Rods are especially important for vision in dim lighting situations. They contain a pigment called rhodopsin (also known as 'visual purple') that is broken down (term: 'bleached') in bright light and regenerated in darker conditions. The break-down of rhodopsin leads to nerve impulses being sent to the brain. However, when all of the rhodopsin is broken-down (bleached) the rods do not function. The activation of the rods by regeneration of rhodopsin in dim light, together with the enlargement of the pupil of the eye, are the key processes associated with dark adaption (i.e. the adjustment made by the eyes for optimal vision in dim light). The opposite processes are associated with light adaption.
Each human eye contains approx. 125 million rods - compared with only approx. 6-7 million cones.
Structure of the rods
The rods are of approx. uniform size and are arranged perpendicularly to the surface of the layer of the retina in which they are located.
Each rod is composed of an outer and an inner portion, which are of about equal length. These segments have different properties of refraction (i.e. the extent to which they bend the light passing through them), and interaction with colouring reagents (concerning staining by various chemicals). They also have slightly different physical structures, for example the outer segments are marked by transverse straiae and have faint longitudinal markings. Rhodopsin is only located in the outer segments of the rods.
More about Ophthalmology:
This section includes short definitions and descriptions of the parts of the eye.
For other descriptions in this category, choose from the list to the left (but note that this is not a complete / exhaustive list).
Other related sections include:
- A labelled diagram of the eye
- A concise description of the human retina with brief descriptions of the parts of the retina
- Diseases and disorders of the human eye and the human visual system
- Clinical and surgical procedures re. eyes and human visual system
For further information see also our pages of books about ophthalmology.