Where are the rod cells located in the retina; what type of light are the rods specialized to receive; what quality of image do they foster? Describe the structure of a rod cell and the process by which light is received, translated into a chemical signal, and transferred to the subsequent neurons in the retina on its way to the brain.
Concentrated most heavily at the periphery of the retina, rod cells are the most sensitive to light enabling humans to see in conditions with very low lighting. Yet, the images the rods foster are grainy and poorly defined as well as being only in shades of gray since rod cells cannot detect color. Each rod cell has an inner segment which contains the nucleus and synaptic body and a cylinder-shaped outer segment. The outer segment is a specialized, non-motile cilium which contains light-sensitive pigment in the vesicles on its surface. The pigment, rhodopsin, is composed of a light-absorbing compound called retinal (derived from vitamin A) and the protein opsin. When light strikes rhodopsin, retinal and opsin are split and the isomerized retinal causes the polarity of the outer segment's plasma membrane to change. Calcium is then released from vesicles on the surface of the outer segment. The calcium causes a transmitter substance to be released from the inner segment at its synapse with the sensor neuron generating an impulse. In the dark, retinal bonds again with opsin to reform rhodopsin.
