You can only see the physical differences between an Alzheimer’s brain and a healthy brain through scans, but the effects show up in everyday life. Alzheimer’s is the most common cause of dementia. Over time, it slowly changes how the brain looks and works, causing problems with memory, reasoning, and communication.

At first, the symptoms may be mild. But as the disease progresses, the damage becomes more severe. Understanding how Alzheimer’s affects the brain can help you better understand what’s happening to someone living with the condition.

Here are five key differences between a healthy brain and one with Alzheimer’s, including how these changes affect thinking and behavior, and how the brain looks on different scans.

1. Brain Shrinkage

As more neurons die, the brain shrinks further, especially in the final stage of Alzheimer’s. The brain shrinks as a normal part of healthy aging. But in a healthy brain, neurons (nerve cells) don’t significantly decrease with age. In Alzheimer’s, however, the brain shrinks much more because many nerve cells are damaged or die.

In Alzheimer’s, cell damage usually starts in the hippocampus and entorhinal cortex, which are key parts of the brain responsible for memory. The hippocampus helps form new memories, but it’s less involved in keeping older ones. That’s why someone in the early stage of Alzheimer’s may remember things from years ago but forget what happened earlier today.

The damage moves to the cerebral cortex, which is the area in charge of reasoning, behavior in social situations, and language. As the cortex becomes smaller from damage, memories from a long time ago are also lost.

As damage spreads and more neurons die, the brain continues to shrink, most noticeably in the final stage of Alzheimer’s. Loss of brain tissue due to Alzheimer’s can be seen on magnetic resonance imaging (MRI) scans. There, healthcare providers can see widening gaps in the cerebral cortex from cell damage.

2. Fewer Neurons and Synapses

Neurons are the brain cells most affected by Alzheimer’s disease. Compared to a healthy brain, an Alzheimer’s brain has far fewer neurons and synapses. Synapses are tiny connections between neurons that let them talk to each other.

Alzheimer’s disease causes disruptions to neurons’ ability to communicate and repair themselves. Neurons send messages using tiny bursts of electricity and special chemicals. These messages help control your senses, movement, and heart rate, and more. Alzheimer’s disrupts how these electrical signals travel, which can make it harder for the brain to form memories and thoughts. In people with Alzheimer’s disease, neurons stop working and lose their connections, which makes it harder for the brain to function. This breakdown is one of the disease’s most harmful effects.

The loss of neurons and their connections, which are key to daily functioning, makes it extremely difficult or impossible for people with Alzheimer’s to care for themselves. It also makes it hard to communicate with and remember people they know, including loved ones.

Scientists believe plaques and tangles in the brain play a major role in how Alzheimer’s damages and kills brain cells, but they are still studying exactly how this happens.

3. Amyloid Plaques

Amyloid plaques in the brain are considered one of the main features of Alzheimer’s. Amyloid plaques are made up of beta-amyloid proteins. Beta-amyloid is a small piece of a larger protein called amyloid precursor protein, or APP. APP is normally found in the body, but when beta-amyloid builds up, it can cause problems in the brain.

Small clumps of beta-amyloid may be even more harmful than larger plaques. They can make it harder for brain cells to communicate and may cause the immune system to respond in a way that may damage neurons.

In the early stages of Alzheimer’s, amyloid plaques build up in parts of the brain that control memory, learning, thinking, and planning. As the disease gets worse, these plaques spread to areas that control communication and spatial awareness — the ability to know where your body is in relation to your surroundings. Amyloid plaques can be seen on a positron emission tomography (PET) scan, a type of imaging test.

Scientists continue to look at how amyloid plaques contribute to Alzheimer’s. Research has shown that amyloid causes two different proteins in neurons to pair together. The pairing of these two proteins has been linked to about half of the genetic changes that happen with Alzheimer’s. This protein pair also causes tangles of tau protein to grow in neurons.

4. Tangles of Tau Protein

Like amyloid plaques, tangles of tau proteins are one of the main features of Alzheimer’s. In healthy neurons, a protein called tau provides structure to neurons, allowing necessary nutrients and molecules to move through them. In Alzheimer’s, tau changes in a way that causes it to twist and clump together, forming tangles inside neurons. These tangles block nutrients from reaching the neurons, leading to cell death. Tangles, along with sticky clumps called amyloid plaques, are key signs of Alzheimer’s.

Tangles can also be seen on a PET scan.

Like amyloid plaques, tangles form in parts of the brain that control learning, memory, thinking, and planning in the early stages. Later, in mild to moderate Alzheimer’s, tangles affect parts of the brain responsible for communication and spatial awareness.

5. Poor Glucose Metabolism in the Brain

Glucose metabolism refers to the breakdown of glucose (a type of sugar) in the body. The body uses the broken-down glucose for energy. In a healthy brain, brain cells can adequately break down glucose so that the cells can use it for fuel. In an Alzheimer’s brain, glucose metabolism decreases in two types of brain cells, astrocytes and microglia.

Research suggests that amyloid and tau proteins can cause certain brain cells, called astrocytes, to work in ways that reduce glucose metabolism (how well the brain breaks down sugar). When glucose isn’t broken down properly, neurons don’t get the energy they need to work properly. Studies have found a link between problems with glucose metabolism and cognitive impairment — trouble with memory, learning, decision-making, or concentration.

Making sure the brain can break down glucose properly helps neurons do their job. In a study with mice, scientists found that blocking the enzyme that slows down glucose metabolism improved the mice’s spatial memory — their ability to remember where things are and how to move around. It also helped improve connections between neurons. When researchers tested this in people with Alzheimer’s, they found that blocking the same enzyme helped the brain break down sugar better. This gave the neurons the nutrients they needed. Some small studies in people with Alzheimer’s showed similar results, but more research is needed to know if this could become a safe and effective treatment.

PET scans can show parts of the brain where there are issues with metabolism, and neurons aren’t getting the nutrients they need. The worse Alzheimer’s gets, the more areas of the brain become affected by poor glucose metabolism.

Find Support From Your Doctor

Talk with your doctor if you have questions about how Alzheimer’s is affecting you or someone you care about. They can help you understand what’s causing certain symptoms, like trouble talking or not recognizing people. Sometimes, knowing what’s happening in the brain can make these symptoms a little easier to manage.

The effects of Alzheimer’s can feel scary or overwhelming, but remember that you’re not alone. A healthcare provider can give you the information you need about what to expect — and what can be done to help treat Alzheimer’s.

Talk With Others Who Understand

On myALZteam, the social network for people with Alzheimer’s disease and their loved ones, more than 86,000 members come together to ask questions, give advice, and share their stories with others who understand life with Alzheimer’s.

What differences have you learned about in an Alzheimer’s brain vs. a healthy brain? How have the brain effects of Alzheimer’s affected you or a loved one? Share your experience in the comments below, or start a conversation by posting on your Activities page.