Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 79

How do genes know to adaptively respond to specific environmental agents with
whom they have no physical contact? Speaking metaphorically, genes are unilin-
guals; they understand the language of biology, but they are not proficient in the
external environment language. The fact that genes respond adaptively to external
influences, which they do not “see” and do not “understand,” suggests that the exter-
nal influences reach genes interpreted in another language. An interpreter of the
environmental influences comes into play, and, via communications channels, pro-
vides them with instructions on how to respond in the familiar biological language.
The interpreter is the nervous system, which processes environmental data to
generate the epigenetic information provided to genes in the form of chemicals or
instructions for switching genes on and off and determining the spatiotemporal pat-
terns of gene expression.
Now, let us try to outline the pathway from these environmental genetically unin-
telligible variables to genes. This is a neural pathway because it could not be any-
thing else. The nervous system is the only system that, owing to its omnipresence
down to the cell level, can receive data on both the external and internal environ-
ment and, via afferent pathways, send them to the CNS. The first thing the CNS does
with the wealth of the incoming data is to filter them or separate “the wheat from the
chaff”—to separate the meaningful or significant data from the insignificant data.
Ignoring the “insignificant” data, the CNS takes the rest for challenges to work
out, which it labels as stimuli (Latin for goad ). Once qualified as a stimulus, the
environmental variable is processed in a specific neural circuit. The processing ends
with a neural output in the form of an electrical/chemical signal assigned for activat-
ing a specific signal cascade.
In order to be taken as a stimulus in metazoans, an internal/external agent must
exceed a certain threshold as measured by a set point determined in the brain.
Accordingly, a stimulus may be functionally defined as any variable that, above or
below an intrinsically determined set point, causes an adaptive response.
Confusion often arises in relation to the nature of external stimuli from the causal
and the informational standpoints. Because external stimuli precede in time the intra-
generational and transgenerational changes, they are considered to be causes of those
changes. For the appearance of an adaptive change, stimuli are necessary but not suf-
ficient conditions. An approaching fox may be taken as a stimulus by a bird, who
immediately decides to take flight, while a horse may ignore it. The decisions are
made in the brains of the bird and the horse based on predicted dangers or rewards.
The cause is, thus, within the animal itself, rather than in the external stimulus,
although the external stimulus is a necessary condition for the animal's decision.
That different organisms may respond differently to different stimuli suggests that
the external agent per se is not information or instruction for the organism. It is the
interpretation of that stimulus in the brain that generates the epigenetic information
that via signal cascades instructs target cells to adaptively respond to the external
stimulus. From the communications theory view, the processing of the stimulus in
the brain reduces the uncertainty and increases the probability of the activation of a
specific gene to 1, by selectively activating only one of a number of available signal