Rods

Rods are terms used for several related concepts in biology, microbiology, and physics, described below in a concise overview.

In biology, rod cells are photoreceptor cells located in the retina of the eye. They are highly sensitive to light and provide vision in low-light conditions, but do not detect color. Rods contain the photopigment rhodopsin and are concentrated around the periphery of the retina, with humans having about 120 million rods compared with about 6 to 7 million cones. Signals from rods converge onto bipolar cells, yielding high sensitivity but lower visual acuity. In darkness, rod cells are depolarized; when light is absorbed by rhodopsin, a phototransduction cascade reduces neurotransmitter release, leading to a neural signal. Rods play a major role in scotopic (low-light) vision and require pigment regeneration after light exposure, enabling adaptation to changing illumination.

In microbiology, rod-shaped bacteria (bacilli) are a common cellular morphology. These organisms vary widely in Gram staining properties, metabolism, and ecological niche. Some rods form spores, such as Bacillus and Clostridium species, while others are non-spore-forming. Rod morphology is a foundational characteristic used in microscopic identification alongside staining patterns and biochemical tests.

In physics and engineering, a rod is a slender, elongated cylindrical or prismatic object used to transmit force, serve as a support element, or act as a part of a measuring or experimental apparatus. Rods appear in a wide range of scales, from microscopic probes to large structural components.

Other uses include the generic meaning of a straight rod or a surname.