ROD

type of photoreceptive cell in the retina of the eye in vertebrate animals. Rod cells function as specialized neurons in the conversion of visual stimuli in the form of photons (particles of light) into electrical stimuli that can be processed by the central nervous system (phototransduction). Rod cells are stimulated by light over a wide range of intensities and are responsible for perceiving the size, shape, and brightness of visual images. They do not perceive colour and fine detail, tasks performed by the other major type of light-sensitive cell, the cone. Rod cells are much more sensitive to light than cones and are also much more numerous; the human eye contains about 130 million rods and about 7 million cones. Rod cells have an elongated structure and consist of four distinct regions: an outer segment, an inner segment, the cell body, and the synaptic region. The outer segment contains the phototransduction apparatus. It is composed of a series of closely packed membrane disks that contain the photoreceptor molecule rhodopsin. The synaptic region is the site where the rod cell relays its information to a bipolar neuron in the retina. The bipolar neurons connect with ganglion neurons whose axons form the fibres of the optic nerve. Rhodopsin is made up of a protein called opsin and a photosensitive chemical derived from vitamin A, 11-cis-retinal. Photons of light entering the eye cause the 11-cis-retinal to undergo isomerization, forming all-trans-retinal. This isomerization activates the opsin protein, which then interacts with and activates a G-protein called transducin. The association of opsin with transducin couples the external stimulus of light to an internal biochemical pathway that ultimately alters the release of neurotransmitters from the synaptic region of the cell and thus changes the firing of the bipolar neurons and the electrical impulses sent along the optic nerve to the brain. Rhodopsin molecules are broken down in sunlight or other bright viewing conditions. This breakdown prevents the overstimulation of the rod cells and makes them less sensitive to a bright environment. In dim light there is little breakdown of rhodopsin, and its persisting high concentration allows for better vision in a dark environment. Dark-adapted vision in humans is basically devoid of colour because it depends almost entirely on the functioning of rods. Compare cone. old English measure of distance equal to 16.5 feet (5 m). That length is sometimes also called a perch. The word rod is an Anglicization of the Dutch rood, which is traced to the German rute. The Dutch rood referred to a land area of 40 square rods, equal to one-quarter acre, or 10,890 square feet (1,012 square m). It also denoted just one square rod, or 272.25 square feet (25.29 square m). The rood also was a British linear unit, designating any of a wide range of lengths from 16.5 to 24 feet. At 16.5 feet (5 m) it was identical with the common surveyor's measure, the rod. The German rute was much shorter, about 12.36 to 12.47 feet (3.77 to 3.8 m).