What do we call those cells in the retina that respond to low levels of light and allow seeing in the dark?

Our amazing sense of sight relies on a complex network (Review) of specialized cells within the retina at the back of our eyes. These photoreceptor cells are responsible for converting incoming light into electrical signals that our brain interprets as images. When we move from a brightly lit environment into a dim one, a particular type of these cells takes over, enabling us to perceive shapes and movement even when very little light is available.

These highly sensitive photoreceptors are called rods. Each human eye contains approximately 90 to 120 million rods, vastly outnumbering their counterparts, the cones. Rods are uniquely adapted for scotopic vision, which is our vision in low-light conditions. They are most effective in detecting shades of gray, motion, and contribute significantly to our peripheral vision, but they do not perceive color or fine detail, which are tasks handled by cones in brighter light.

The incredible sensitivity of rods stems from a light-absorbing pigment called rhodopsin. When even a tiny amount of light strikes a rod cell, it causes a chemical change in rhodopsin, initiating a signal to the brain. This process, however, temporarily "bleaches" the rhodopsin, making it inactive. This is why it takes several minutes for your eyes to fully adjust to darkness – your rods need time to regenerate their supply of active rhodopsin, a process known as dark adaptation. Animals with exceptional night vision, such as owls, possess retinas that are almost entirely composed of rods, highlighting their crucial role in navigating nocturnal worlds.