Visual Transduction in Photoreceptors

Study Notes

Photopigments are found in the outer segment membranes of rods and cones, consisting of opsin (protein) and retinal (derivative of vitamin A).
In darkness, open NA+/CA++ channels lead to glutamate release, causing membrane depolarization with open K+ channels contributing to this net effect.
Light Activation Process:
- Light converts opsin and retinal, activating the G protein transducin.
- This subsequently activates phosphodiesterase, decreasing cGMP levels.
- Closure of NA+/CA++ channels occurs while K+ channels remain open, resulting in hyperpolarization of the retinal.

Rhodopsin, formed by retinal and opsin, is pivotal for light absorption.
A single rhodopsin molecule can activate substantial amplification responses:
- Absorbing one photon activates 500 transducin molecules.
- This leads to activation of 500 phosphodiesterase, hydrolyzing 10^5 cGMP molecules and closing 250 cation channels.
- Prevents entry of 10^6 to 10^7 Na+ ions per second.

Agonist binding initiates the activation of G-proteins, leading to:
- Phosphorylation which promotes arrestin binding.
- Clathrin-coated pits facilitate clustering and endocytosis.
- Dissociation of agonist enables dephosphorylation and sorting between recycling and lysosomal pathways.

In dark conditions, trans-retinal binds with opsin to form rhodopsin, activating guanylate cyclase (GC) to increase cGMP, thus opening Na+ channels and depolarizing rod cells.
In light, cis-retinal transforms to trans-retinal, dissociating from opsin, activating transducin, activating phosphodiesterase (PDE) which lowers cGMP, closing Na+ channels, and hyperpolarizing rod cells, reducing glutamate release.

A powerful signal: one photon can block entry of one million Na+ ions due to the amplification processes.

Electromagnetic Spectrum: Wavelengths range from 400 to 700 nanometers for visible light.
Cornea contributes 80% to focusing; the lens provides the remaining 20% via adjustments for distance through ciliary muscle contraction.
Blind Spot: Area where the optic nerve exits the eye, lacking receptors. The brain compensates by filling in this gap, often unnoticed due to overlap from the other eye.