If you have ever tried to read a medical article about Alzheimer’s disease and ended up more confused than when you started, you are not alone. The research literature is dense with terms like amyloid-beta, neurofibrillary tangles, and hippocampal atrophy. These are real and important concepts, but they do not have to be intimidating. Understanding what is actually happening in the brain during dementia can help families make sense of what they are seeing in a loved one and why certain symptoms develop in the order they do.
This post walks through the biology plainly.
The Brain Is a Communication Network
Before getting into what goes wrong, it helps to understand how the brain works at a basic level. The brain contains roughly 86 billion neurons, which are nerve cells that communicate with each other through connections called synapses. When you remember a name, recognize a face, or decide what to have for breakfast, what is happening underneath is a cascade of electrical and chemical signals traveling across billions of these connections at remarkable speed.
Dementia, in most of its forms, is fundamentally a disease of communication failure. The neurons stop talking to each other properly, and eventually they die. What varies between different types of dementia is the reason the communication breaks down and which parts of the brain are affected first.
In Alzheimer’s: Two Kinds of Damage
Alzheimer’s disease is caused by the buildup of two abnormal proteins in the brain. Both have been studied extensively, and both cause measurable harm to neurons and the synapses between them.
The first is amyloid-beta. Under normal conditions, amyloid-beta is a protein fragment produced naturally in the brain and cleared away as part of routine cellular maintenance. In Alzheimer’s disease, this clearance process fails. The protein accumulates and clumps together outside neurons, forming what researchers call amyloid plaques. A 2025 review published in Cell Death & Disease describes how these plaques disrupt communication between nerve cells and trigger inflammatory responses, which then cause further neuronal damage. The brain, trying to respond to what it perceives as a threat, ends up contributing to the destruction.
The second is tau. Tau is a protein whose normal job is to stabilize the internal structure of neurons, essentially acting as scaffolding that holds the cell together and supports the transport of nutrients along its length. In Alzheimer’s disease, tau becomes chemically altered, detaches from the scaffolding, and begins to clump inside neurons, forming what are called neurofibrillary tangles. Research published in Brain by Oxford Academic describes how tau tangles interfere with the internal transport systems of neurons, cutting off the flow of nutrients and eventually killing the cell. The tangles also spread from neuron to neuron as the disease progresses, which is part of why Alzheimer’s expands across the brain over time.
The two types of damage interact and amplify each other. Amyloid plaques appear to accelerate the formation of tau tangles, and the combined presence of both drives faster neurodegeneration than either would cause alone.
Why Memory Goes First
The reason memory is typically the first thing affected in Alzheimer’s disease comes down to geography. The disease tends to begin in the hippocampus, a small, curved structure deep in the brain that plays a central role in forming new memories.
The hippocampus is essentially where short-term experiences get encoded into longer-term storage. When it is damaged, new information stops sticking. A person can remember events from 30 years ago with vivid clarity while being unable to recall a conversation from this morning. This is not selective or deliberate, it is a direct consequence of where the disease starts.
Research published in a systematic review in PMC confirms that hippocampal atrophy, the physical shrinkage of the hippocampus as neurons die, is among the earliest detectable structural changes in Alzheimer’s disease and is strongly correlated with the progression of memory impairment. Brain imaging studies can often visualize this shrinkage years into the disease course.
The Spread Across the Brain
Alzheimer’s disease does not stay in the hippocampus. Over time, the damage spreads outward into other regions of the brain, which is why symptoms become broader and more severe as the disease progresses.
As it moves into the temporal lobes, language becomes affected. Word-finding difficulties, trouble following conversations, and eventually the loss of the ability to speak meaningfully are all connected to damage in these regions. As the frontal lobes become involved, executive function deteriorates: planning, judgment, impulse control, and the ability to manage complex tasks all decline. In the late stages, the disease has affected so much of the cortex that basic motor functions and the ability to swallow begin to fail.
A 2025 study published in Brain Communications tracking brain atrophy patterns in people at risk for Alzheimer’s disease found that this spread follows predictable pathways along neural connections, meaning the disease propagates through the brain’s own communication network, using the same routes neurons rely on to talk to each other.
How Other Types of Dementia Differ
The protein-buildup story described above applies primarily to Alzheimer’s disease. Other types of dementia damage the brain through different mechanisms, which is part of why their symptoms look different.
In vascular dementia, the problem is blood supply rather than protein accumulation. Small strokes or chronic reductions in blood flow deprive regions of the brain of oxygen, killing neurons in affected areas. Because strokes can occur anywhere, the symptoms of vascular dementia depend heavily on which regions are deprived of blood. The progression often looks stepped rather than gradual, with relatively stable periods punctuated by noticeable declines following additional vascular events.
In frontotemporal dementia, the damage begins in the frontal and temporal lobes rather than the hippocampus. These are the regions that govern personality, behavior, social judgment, and language. Because memory formation in the hippocampus is spared early on, people with frontotemporal dementia often show dramatic personality changes and behavioral symptoms before they show meaningful memory loss. This can make early diagnosis particularly difficult, as the symptoms can initially look like depression, a psychiatric disorder, or a personality change with no obvious cause.
In Lewy body dementia, a different protein, called alpha-synuclein, accumulates inside neurons in clusters called Lewy bodies. These affect neurons involved in cognition, behavior, movement, and the autonomic nervous system, which is why Lewy body dementia produces a distinctive combination of cognitive fluctuations, visual hallucinations, and Parkinson’s-like movement symptoms.
What the Damage Looks Like From the Outside
Understanding the biology helps explain why dementia symptoms are not random. Forgetting recent events while remembering old ones reflects hippocampal damage. Getting lost while navigating a familiar route reflects damage to spatial memory systems in the parietal and temporal cortex. Changes in personality or social behavior reflect frontal lobe involvement. Difficulty finding words reflects temporal lobe damage.
The brain is organized, and dementia dismantles it in patterns. Recognizing those patterns does not change the diagnosis, but it can make the experience of watching a loved one’s decline feel a little less bewildering.
If you want to understand how this biological progression maps onto observable stages of the disease, our post on the 7 Stages of Dementia breaks down the Global Deterioration Scale in plain language, including what families can expect as each region of the brain becomes increasingly affected.
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Sources
● Guo T, et al. Amyloid-beta and Tau in Alzheimer’s Disease: Pathogenesis, Mechanisms, and Interplay. Cell Death & Disease. 2025.
● Vogel JW, et al. Data-driven study of Alzheimer’s disease related amyloid and tau pathology progression. Brain. 2023;146(12):4935-4948.
● Tremblay C, et al. Uncovering atrophy progression pattern and mechanisms in individuals at risk of Alzheimer’s disease. Brain Communications. 2025;7(2):fcaf099.
● Lach HJ, et al. The relationship between hippocampal changes in healthy aging and Alzheimer’s disease: a systematic literature review. PMC. 2024.
● Arnsten AFT, et al. An integrated view of the relationships between amyloid, tau, and inflammatory pathophysiology in Alzheimer’s disease. Alzheimer’s & Dementia. 2025.
● Alzheimer’s Association. 2024 Alzheimer’s Disease Facts and Figures. Alzheimer’s & Dementia. 2024.