Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 195

First, to clarify the concept of stimulus as it will be used for the purpose of this
discussion, it is an environmental action or condition that provokes a specific change
in the phenotype. Living organisms are under the constant action of numerous
environmental abiotic and biotic factors, but not all environmental factors serve as
stimuli in metazoans. Moreover, a factor that serves as a stimulus for one species
does not necessarily serve as a stimulus for another. Internal factors also may act as
stimuli.
This last empirical observation is essential for understanding the nature of the
stimulus and the organism-environment relationship. It implies that an environmen-
tal factor per se is not a stimulus. Whether an environmental factor or condition will
be taken as a stimulus or not depends primarily on how it is perceived in the brain
more than it does on the nature of the environmental factor itself. The decision to
categorize it as a stimulus or not is made in the CNS. This is why the same agent is
counted as a stimulus by organisms of one species but not by others. The organism
responds adaptively when it perceives the agent as a stimulus.
How does the organism decide whether to take an agent/action as a stimulus or
“ignore” it as “noise”? The answer requires an understanding of the pathway that
links the external/internal stimulus to its perception in the CNS. In metazoans, certain
cell types and organs (sense organs) are specialized to detect changes in the envi-
ronment. The sensory organs and sensory cells are bombarded with streaming exter-
nal factors and the organism must separate the environmental noise from the stimuli.
Sensory cells have a background action potential that changes under the influence
of external factors. They conduct this environmental data to respective centers in the
CNS, where the degree of the environmental change is compared to species-specific
set points. When the intensity of the change exceeds (or drops below) the limits of
the set point, the organism responds with a phenotypic change. This is a quantitative
criterion of the categorization of environmental factors and actions as stimuli. There
are other factors in the environment, whose perceptions are taken as stimuli, regard-
less of their quantity (predators, light, darkness, etc.) and automatically elicit a phe-
notypic response.
Let us look at a few of the many examples. When the environmental temperature
elevates to levels that challenge normal body temperature, a number of phenotypic
changes occur in the organism to counteract and maintain normal temperature by
decreasing thermogenesis. This implies that the organism has detected an increase in
the body temperature above the species-specific set point, which in vertebrates, for
example, resides in the hypothalamus. The increase in body temperature is assessed
in the neurons of the preoptic area (POA). Besides the POA, brain stem centers and
the spinal cord are also involved in the neuroendocrine mechanisms that slow down
thermogenesis by inhibiting thyroid hormones and by increasing heat loss through
vasodilatation, sweating, and panting. Finding places to cool off is often another ben-
eficial adaptive behavioral change.
Morphological adaptations are characteristics of many lower metazoans (rotifers,
snails, etc.). Upon detecting visually or olfactorily, the presence of predators in the
environment may rapidly generate morphological changes that make it difficult to
be eaten by predators. Curiously, the freshwater snail, Helisoma trivolis , specifies