Biology Reference
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substance
T
, which then initiated the nerve impulse. The model was
expressed in the following formulas:
S
↔ (
P
+
A
)
L
|| (
P
+
A
) || →
T
in which
||
(
P
+
A
)
||
means catalysis by
P
or
A
or both.
It should be noted that Hecht's (
1921
/1922) model for sensitivity
regulation presumed that sensitivity variations during dark and light
adaptation were determined in quite different ways. In both cases
the sensitivity was assumed to depend on the amount of photoprod-
ucts
P
and
A
, but the mechanism that determined this amount was
different for dark and light adaptation. During dark adaptation the
sensitivity increase was due to the gradual and continuous decrease
of the
P
and
A
material used in rebuilding the
S
substance, while, on
the other hand, the concentration of
P
and
A
during light adaptation was
assumed to depend on two opposing processes: a continuous increase
due to the decomposition of the light-sensitive substance
S
when
acted upon by light, and a continuous decrease due to the rebuilding
of the
S
substance. Eventually, with constant light level, these two
opposing processes were assumed to reach a balance or equilibrium,
where sensitivity remained constant. If the light-adaptation level, then,
increased, a larger amount of
P
and
A
would be formed by the increased
action of the light, shifting the stationary state to a new point of equilib-
rium with reduced sensitivity.
17.2 Supporting evidence obtained from
invertebrates
Hecht (
1919
/1920a) tested the photochemical assumption of the
model by utilizing the light reflex of the siphon of an invertebrate
animal (the common North-Atlantic long-neck clam
Mya arenaria
,
a shelled bivalve mollusc). When the siphon of
Mya
is stimulated
by light, it is shortened and its tip, thereby, moved toward the shell.
The reaction time of this light reflex was assumed to consist of two
distinct periods: a first sensitization and a second latent period. The
