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There
are two basic ways to set up a nervous system: use a diffuse
organization of interacting neurons, perhaps with many small
interneuron-rich processing centers, or centralize
the majority of the processors and run the system from there.
Diffuse systems are common in animals with
radial symmetry,
where stimuli could be coming from any direction.
Cnidarians have the
simplest of these diffuse systems, a nerve net,
where impulses may run in both directions on the same axons (this is
always functionally possible, but other systems organize one-way
systems).
Echinoderms have a diffuse pentaradial system. Octopi have
become famous for the complexity of processors in each of their
arms, but theirs is still technically a centralized system.
Bilateral
animals, with their tendency to
always lead with / encounter new stiluli with a front end, show
cephalization,
a concentration of sense receptors and processors at the anterior
end. Since most major animal groups are bilateral, most have
centralized systems, with outlying areas called a peripheral
system. When processors are somewhat small, each
processor is called a ganglion; larger
processors are brains. Systems tend to have
clusters of processors distributed through the system, especially in
areas that require their own processing, such as locomotory
structures (for instance, insects have large ganglia near their legs
and wings). Processing seems to rise in
complexity from simple linear systems, through
nodal systems that handle clusters of processing in
a somewhat linear order, to multiparallel systems
that can handle many processing duties of one stimulus all at the
same time.
Intelligence
is a term with no clear definition, and
so it is "tested" in many ways, looking for many different,
occasionally contradictory, abilities.
If one is looking for basic problem-solving capabilities,
animals with higher-level intelligence are often social
animals, which naturally live in groups whose individuals
must be continually dealt with. In the
mollusks, the
cephalopods have some very good problem solvers, even
though an octopus is not particularly social.
In the
arthropods, species that show particular intelligence
are not only social but often behave as if the entire colony is
itself a brain; researchers studying ants or bees generally
don't see high-level processing in individual insects, but find that
they as groups interact in a way that may mimic how neurons interact in large
brains.
Bilateral systems in larger animals almost always have a medial
trunk of interneurons, nerve cords, running the
length of the animal, doing some reflex
action processing but also providing a maximum-speed
highway relaying impulses in and out of the main processing centers.
In many invertebrates, such as the
annelids and
the arthropods,
the nerve cords, in a pair, run down just inside
the ventral surface of the animals. In
chordates,
a single, technically hollow nerve cord runs just
inside the dorsal surface of the animal, often
associated with a skeletal vertebral column, where
it is called the spinal cord.
Vertebrate central processors are
commonly subdivided into
substructures with particular specialties.
The spinal cord connects to a hindbrain, consisting
of a medulla that coordinates basic functions such
as heartbeat, breathing rate, and digestive movements, and a
cerebellum, which coordinates responses
the higher processors have decided are necessary. The
midbrain is involved in processing the primary
senses, which vary from animal to animal, and sending
somewhat analyzed imagery up the system.
The forebrain,
which varies greatly among subgroups, includes the thalamus,
sort of the central processor, to make sense of the sensory input,
access memories for analysis and process new experiences for
memorization. It seems to be active in different ways during
wakefulness and sleep, using sleep to move recent experiences into
permanent memory. The hypothalamus is said to
process "primitive urges," such as hunger, fear, and sex, as well as
being a major monitoring system for virtually any body factor which
can be tracked through blood-borne factors. It directly
connects to the hypophysis / pituitary gland to control the
endocrine system, as described above.
A
large part of the forebrain, at least in mammals, is the
cerebrum. In this section is the
paleocortex,
also called the
limbic system, where vital
functions are monitored. A lot of emotional reactions
are developed here, and it often is said to be the source of the
unconscious mind, processing that affects decisions
but which the individual may not be fully aware of. It
interacts strongly with the hypothalamus. The
neocortex is the location of most of what's called "higher
level" processing, where experiences are fully analyzed and
responses decided upon to be sent out to the coordinating
structures. |
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