Rod cell
Introduction
Rod cells are a type of photoreceptor found in the retina of the eye that are responsible for vision in low light conditions. They are one of the two types of photoreceptor cells, the other being cone cells, which are responsible for color vision and function best in relatively bright light. Rod cells are more numerous than cone cells in the retina, with about 120 million rod cells compared to 6 million cone cells.
Structure
Rod cells are named for their cylindrical shape, which is distinct from the conical shape of cone cells. Each rod cell consists of four distinct regions: an outer segment, an inner segment, the cell body, and the synaptic terminal.
The outer segment contains a stack of membranous disks, which are rich in the photopigment rhodopsin. Rhodopsin is responsible for the light-absorbing properties of rod cells. The inner segment contains the cell's organelles and metabolic machinery. The cell body contains the nucleus and other typical cellular components. The synaptic terminal forms synapses with other neurons in the retina.
Function
Rod cells are highly sensitive to light, allowing them to respond even in extremely dim light conditions. This makes them crucial for night vision. However, rod cells do not perceive color and have low spatial acuity, meaning they do not contribute to detailed or color vision. These tasks are performed by the less sensitive but more acuity-focused cone cells.
When light hits the rhodopsin in the outer segment of a rod cell, it triggers a chemical reaction that changes the shape of the rhodopsin molecule. This change in shape initiates a series of reactions that result in an electrical signal being sent to the brain, where it is interpreted as vision.
Distribution in the Retina
The distribution of rod cells in the retina is not uniform. They are absent from the fovea, the central region of the retina where visual acuity is highest. Instead, they are most densely packed in a ring-shaped area surrounding the fovea. This distribution contributes to the phenomenon of night blindness, where the center of the visual field is darker than the periphery in low light conditions.
Clinical Significance
Defects or damage to rod cells can lead to various vision disorders. For example, retinitis pigmentosa is a genetic disorder that causes the rod cells to deteriorate over time, leading to night blindness and peripheral vision loss. Age-related macular degeneration also affects the rod cells, leading to a gradual loss of central vision.