Photoreceptorsrods

Rod photoreceptors, or rods, are one of the two main types of photoreceptor cells in the vertebrate retina. They are specialized for scotopic, or low-light, vision and contribute to sensitivity and motion detection, but they provide limited spatial detail and no color information.

Rods are cylindrical cells located predominantly in the peripheral retina, with the highest density outside the central retina. The central retina (fovea) is rich in cones and contains few or no rods. In humans, about 120 million rods are present compared with about 6 million cones.

Rods have an outer segment filled with stacked membranous discs containing rhodopsin; an inner segment with mitochondria and other organelles; and a synaptic terminal that connects to rod bipolar cells. The outer segments continuously renew and shed discs. The photopigment rhodopsin is a light-sensitive GPCR formed by opsin and 11-cis-retinal.

Photon absorption activates rhodopsin, which activates transducin, then phosphodiesterase, lowering cGMP, causing closure of cGMP-gated cation channels, leading to hyperpolarization and reduced glutamate release. In darkness, photoreceptors are depolarized and release glutamate.

Functional properties include high sensitivity to low light, a slow response to changes, good detection of contrast and motion, and limited spatial resolution. They cannot distinguish colors, and night vision adapts as rod rhodopsin regenerates over minutes to hours.

Rod dysfunction can cause nyctalopia; degenerative diseases such as retinitis pigmentosa and vitamin A deficiency can affect rods. Aging also leads to gradual rod loss. Rods are distributed throughout most vertebrate retinas and are especially dense in nocturnal species, supporting broad-field vision beyond the central, cone-dominated fovea.