction.

Visual Cycle and Rhodopsin Regeneration

• After bleaching, all-trans retinal is converted back to 11-cis retinal by retinal isomerase in the retinal pigment epithelium.
• Regeneration of rhodopsin can occur in both light and dark.
• The overall process balances bleaching and regeneration, forming the visual cycle.

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 — G-protein activated by metarhodopsin II; activates PDE.
• Phosphodiesterase (PDE) — enzyme that converts cGMP to GMP.
• cGMP — cyclic guanosine monophosphate; keeps sodium channels open.
• Dark current — inward flow of sodium in the dark, keeping photoreceptors depolarized.

Action Items / Next Steps

• Review diagrams of the phototransduction cascade.
• Memorize the sequence of molecular events in phototransduction.
• Learn key differences between photoreceptors and other sensory receptors.
• Prepare for questions on the visual cycle and its regulation.