Biology Reference
In-Depth Information
For simplicity, this mechanism is illustrated using the case of higher metazo-
ans, whose integrated control system is known best. In a typical case, in homeo-
thermic animals, such as birds and mammals, the environmental temperature and
other environmental agents such as the photic signals, food, presence of preda-
tors, preys, and other social stimuli are detected by the animal's senses of vision,
hearing, smell, and so on. All of the information related to these external stimuli
is received by sensory neurons, which convert them into specific electrical sig-
nals and transmit them to respective centers of the brain for further processing.
The processing of the electrical input in these brain centers interprets the stimulus
(i.e., its perception and categorization). The electrical spikes train from the recep-
tive sensory neurons is processed, perceived, categorized, and interpreted in neural
circuits to provide it a meaning (i.e., to find out what the stimulus means for the
organism). The final stage of neural processing is determining the appropriate adap-
tive response to the anticipated effects of the stimulus itself or of the environmen-
tal effects it might presage. The final product of the neural processing is an output
that, in the form of a chemical signal, triggers an “adaptive” signal cascade, ulti-
mately leading to the expression of one or a number of specific genes related to the
specific adaptation. The signal cascade usually induces expression of genes not in
all cells of the organism but in particular cells, tissues, or organs. This implies the
presence of another mechanism for preventing the expression of genes in other cells
(see p. 200, Restricting Gene Expression to Relevant Cells Alone: Binary Neural
Control of Gene Expression).
The processing is a neural codification or a translation of environmental stimuli
into messages that are intelligible to genes. The output of the processing comes in
the form of a chemical that triggers a specific signal cascade.
The above explanation raises questions with possible teleological implications.
Indeed, even the idea that the unconscious brain can find these adaptive responses seems
to unavoidably imply a purpose in neural processing and, syllogistically, the action of a
teleological factor. A discussion of this subject is outside the scope of this topic.
It is of paramount importance to animal evolution that the nervous system by
using specific signal cascades or gene regulatory networks (GRNs) can adap-
tively and “at will” relate naturally unrelated external agents to virtually any gene.
This manipulative expression of genes, as opposed to the classical modes of gene
expression, represents a landmark in the history of life that contributed essentially
to the unprecedented explosive evolution in the animal world during and after the
Cambrian explosion.
This mechanism enables the brain to choose from the available off-the-shelf sig-
nal cascades and GRNs leading to phenotypic results that adapt the organism to the
environmental stimulus. The neural processing is very costly, but its evolution proves
that the advantages it offered outweighed its price tag.
The neural manipulation of gene expression in animals is illustrated here with a
few examples.
Astatotilapia burtoni is a cichlid fish in Lake Tanganyika, East Africa. Its males
appear in dominant or subordinate forms, according to their social status. Dominant
males are brightly colored, whereas subordinates' colors are dull and they flee when
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