in partial fulfil]ment of the - MSpace - University of Manitoba

Chapter I
REVIEW OF LITERATURE
8.1 BIOCHEMTCÀL ÀND PHYSIOIOGICAt BÀSIS OF DÀRK ADÀPTÀTION TESTING
The retina of the eye contains a number of specialized cells known as
rods and cones. The rods are specialized for scotopic vision (night
time vision), and the cones for photopic (day light) vision. Both rods
and cones contain photosensitive chemicals, which assist in scotopic and
photopic vision, respectively. The photosensitive chemical in the rods
is known as rhodopsin. Scotopic vision (dark adaptation) is dependent
on the concentration of rhodopsin in the reLina. Low rhodopsin concentration
results in night blindness (poor dark adaptation). This is
because the sensitivity of Lhe rods to light can be altered tremendously
by only sJ.ight changes in rhodopsin level (Guyton , 1976).
During photopic vision, if the retina remains exposed to intense
light for a long time, most of the stored rhodopsin r+ill be converted
inLo retinene and evenLually, into retinol (rigure 6). Therefore, the
concenLraLion of rhodopsin in the rods decreases greatly. This condition
is also true in the cones in that the photosensitive chemical of
the cones decreases during dark-adaptation. The reconversion of retinol
into retinene and then to rhodopsin is a slow process and the sensitivity
of the relina lo light is reduced for a period of time.
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