Biology Reference
In-Depth Information
16
Early photochemical
explanations
A first clue to an understanding of the sensitivity regulation
mechanism in rods was given by the discovery by Boll in 1876 that
the photopigment rhodopsin, situated in the outer segment of the rod
receptors, was bleached by light and regenerated in the dark. He also
showed that the bleaching effect of light depended on the wavelength
used (Boll,
1878
).
Influenced by this great discovery, Kühne, in an extensive
research work, provided strong evidence in favour of the view that the
sensitivity difference between rods and cones had a photochemical
basis (Kühne,
1877a
,
b
,
1877
-1878, 1879). He investigated the
bleaching and regeneration processes of rhodopsin in much more
detail than Boll and made an important theoretical contribution with
his influential 'Optochemische' hypothesis, where he presumed that
the phototransduction in both rods and cones was photochemical
in nature. Accordingly, he presumed that the apparent colourless
cone receptors contained photochemical substances, and that these
substances became involved in visual processing under daylight
conditions. Indeed, in opposition to Schultze's (
1866
) duplicity theory,
he presumed that even rods were activated by photochemical, colour-
less substances in daylight.
Furthermore, Kühne (
1879
) made an important distinction
between the photochemical substances and their photoproducts,
and argued that it was the photoproducts, not the photosensitive
substances that generated the neural activity in the retina. Moreover,
he discovered that rhodopsin may regenerate in two quite different
ways: a rapid anagenese from photoproducts of rhodopsin and a
slower neogenese from new substances formed after rhodopsin had
been bleached. The relative contribution of these two processes was
