Rod cell

A rod cell, or rod, is a type of photoreceptor cell in the retina of the eye. It is responsible for vision at low light levels (scotopic vision). Rod cells are highly sensitive to light, allowing us to see in dim conditions, but they do not mediate color vision and have poor spatial acuity.

Rod cells are elongated, cylindrical cells. They contain a light-sensitive photopigment called rhodopsin. Rhodopsin is composed of a protein called opsin and a light-absorbing molecule called retinal (a derivative of vitamin A). When light strikes rhodopsin, retinal changes shape, initiating a cascade of biochemical events that ultimately leads to a change in the rod cell's membrane potential. This electrical signal is then transmitted to other neurons in the retina, eventually reaching the brain for processing.

Rod cells are more numerous than cone cells in the human retina, especially in the periphery. This distribution contributes to our ability to detect motion in our peripheral vision, particularly in low light. However, the high convergence of rod cells onto a single retinal ganglion cell results in lower spatial resolution compared to cone-mediated vision.

Disorders affecting rod cell function can lead to night blindness (nyctalopia) or other visual impairments in low light conditions. Retinitis pigmentosa, a group of inherited diseases, often begins with the progressive loss of rod cells, leading to a gradual loss of peripheral and night vision.