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Concept
Rod cell
Summary
Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in peripheral vision. On average, there are approximately 92 million rod cells (vs ~6 million cones) in the human retina. Rod cells are more sensitive than cone cells and are almost entirely responsible for night vision. However, rods have little role in color vision, which is the main reason why colors are much less apparent in dim light.
Rods are a little longer and leaner than cones but have the same basic structure. Opsin-containing disks lie at the end of the cell adjacent to the retinal pigment epithelium, which in turn is attached to the inside of the eye. The stacked-disc structure of the detector portion of the cell allows for very high efficiency. Rods are much more common than cones, with about 120 million rod cells compared to 6 to 7 million cone cells.
Like cones, rod cells have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with another neuron, usually a bipolar cell or a horizontal 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 (abbreviated to ROS), which is pointed toward the back of the eye, contains the light-absorbing materials.
A human rod cell is about 2 microns in diameter and 100 microns long. Rods are not all morphologically the same; in mice, rods close to the outer plexiform synaptic layer display a reduced length due to a shortened synaptic terminal.
In vertebrates, activation of a photoreceptor cell is a hyperpolarization (inhibition) of the cell. When they are not being stimulated, such as in the dark, rod cells and cone cells depolarize and release a neurotransmitter spontaneously. This neurotransmitter hyperpolarizes the bipolar cell.
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