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.
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.
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.
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).
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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