Retina and Phototransduction

Overview

This lecture explains the anatomy and function of the retina, focusing on how rods and cones convert light into neural signals.

The eyeball is the receptor organ for vision.
The retina is a multi-layered structure inside the eye containing sensory cells that detect light.
Photoreceptor cells in the retina are rods and cones.

Rods detect black and white and function best in low light conditions.
Cones detect color and require bright light to function.
Cones are most concentrated in the fovea, the central part of the retina, responsible for sharp, direct vision.
There are three types of cones: blue, green, and red, each sensitive to different wavelengths of light.

Light entering the eye stimulates rods and cones.
In rods, light hits rhodopsin, a light-sensitive molecule.
Rhodopsin breaks into opsin and retinol when exposed to light, a process called bleaching.
After bleaching, rhodopsin must be rebuilt before the rod can respond to light again.
Bleaching triggers a second messenger cascade that causes sodium channels in the rod to close.
Closing sodium channels hyperpolarizes the cell membrane, preventing an action potential in the rod.
The bipolar cell, which receives input from rods, does not receive neurotransmitter (glutamate) and thus fires its own action potential.

In darkness, rods continuously release glutamate, keeping bipolar cells inactive.
When light hits, glutamate release stops, prompting bipolar cells to send a message to the brain.

Retina — Light-sensitive layer at the back of the eye containing photoreceptor cells.
Photoreceptor — A cell in the retina that detects light (rods and cones).
Rod — Photoreceptor cell sensitive to dim light; does not distinguish colors.
Cone — Photoreceptor cell responsible for color vision and sharp central vision.
Fovea — Central area of the retina with the highest density of cones.
Rhodopsin — Light-sensitive pigment in rods that initiates phototransduction.
Bleaching — The breakdown of rhodopsin into opsin and retinol when exposed to light.
Hyperpolarization — An increase in a cell's membrane potential, making it less likely to fire an action potential.
Glutamate — Neurotransmitter released by rods in darkness to bipolar cells.