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.
The rods are of approx. uniform size and arranged
perpendicularly to the surface of the layer/retina. Each rod is composed
of an outer
and an inner portion - which are of about equal length. These segments
have different properties of refraction (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 - e.g. the outer segments are marked by
transverse
straiae and has faint longitudinal markings.
Rhodopsin is only located in the outer segments of
the rods. |
