Why Cones Regenerate Faster Than Rods

Rods and cones are photoreceptors---cells on the back of our eyes (the retina) that turn light into electrical signals. They are an essential part of the vision process. Cones react to color and rods react to dim light.

Types

• Cones come in three types: red, green and blue. Each type of cone reacts strongly to one specific color. Rods are all the same. They react to light, even dim light, of any color.

Time Frame

• Both rods and cones respond to light by firing a pulse along a nerve. After firing, the photoreceptor needs a rest period before it can fire again. This rest period is called "regeneration" and is necessary so the photoreceptor can load up on ions for the next firing.

The Two Theories

• There are two different theories about why cones regenerate faster than rods. Both theories have their backers, but the consensus is that both offer a partial explanation. The theories are called the primitive regeneration theory and the bipolar connections theory.

Primitive Regeneration

• Rods are older evolutionarily and have a primitive regeneration process that is more sensitive to light but slower to regenerate.

Bipolar Connections

• Cones are connected to one, or only a few, of the cells at the next deepest layer (the bipolar layer) of the retina. Rods are connected to many more cells in the bipolar layer. The large number of intercellular connections slows down the regeneration process for rods.