The human eye senses light via specialized nerve cells called photoreceptor cells. There are two types of photoreceptor cells: rods and cones. Together these photoreceptor cells have the ability to sense light, color, movement, and other visual stimuli. Rods function best at low levels of light and are responsible for night (scotopic) vision. These cells are incredibly sensitive and under optimal conditions a single unit of light, or photon, may activate rod photoreceptors. This property of having high sensitivity allows rods to have fine light and motion detection. Rods are far more numerous than cones and account for 95 percent of the photoreceptors in the retina (Lamb, 2015). Rods are located at the edge of the retina (inner surface of the eye), which allows for peripheral vision but does not allow for the ability to focus on fine details.
An important functional component of rods is the protein rhodopsin. Rhodopsin is densely packed inside of rod cells; it is estimated that a single rod cell contains 108, or 100 million, rhodopsin molecules (Milo & Philips, 2015). Rhodopsin is extremely photosensitive (sensitive to light), and its high concentration means that a single photon of light can hit a single rhodopsin molecule in one of the millions of rod cells in an eye to elicit a cellular response.
Rods have different reactions to different levels of light. When a small amount of light hits a rod cell, rhodopsin is activated and, through a series of events known as phototransduction, the signal is transmitted via neurons to the primary visual cortex (the area responsible for processing visual information) located in the occipital lobe of the brain. Once the cell response ends, rods enter a deactivated stage that can last several minutes. There are balanced amounts of activated and deactivated cells in standard lighting conditions, which allows visual perception to proceed normally. An extreme amount of light in the environment can lead to overactivation of rhodopsin, and it can take several minutes for enough cells to “reset” themselves to resume normal visual function. An example of this is walking into a dim room after being in bright sunlight: the strong light deactivates a majority of rod cells, and the room may appear darker than it really is because there are not enough rods available to immediately respond to the dim surroundings.
The high sensitivity of rod cells can also be illustrated in nonhumans. Nocturnal animals rely on rod photoreceptors to hunt prey or spot predators at night. Rod function is preserved across species, meaning humans and animals have similarly functioning rods. The main difference is the presence of the tapetum lucidum under the retina, which “bounces” unabsorbed photons around until they hit a rod rather than leave the eye as in humans (Milo & Philips, 2015). This small anatomical difference allows nocturnal animals to use rods more efficiently.
Rod photoreceptor function can be impaired by a class of genetic diseases called rod-cone dystrophies. Rod-cone dystrophies are usually inherited from a child’s parents but may also occur randomly. Retinitis pigmentosa and Leber’s congenital amaurosis are examples of diseases in which rods and cones either develop abnormally or break down over time. The severity and visual deterioration speed of rod-cone dystrophies can vary greatly between affected individuals. To date, there are no known treatments for such diseases.
Rhodopsin function can be impaired due to vitamin A deficiencies. In the simplest terms, a form of vitamin A functions with rhodopsin for the process of phototransduction. Deficiencies in vitamin A can affect both rods and cones to cause vision problems from night blindness to total vision loss. Many side effects can be reversed with vitamin A supplements as long as permanent damage has not taken place to the photoreceptors or other eye components, such as the epithelial tissue.
See also: Cone Dystrophy; Cones; Visual Fields; Visual Motor System; Visual Perception; Visual System; Visual Threshold
Lamb, T. D. (2015). Why rods and cones? Eye, 1—7. http://dx.doi.org/10.1038/eye.2015.236
Milo, Ron, & Rob Phillips. (2015). Cell biology by the numbers, pp. 47—50, 183—187. New York, NY: Garland Science, Taylor & Francis Group.