The Photoreceptors

This page is meant to familiarize you with the two types of photoreceptor cells found in the outer layers of the human retina, discussed on the previous page. Photoreceptors are so named because they can respond to photons of light. Rods contain the photosensitive pigment rhodopsin. In humans, there's only one type of rod. It's responsible for scotopic vision, which is employed in dim illumination.

There are different flavors of cones, on the other hand. Each contains a different kind of photopsin molecule, and is classified depending on the color of light which that molecule enables it to respond to. The recognition of color occurs in photopic vision, which is the kind your eye uses when light is plentiful.

Below are the anatomical features of note. It's helpful to be reminded that the 'outer' and 'inner' orientations are with respect to the center of the eyeball - therefore the outer segment is located further away from the center, close to the the pigmented epithelium of the retina.

The Outer Segment: contains membranous discs, which have vast numbers of photopigment molecules embedded within them.

The cilium: is the narrow stalk connecting the inner and outer segments to each other.

The Inner Segment: contains the cell nucleus, mitochondria, and all the apparatus a cell needs for protein synthesis and normal functioning.

Synaptic terminal: called a 'spherule' in a rod, and a 'pedicle' in a cone, it contains the synaptic ribbon. This ribbon is an electron-dense structure which, in photoreceptors, releases vesicles of the neurotransmitter glutamate, which is a chemical substance these cells use to communicate.

One difference to note between rods and cones is the organization of the discs containing photosensitive pigment. In both, these membranous discs are stacked. In rods, however, the discs are isolated structures and contained within the cell cytoplasm. In cones, the cell membrane forms many invaginations throughout the outer segment to form the discs, and so the discs are continuous with the cell membrane.

In humans, vision is trichromatic. This means we have three different types of cone - S cones for blue light (short wavelengths), M cones for green (medium wavelengths), and L cones for red (long wavelengths). Other animals have different mixes of rods and cones. The most complex visual system that has so far been discovered belongs to the mantis shrimp, a beautiful, multi-colored crustacean. It has 16 different photoreceptor cells within its eyes.