In: Anatomy and Physiology
Physiology of vision is a complex phenomenon which is still
poorly understood. The main mechanisms involved in physiology of
vision are:
• Initiationofvision(Phototransduction),afunction
of photoreceptors (rods and cones),
• Processing and transmission of visual sensation,
a function of image processing cells of retina and
visual pathway, and
• Visualperception,afunctionofvisualcortexand
related areas of cerebral cortex.
Phototransduction
The rods and cones serve as sensory nerve endings for visual
sensation. light falling upon the retina causes photochemical
changes which in turn trigger a cascade of biochemical reactions
that result in generation of electrical changes. Photochemical
changes occurring in the rods and cones are essentially similar but
the changes in rod pigment (rhodopsin or visual purple) have been
studied in more detail. This whole phenomenon of conversion of
light energy into nerve impulse is known as
phototransduction.
Photochemical changes
The photochemical changes include:
Rhodopsin bleaching. Rhodopsin refers to the visual pigment present
in the rods—the receptors for night (scotopic) vision. Its maximum
absorption spectrum is around 500 nm. Rhodopsin consists of a
colourless protein called opsin coupled with a carotenoid called
retinine (Vitamin A aldehyde or 11-cis-retinal). light falling on
the rods converts 11-cis-retinal component of rhodopsin into
11-cis-retinal through various stages . The all-trans-retinal so
formed is soon separated from the opsin . This process of
separation is called photodecomposition and the rhodopsin is said
to be bleached by the action of light. Rhodopsin regeneration. The
11-cis-retinal is regenerated from the alt-trans-retinal separated
from the opsin (as described above) and vitamin-A (retinal)
supplied from the blood. The 11-cis- retinal then reunites with
opsin in the rod outer segment to form the rhodopsin. This whole
process is called rhodopsin regeneration . Thus, the bleaching of
the rhodopsin occurs under the influence of light, whereas the
regeneration process is independent of light, proceeding equally
well in light and darkness.
Visual cycle. In the retina of living animals, under constant light
stimulation, a steady state must exist under which the rate at
which the photochemicals are bleached is equal to the rate at which
they are regenerated. This equilibrium between the
photodecomposition and regeneration of visual pigments is referred
to as visual cycle.