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In the anatomy of animals, the brain, or encephalon, is the supervisory
center of the nervous system. Although the brain is usually cited
as the supervisory center of vertebrate nervous systems, the same
term can also be used for the invertebrate central nervous system.
In most animals, the brain is located in the head.
The brain controls and coordinates most movement, behavior and
homeostatic body functions such as heartbeat, blood pressure, fluid
balance and body temperature. Functions of the brain are responsible
for cognition, emotion, memory, motor learning and other sorts of
learning.
The brain is primarily made up of two types of cells: glia and
neurons. Glia function primarily to support and protect the neurons.
The neurons carry information in the form of electrical pulses known
as action potentials. They communicate with other neurons in the
brain and throughout the body by sending various chemicals called
neurotransmitters across gaps known as synapses. Small invertebrates
such as insects may have a million neurons in the brain, larger
vertebrate brains have over one hundred billion neurons. The human
brain is particularly complex and large in comparison to human body
size.
Brains in nature
Although many classes of animals have nervous systems, three groups
of animals, with some exceptions, have notably complex brains: the
arthropods (for example, insects and crustaceans), the cephalopods
(octopuses, squid, and similar mollusks), and craniates (vertebrates
and their cousins). The brains of arthropods and cephalopods arise
from twin parallel nerve cords that extend through the body of the
animal. The arthropod brain consists of large optical lobes behind
each eye for visual processing and a central brain with three divisions.
The cephalopod brain has a central group of lobes known as circumesophageal
lobes that are flanked by two large optical lobes on the left and
right (Butler, 2000).
The brains of craniates develop from the anterior section of a
single dorsal nerve cord, which later becomes the spinal cord. In
craniates, the brain is protected by the bones of the skull. Vertebrates
are characterized by increasing complexity in the cerebral cortex
as one moves up the phylogenetic and evolutionary tree. Primitive
vertebrates, like fish, reptiles, and amphibians have cortices with
less than six layers of neurons, a structure known as allocortex
(Martin, 1996). More complex vertebrates like mammals have developed
six-layered neocortex in addition to having some parts of the brain
that are allocortex (Martin, 1996). In mammals, increasing convolutions
of the brain, called gyri, are characteristic of animals with more
advanced brains. These convolutions evolved to provide more surface
area for a greater number of neurons while keeping the volume of
the brain compact enough to fit inside the skull.
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