How Does Alzheimer's Disease Develop?

Alzheimer's disease is a neurological condition, which means it affects the brain. Over time, it causes the brain to shrink and brain cells (neurons) to die. These changes damage memory first, then language and independent functions.

We do not know the exact causes of Alzheimer's disease. But we know certain changes in the brain lead to symptoms. These changes include the formation of protein clumps that block neuron functions. These clumps are called "amyloid plaques" and "neurofibrillary tangles." Scientists are currently exploring what causes the clumps and how they lead to symptoms.

What causes Alzheimer’s disease?

We do not know exactly what causes Alzheimer's. But we do know that genetic, lifestyle, and environmental factors all contribute to the disease's development. For example, older age and other chronic conditions increase the risk of Alzheimer's.¹

We do not understand why these factors lead to Alzheimer's disease. The explanations are likely quite complex. But scientists are quickly uncovering more about causes and risk factors. Their discoveries will change how we understand and treat Alzheimer's disease.

What does Alzheimer's disease do to the brain?

A healthy human brain contains billions of neurons. Neurons send information to each other and the rest of the body. Alzheimer's disease prevents neurons from communicating with each other.²

When neurons stop communicating with each other, they start to die. People with Alzheimer's disease lose large numbers of neurons. This causes the brain to shrink by the late stages of the disease.^1,2

Damage to neurons happens in a predictable pattern. Usually, neurons in parts of the brain involved in memory are the first to stop working. Then, the disease affects parts of the brain responsible for language and social behavior. Over time, it causes an inability to live independently. Alzheimer's disease ultimately ends in death.^1,2

What are amyloid plaques and neurofibrillary tangles?

Scientists are trying to understand exactly what happens inside the brain of people with Alzheimer's disease. But it is hard to tell whether changes in the brain are causes or results of the disease. Research is mostly focused on 2 proteins that clump together in the brain:^1,3,4

• Plaques (clumps of beta-amyloid proteins)
• Neurofibrillary tangles (clumps of tau proteins)

Other changes to the brain also happen in Alzheimer's disease. For example, chronic inflammation and blood flow issues also contribute to the disease. A complex combination of these factors likely plays a role in disease development.²

Amyloid plaques

Beta-amyloid proteins form from a larger protein called "amyloid precursor protein" (APP). APP is an important part of the surface of neurons. But in Alzheimer's disease, APP gets broken down abnormally into beta-amyloid pieces.³

These beta-amyloid proteins tend to clump together. When this happens, they form structures called "plaques." Plaques collect between neurons and block communication. They also cause immune cells to surround the plaques, which leads to inflammation.³

Neurofibrillary tangles

In healthy neurons, a protein called "tau" helps stabilize networks that give cells their shape. In Alzheimer's disease, abnormal tau proteins detach from these networks and clump together. This forms threads or "neurofibrillary tangles" inside neurons. These tangles prevent neurons from communicating with each other.⁴

The formation of amyloid plaques seems to trigger changes in tau that cause it to clump. Once amyloid plaques reach a certain level, tau tangles begin to spread throughout the brain.⁴