| The brain is our
control center. It controls our
whole body, including involuntary
tasks such as blinking,
breathing, and your heart
beating. It also receives and
produces countless signals from
your body. So examples of the
kinds of signals it receives are
sights, from your eyes, sounds
from your ears, and heat from
your nerves. It also controls all
of the human emotions, including
love, hate, and fear. From the outside
the brain appears as three
distinct, but connected parts.
These are the cerebrum, the
cerebellum, and the brain stem.
The brain stem is where the brain
connects to your spinal cord, the
cerebrum controls much of your
voluntary activities, and the
cerebellum deals mostly with
movement and involuntary motions.
The
cerebrum is the largest part of
the human brain, making up
approximately 85 percent of the
brain's weight; its large surface
area (cortex) and intricate
development account for the
superior intelligence of humans,
compared with other animals. The
cerebrum is divided by a
longitudinal fissure
(indentation) into right and
left, mirror-image hemispheres.
The corpus callosum is the
slab of white nerve fibers that
connects these two cerebral
hemispheres and transfers
important information from one to
the other.
Each
cerebral hemisphere consists of
an outer layer of gray matter
called the cerebral cortex, about
3 to 4 mm thick. The cortex is
composed of layers of
unmyelinated (non sheathed) cells,
which in turn cover an inner mass
of myelinated (white-sheathed)
fibers called white matter.
Myelinated fibers connect the
cerebrum with other parts of the
brain (projection fibers); the
front of the brain to the back
portion; different areas on the
same side of the cerebrum
(association fibers); and one
side of the brain to the other
side (commissural fibers).
The
cerebellum lies in the hind part,
of the cranium, underneath the
cerebrum. Like the cerebrum, it
is made up of gray, unmyelinated
cells on the exterior and white,
myelinated cells in the interior.
It is composed of two hemispheres
that are connected by white
fibers called the vermis.
Three bands of fibers called the
cerebellar peduncles connect the
cerebellum to other parts of the
brain. The cerebellum is linked
with the midbrain by the superior
(top) peduncle, with the pons by
the middle peduncle, and with the
medulla by the inferior (bottom)
peduncle.
The
cerebellum is essential to the
control of movement of the human
body in space. It acts as a
reflex center for the
coordination and precise
maintenance of equilibrium. Thus,
all motor activity, from hitting
a baseball to fingering a violin,
depends on the cerebellum.
The
brain stem is actually composed
of many components, that you can
learn more about at the bottom of
this page. Its main functions are
many of your involuntary
functions, your heart beat, blood
pressure, and involuntary
breathing all happen in this part
of the brain.
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