Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 278

all vertebrate organs. The vertebrate head is considered an evolutionary addition to
the body of protochordates ( Gans and Northcutt, 1983 ). In the head's case, the cra-
nial crest took over most of the functions of the mesoderm.
On the Generation of Epigenetic Information in the CNS
We have seen that the animal organism responds to external and internal stim-
uli adaptively, by activating signal cascades and GRNs that tend to avoid possible
harmful effects of the stimuli. We have also seen that the stimuli are received, per-
ceived, and “classified” as such in the CNS where the relevant signal cascades start.
Adaptive responses are neither random nor default; hence the animal uses informa-
tion to select and perform them. The fact that perception of stimuli and the activation
of cascades starts in the CNS suggests that there may also be the source of informa-
tion for adaptive responses.
The data/information on the internal or external environment are received and
processed in the synapses of the sensory cells. This induces the movement of ions
across neuronal membranes, leading to potential differences. Thus sensory cells
convert external stimuli into electronic messages presented as electrical spikes. The
messages are transmitted to the CNS via afferent pathways. They may be encoded
in the form of the spike frequencies, spike duration, number, or patterns ( Stafford,
2010 ). In the CNS, the sensory information is processed further in various centers
until an output is produced in the form of a chemical signal (neurotransmitter, neuro-
modulator, etc.).
Not all data on external or internal agents are taken for a stimulus requiring a
response. The nervous system is bombarded with a myriad of data on the external
and internal environment and it must distinguish between the normal innocuous
variables and variables that pose problems. It is in the CNS where the distinction is
made and the chaff is separated from the wheat. What is classified as innocuous is
neglected as “noise.” The rest are classified as stimuli requiring response.
The CNS makes these decisions by comparing the data on environmental varia-
bles with respective set points: when the variable is outside the limits of the set point,
it is taken as a stimulus or as a problem requiring a solution. The processing of the
stimulus then follows. So, for example, a small drop of 1°C in the environmental
temperature is received by skin thermoreceptors and transmitted to a specific center
in the hypothalamic preoptic area (POA), where it is ignored as “noise: unworthy of
a response.” But when the temperature drop is greater, say 10°C, it falls outside the
set point and is classified as a stimulus to which the CNS responds adaptively: the
POA stimulates the thermogenesis (other CNS centers such as the brainstem and spi-
nal cord are also involved in thermogenesis) by activating the production of thyroid
hormones (TH) by the thyroid gland via the following signal cascade:
Secretion by the hypothalamic neurons of the thyroid-releasing hormone
(TRH)→Secretion of thyroid-stimulating hormone (TSH) by the pituitary→Secretion of
TH by the thyroid glands.