Tuesday, August 24, 2010

Rods (Human Eye)

Rods are one of the two types photoreceptor cells which are found in the retina of the eye. Being used in peripheral vision, they are activated by less intense light than are the cone cells. Rods are named for their cylindrical shape and are concentrated at the outer edges of the retina. On average, there are approximately 95 million rod cells in the human retina. Rods have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with another neuron, for example a ganglion cell (bipolar cell). The inner and outer segments are connected by a cilium, which lines the distal segment. The inner segment contains organelles and the cell's nucleus, while the rod outer segment, which is pointed toward the back of the eye, contains the light-absorbing materials.

Rods are almost entirely responsible for night vision. Although they are more sensitive light than cones, rod cells are not sensitive to color. A rod is sensitive enough to respond to a single photon of light, and is about 100 times more sensitive to a single photon than cones. Rods require less light to function than cones, they are therefore the primary source of visual information at night (scotopic vision). Cone cells, on the other hand, require tens to hundreds of photons to become activated. Additionally, multiple rod cells converge on a single interneuron, collecting and amplifying the signals.