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The Three Main Players
- The cerebral hemispheres accounts for 85
percent of the brain's weight. The billions of neurons in the
two hemispheres are connected by a thick bundle of nerves called
the corpus callosum.
Scientists now think that the two hemispheres differ not so much
in what they focus on (the "logical versus artistic" notion),
but how they process information. The left hemisphere appears to
focus on the details (such as recognizing a particular face in a
crowd). The right hemisphere focuses on the broad background
(such as understanding the relative position of objects in a
space). The cerebral hemispheres have an outer layer called the
cerebral cortex. This is where the brain
processes sensory information received from the outside world,
controls voluntary movement, and regulates conscious thought and
mental activity.
- The cerebellum takes up a little more than 10
percent of the brain. It's in charge of balance and
coordination. The cerebellum also has two hemispheres. They are
always receiving information from the eyes, ears, and muscles
and joints about the body's movements and position. Once the
cerebellum processes the information, it works through the rest
of the brain and spinal cord to send out instructions to the
body. The cerebellum's work allows us to walk smoothly, maintain
our balance, and turn around without even thinking about
it.
- The brain stem sits at the base of the brain.
It connects the spinal cord with the rest of the brain. Even
though it's the smallest of the three main players, its
functions are crucial to survival. The brain stem controls the
functions that happen automatically to keep us alive - our heart
rate, blood pressure, and breathing. It also relays information
between the brain and the spinal cord, which then sends out
messages to the muscles, skin, and other organs. Sleep and
dreaming are also controlled by the brain stem.
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Other Crucial Parts
Several other essential
parts of the brain lie deep inside the cerebral
hemispheres:
- The limbic system links the brain stem with
the higher reasoning elements of the cerebral cortex. It
controls emotions and instinctive behavior. This is also where
the sense of smell is located.
- The hippocampus is important for learning and
short-term memory. This part of the brain is considered to be
the site where short-term memories are converted into long-term
memories for storage in other brain areas.
- The thalamus receives sensory and limbic
information, processes it, and then sends it to the cerebral
cortex.
- The hypothalamus is a structure under the
thalamus that monitors activities like body temperature and food
intake. It issues instructions to correct any imbalances. The
hypothalamus also controls the body's internal clock.
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The Brain in Action
New imaging techniques
allow scientists to monitor brain function in living people. This
is opening up worlds of knowledge about normal brain function and
how it changes with age or disease.
One of these
techniques is called positron emission tomography, or PET
scanning. PET scans measure blood flow and glucose metabolism throughout the brain. (For more
on metabolism see the section Neurons and
Their Jobs) When nerve cells in a region of the brain
become active, blood flow and metabolism in that region increase.
These increases are usually shown as red and yellow colors on a
PET scan. Shades of blue and black indicate decreased or no
activity within a brain region. In essence, a PET scan produces a
"map" of the active brain.
Scientists use PET scans to see what happens in the brain
when a person is engaged in a physical or mental activity, at
rest, or even sleeping or dreaming. Scientists can also inject
chemicals tagged with a tracer that will "light up" on PET scans.
These tracers can track the activity of brain chemicals, for
example neurotransmitters such as dopamine and
serotonin. Some of these neurotransmitters are altered with age,
disease, and drug treatment.
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The Aging Brain
As a person gets older,
changes occur in all parts of the body, including the
brain:
- Some neurons shrink, especially large ones in areas
important to learning, memory, planning, and other complex
mental activities.
- Tangles and plaques develop in neurons and surrounding
areas, though in much smaller amounts than in AD (see the
section Plaques and
Tangles for more information).
- Damage by free radicals increases (free radicals are
a kind of molecule that reacts easily with other molecules; see
the section Oxidative Damage from Free Radicals for more on
these molecules).
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What
is the impact of these changes? Healthy older people may notice a
modest decline in their ability to learn new things and retrieve
information, such as remembering names. They may perform worse on
complex tasks of attention, learning, and memory. However, if
given enough time to perform the task, the scores of healthy
people in their 70s and 80s are often the same as those of young
adults. As they age, adults often improve their vocabulary and
other forms of verbal knowledge.
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