A type of light-sensitive nerve cell found in the vertebrate retina. Rods do not discriminate fine detail or color differences, but they are sensitive to very dim light, which cones are not. Whereas tens or hundreds of photons are needed to activate a cone, a single photon is all it takes to produce a response from a rod cell. Because of this sensitivity, rods are responsible for night vision. Loss of them result in the condition known as night blindness.