High resolution

Module 10: Figure rod and cone structure

The structural organization of rods and cones.

Rods and cones are specialized neuronal cells designed to detect light and to relay information to the nervous system by releasing transmitters from the synaptic region. The photon-detection region is localized in the outer segment, where there is a large increase in the membranes containing components of visual transduction. In the case of rods, there are large numbers of membrane discs that are stacked on top of each other. Membrane amplification in the cones is achieved by invaginations of the plasma membrane. Cellular organelles such as the mitochondria, endoplasmic reticulum (ER) and Golgi are packed into the inner segment, and this may ensure that they do not interfere with the optics of the outer segment. Rhodopsin located on the membranes in the outer segment absorbs light, which triggers a phototransduction cascade that decreases the amount of transmitter being released from the synapse at the opposite end of the cell. The box indicates a region of the outer segment of the rod that is magnified in Module 10: Figure phototransduction overview to illustrate the process of visual transduction.