Alzheimer's Disease: Unraveling the Mystery

Part 1: The Basics of the Healthy Brain

To understand AD, it is important to know a bit about the brain. This part of Unraveling the Mystery gives an inside view of the normal brain, how it works, and what happens during aging.

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

The brain is made of nerve cells and several other cell types. Nerve cells also are called neurons. The neurons of all animals function in basically the same way, even though animals can be very different from each other. Neurons survive and function with the help and support of glial cells, the other main type of cell in the brain. Glial cells hold neurons in place, provide them with nutrients, rid the brain of damaged cells and other cellular debris, and provide insulation to neurons in the brain and spinal cord. In fact, the brain has many more glial cells than neurons—some scientists estimate even 10 times as many.

Another essential feature of the brain is its enormous network of blood vessels. Even though the brain is only about 2 percent of the body’s weight, it receives 20 percent of the body’s blood supply. Billions of tiny blood vessels, or capillaries, carry oxygen, glucose (the brain’s principal source of energy), nutrients, and hormones to brain cells so they can do their work. Capillaries also carry away waste products.

The Brain’s Vital Statistics

cross-section image of the brainAdult weight
about 3 pounds

Adult size
a medium cauliflower

Number of neurons
about 100,000,000,000 (100 billion)

Number of synapses
(the gaps between neurons)
about 100,000,000,000,000 (100 trillion)

Number of capillaries
(tiny blood vessels)
about 400,000,000,000 (400 billion)


Publication Date: September 2008
Page Last Updated: January 22, 2015

Thumbnail of Alzheimer's Disease: Unraveling the Mystery