Hemispheres of the brain constitute 85 percent of the weight of the brain. The billions of neurons in the two hemispheres are connected by a thick bundle of nerves called the corpus callosum. Scientists believe that the two hemispheres differ not so much in focus (the idea of "logical" versus "artistic"), but how to 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 generalities (such as understanding the relative position of objects in space). The cerebral hemispheres have an outer layer called cortex. This is where the brain processes sensory information received from the outside world, controls voluntary movements and regulates conscious thought and mental activity.
* The cerebellum covering a little over 10 percent of the brain. Is in charge of balance and coordination. The cerebellum has two hemispheres that are always receiving information from the eyes, ears, muscles and joints about the movements and body position. Once the brain processes this information, it travels through the rest of the brain and spinal cord to issue instructions to the body. The work of the cerebellum allows us to walk smoothly, maintain our balance and turn around without having to think to do so.


* The brain stem is located at the base of the brain. Spinal cord connects with the rest of the brain. Although it is the smallest of the three main actors, their roles are crucial to survival. The brain stem controls automatic functions that keep us alive, our heart rate, blood pressure and respiration. It also relays information between the brain and spinal cord, which then sends messages to muscles, skin and other organs. Rest and sleep are also controlled by the brainstem.


Other key parts

There are other essential parts of the brain that are located inside the brain:

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The limbic system or visceral brain stem-links with elements of higher reasoning of the cerebral cortex. Controls emotions and instinctive behavior. It is also where lies the sense of smell.
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The hippocampus is an important part of learning and memory in the short term. It is considered as part of the brain where short-term memories become long-term memories, and then stored in other areas of the brain.
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The thalamus receives sensory and limbic information, processes it, and then sends it to the cerebral cortex.
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The hypothalamus is a structure under the thalamus that monitors activities such as body temperature and food consumption. Instructed to correct any misalignment. The hypothalamus also controls the body's internal clock.

The brain in action

New imaging techniques allow scientists to monitor brain function in people alive. This has opened 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 scan (PET, acronym in English). PET measures blood flow and glucose metabolism throughout the brain (for more information on the metabolism see the neurons and their functioning). When nerve cells in a brain region is activated, blood flow and metabolism increase in these regions. These increases are usually red and yellow colors on a PET. Black and blue shades indicate a decrease or lack of activity within a brain region. Essentially, a PET produces a "map" of the brain active.

Scientists use PET scans to see what happens in the brain when a person makes a mental or physical activity, rest, sleep or dream. Scientists also can inject chemicals marked with a crawler that will light up during PET. These trackers can monitor the activity of brain chemicals, neurotransmitters such as dopamine and serotonin. Some of these neurotransmitters are altered with age, disease and medicine.

The aging brain

As a person ages, changes occur in all parts of the body, including the brain:

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       Some neurons shrink, especially in larger areas important for learning, memory, planning and other complex mental activities.
     * Knots and plaques develop in neurons and surrounding areas, though in much smaller than when a person has Alzheimer's disease (for details see the section amyloid plaques).
     * The damage caused by free radicals increases (free radicals are a class of molecule that reacts easily with other molecules for more information on these molecules refer to the section oxidative damage from free radicals).

What is the impact of these changes? Healthy older people may experience a moderate decrease in the ability to learn new things and remember information, such as remembering names. Also, can perform poorly on complex tasks related to attention, learning and memory. However, if given enough time to fulfill these tasks, the scores of healthy people aged between 70 and 80 years is often the same as that of younger adults. As these people age, adults often improve their vocabulary and other verbal knowledge.