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Three Pounds, Three Parts

Your brain is your most powerful organ, yet weighs only about three pounds. It has a texture similar to firm jelly.

The brain has three main parts:


The cerebrum fills up most of your skull. It is involved in remembering, problem solving, thinking, and feeling. It also controls movement.


The cerebellum sits at the back of your head, under the cerebrum. It controls coordination and balance.

brain stem

The brain stem sits beneath your cerebrum in front of your cerebellum. It connects the brain to the spinal cord and controls automatic functions such as breathing, digestion, heart rate and blood pressure.

Supply Lines

Your brain is nourished by one of your body’s richest networks of blood vessels. When you are thinking hard, your brain may use up to 50 percent of the fuel and oxygen.

Brain Arteries

With each heartbeat, arteries carry about 20 to 25 percent of your blood to your brain, where billions of cells use about 20 percent of the oxygen and fuel your blood carries.

Brain Vessels

The whole vessel network includes veins and capillaries in addition to arteries.

The Cortex: “Thinking Wrinkles”

Your brain’s wrinkled surface is a specialized outer layer of the cerebrum called the cortex. Scientists have “mapped” the cortex by identifying areas strongly linked to certain functions.

Cortex RegionsCortex RegionsCortex RegionsCortex RegionsCortex RegionsCortex RegionsCortex Regions

View the specific regions of the cortex:

  • Interpret Sensations From Your Body
  • Processing Sights
  • Processing Sounds
  • Processing Smells
  • Thoughts, Problem Solving & Planning
  • Forming & Storing Memories
  • Controlling Voluntary Movement

Left Brain/Right Brain

Your brain is divided into right and left halves. Experts are not certain how the “left brain” and “right brain” may differ in function. In most people, the language area is chiefly on the left.

The left half controls movement on the body’s right side.
The right half controls the body’s left side.

The Neuron Forest

Neurons are the chief type of cell destroyed by Alzheimer’s disease.

Neuron Cells

An adult brain contains about 100 billion nerve cells.

Neuron Cells

Branches connect the nerve cells at more than 100 trillion points. Scientists call this dense, branching network a “neuron forest.”

Neuron Cells

Signals traveling through the neuron forest form the basis of memories, thoughts, and feelings.

Cell Signaling

The real work of your brain goes on in individual cells. The neurotransmitters travel across the synapse, carrying signals to other cells. Scientists have identified dozens of neurotransmitters. Alzheimer’s disease disrupts both the way electrical charges travel within cells and the activity of neurotransmitters.

Cell Electrical Charge1

Signals that form memories and thoughts move through an individual nerve cell as a tiny electrical charge.

Cell Synapses2

Nerve cells connect to one another at synapses.

Cell Neurotransmitters3

When a charge reaches a synapse, it may trigger release of tiny bursts of chemicals called neurotransmitters.

Signal Coding

100 billion nerve cells. 100 trillion synapses. Dozens of neurotransmitters. This “strength in numbers” provides your brain’s raw material. Over time, our experiences create patterns in signal type and strength. These patterns of activity explain how, at the cellular level, our brains code our thoughts, memories, skills and sense of who we are.

The positron emission tomography (PET) scan on the left shows typical patterns of brain activity associated with:

  • Reading Words
  • Hearing Words
  • Thinking About Words
  • Saying Words


Alzheimer’s Changes the Whole Brain

Alzheimer’s disease leads to nerve cell death and tissue loss throughout the brain. Over time, the brain shrinks dramatically, affecting nearly all its functions.

These images show:

Brain Tissue

A brain without the disease.

Brain Tissue

A brain with advanced Alzheimer’s.

Brain Tissue

How the two brains compare.

Under the Microscope

Scientists can also see the terrible effects of Alzheimer’s disease when they look at brain tissue under the microscope. Scientists are not absolutely sure what causes cell death and tissue loss in the Alzheimer’s brain, but the plaques and tangles in the figures below are prime suspects.

Alzheimer’s tissue has many fewer nerve cells and synapses than a healthy brain.

Brain Tissue

Plaques, abnormal clusters of protein fragments, build up between nerve cells.

Brain Tissue

Dead and dying nerve cells contain tangles, which are made up of twisted strands of another protein.

More About Plaques


Plaques form when protein pieces called beta-amyloid (BAY-tuh AM-uh-loyd) clump together. Beta-amyloid comes from a larger protein found in the fatty membrane surrounding nerve cells.


Beta-amyloid is chemically “sticky” and gradually builds up into plaques.


The most damaging form of beta-amyloid may be groups of a few pieces rather than the plaques themselves. The small clumps may block cell-to-cell signaling at synapses. They may also activate immune system cells that trigger inflammation and devour disabled cells.

More About Tangles

Tangles destroy a vital cell transport system made of proteins. This electron microscope picture shows a cell with some healthy areas and other areas where tangles are forming.

In healthy areas:

  • Orderly, parallel strands for delivering key materials to the cells
  • A protein called tau helps keep the strands straight

In areas where tangles are forming:

Nutrients and other essential supplies can no longer move through the cells, which eventually die.

  • Tau collapses into twisted strands called tangles
  • The strands can no longer stay straight and disintegrate

Progression Through the Brain

Plaques and tangles (shown in the blue-shaded areas) tend to spread through the cortex in a predictable pattern as Alzheimer’s disease progresses. The rate of progression varies greatly. On average, a person with Alzheimer’s lives four to eight years after diagnosis, but can live as long as 20 years, depending on other factors. The course of the disease depends in part on age at diagnosis and whether a person has other health conditions.

Alzheimer's Brain Progression

Earliest Alzheimer’s – changes may begin 20 years or more before diagnosis.

Alzheimer's Brain Progression

Mild to moderate Alzheimer’s stages – generally last from 2 – 10 years.

Alzheimer's Brain Progression

Severe Alzheimer’s – may last from 1 – 5 years.

Earliest Alzheimer’s Stages

In the earliest stages, before symptoms can be detected with current tests, plaques and tangles begin to form in brain areas involved in:

Earliest Alzheimer's Stages

Learning and Memory

Earliest Alzheimer's Stages

Thinking and Planning

Mild to Moderate Alzheimer’s

In mild to moderate stages, brain regions important in memory and thinking and planning develop more plaques and tangles than were present in early stages. As a result, individuals develop problems with memory or thinking serious enough to interfere with work or social life. They may also get confused and have trouble handling money, expressing themselves and organizing their thoughts. Many people with Alzheimer’s are first diagnosed in these stages.

As Alzheimer’s progresses, individuals may experience changes in personality and behavior and have trouble recognizing friends and family members.

Plaques and tangles also spread to areas involved in:

Mild to Moderate Stages

Speaking and Understanding Speech

Mild to Moderate Stages

Your sense of where your body is in relation to objects around you

Severe Alzheimer’s Disease

In advanced Alzheimer’s disease, most of the cortex is seriously damaged. The brain shrinks dramatically due to widespread cell death. Individuals lose their ability to communicate, to recognize family and loved ones and to care for themselves.

Severe Alzheimer's Stage