Rods are one of two types
of light-sensitive photo-detector cells found in the "Jacob's Membrane"
(Layer 9.) of the retina
of the human eye. The other type of photo-detector cells are called cones.
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.