Alzheimer's Disease : Inside the Human Brain

by National Institute on Aging

(Page 2 of 16)

A Walking Tour Through the Brain

The brain is a remarkable organ. Seemingly without any effort, it allows us to carry out every element of our daily lives. It manages many of the body functions that happen without our knowledge or direction, such as breathing, blood circulation, and digestion. It also directs all the functions we carry out consciously. We can speak, move, see, remember, feel emotions, and make decisions because of the complicated mix of chemical and electrical processes that take place in our brains.

Our brains are made of nerve cells and lots of other cell types. Nerve cells are also called neurons. The neurons of all animals function in basically the same way, even though animals can be very different from each other. What sets people apart from other animals is the huge number of nerve cells we have in the cerebral cortex, regions of which are proportionally much larger in humans than in any other animals. These regions are the parts of the brain where cognitive functions, like thinking, learning, speaking, remembering, and making decisions, take place. The many interconnections among the nerve cells in these regions also make us different from other animals.

To understand Alzheimer's disease, it's important to know a bit about the brain. Part 1 of Unraveling the Mystery first gives an inside view of the normal brain, how it works, and what happens during aging. Then, it shows what happens to the brain in Alzheimer's and how the disease slowly destroys a person's mental and physical capacities.

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. 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.

About the Author

www.nia.nih.gov
NIA, one of the 27 Institutes and Centers of NIH, leads a broad scientific effort to understand the nature of aging and to extend the healthy, active years of life. In 1974, Congress granted authority to form NIA to provide leadership in aging research, training, health information dissemination, and other programs relevant to aging and older people.