08-11-2012, 01:08 PM
Brain
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The brain is the center of the nervous system in all vertebrate and most
invertebrate animals—only a few invertebrates such as sponges,
jellyfish, adult sea squirts and starfish do not have one, even if diffuse
neural tissue is present. It is located in the head, usually close to the
primary sensory organs for such senses as vision, hearing, balance,
taste, and smell. The brain of a vertebrate is the most complex organ of
its body. In a typical human the cerebral cortex (the largest part) is
estimated to contain 15–33 billion neurons,[1] each connected by
synapses to several thousand other neurons. These neurons
communicate with one another by means of long protoplasmic fibers
called axons, which carry trains of signal pulses called action potentials
to distant parts of the brain or body targeting specific recipient cells.
Anatomy
The shape and size of the brains of different species vary greatly, and
identifying common features is often difficult.[3] Nevertheless, there
are a number of principles of brain architecture that apply across a
wide range of species.[4] Some aspects of brain structure are common
to almost the entire range of animals species;[5] others distinguish
"advanced" brains from more primitive ones, or distinguish vertebrates
from invertebrates.
Cellular structure
The brains of all species are composed primarily of two
broad classes of cells: neurons and glial cells. Glial
cells (also known as glia or neuroglia) come in several
types, and perform a number of critical functions,
including structural support, metabolic support,
insulation, and guidance of development. Neurons,
however, are usually considered the most important
cells in the brain.[7]
The property that makes neurons unique is their ability
to send signals to specific target cells over long
distances.[8] They send these signals by means of an
axon, which is a thin protoplasmic fiber that extends
from the cell body and projects, usually with numerous
branches, to other areas, sometimes nearby, sometimes
in distant parts of the brain or body. The length of an
axon can be extraordinary: for example, if a pyramidal
cell of the cerebral cortex were magnified so that its
cell body became the size of a human body, its axon,
equally magnified, would become a cable a few
centimeters in diameter, extending more than a
kilometer.
Invertebrates
This category includes arthropods, molluscs, and numerous types of
worms. The diversity of invertebrate body plans is matched by an
equal diversity in brain structures.[22]
Two groups of invertebrates have notably complex brains: arthropods
(insects, crustaceans, arachnids, and others), and cephalopods
(octopuses, squids, and similar molluscs).[23] The brains of arthropods
and cephalopods arise from twin parallel nerve cords that extend
through the body of the animal. Arthropods have a central brain with
three divisions and large optical lobes behind each eye for visual
processing.[23] Cephalopods such as the octopus and squid have the
largest brains of any invertebrates
Vertebrates
The first vertebrates appeared over 500 million years ago (Mya), during the
Cambrian period, and may have resembled the modern hagfish in form.[33]
Sharks appeared about 450 Mya, amphibians about 400 Mya, reptiles about
350 Mya, and mammals about 200 Mya. No modern species should be described
as more "primitive" than others, strictly speaking, since each has an equally long
evolutionary history—but the brains of modern hagfishes, lampreys, sharks,
amphibians, reptiles, and mammals show a gradient of size and complexity that
roughly follows the evolutionary sequence. All of these brains contain the same
set of basic anatomical components, but many are rudimentary in the hagfish,
whereas in mammals the foremost part (the telencephalon) is greatly elaborated
and expanded.