Basic Neurosciences · Paper A
Alzheimer's disease pathology
Alzheimer's disease is a build-up disease, not a wear-out disease. Two abnormal proteins — sticky amyloid plaques outside neurons and tangled tau fibres inside them — accumulate for years before symptoms appear, while a specific population of acetylcholine-making neurons quietly dies off. Because the damage starts in the hippocampusA brain structure central to forming new memories.Read more → (the brain's memory-recording structure) and only later spreads outward, the first sign is nearly always memory loss, arriving smoothly and gradually rather than in sudden steps. The exam question is almost never 'is this dementia' — it is which of several distinct diseases, each with its own protein, its own timeline and its own tell-tale extra feature, actually produced this patient's picture.
Picture a house nobody has cleaned in twenty years. Two kinds of rubbish pile up: sticky debris left outside the rooms, and knotted wiring collapsing inside the walls. Slowly the electricity supply fails — not because someone cut the power, but because the wiring itself is failing room by room. That house is a simplified picture of Alzheimer's disease: sticky protein clumps building up outside brain cells, tangled protein fibres collapsing inside them, and a specific power supply — the brain chemical acetylcholine — quietly failing as a result. This page builds that picture from zero, then uses it to separate Alzheimer's disease from every other cause of a failing memory the exam likes to test.
Alzheimer's disease is a progressive brain disease caused by two abnormal proteins building up in and around brain cells (neurons), which gradually kills those cells and starves the brain of a chemical messenger it needs. The first protein, beta-amyloid, builds up OUTSIDE neurons as sticky clumps called plaques. The second, tau, builds up INSIDE neurons as tangled fibres called neurofibrillary tangles. Together they kill neurons fastest in the hippocampus (the brain structure that files new memories away), which is why memory loss is almost always the first thing anyone notices.
Here is the trap almost every exam candidate falls into: picturing dementia as one uniform decline, and reaching for Alzheimer's disease whenever a stem mentions an older patient with memory trouble. Alzheimer's disease has a specific SIGNATURE among the causes of dementia — smooth, continuous decline over years, starting with memory, with no sudden drops and no early movement problems. A patient who deteriorates in sudden steps after each small stroke, or who becomes disinhibited while their memory stays intact, is very likely showing you a different disease wearing the same "dementia" label — and the exam is testing whether you notice the difference, not whether you can spell Alzheimer's.
Start from what a healthy neuron needs. Amyloid precursor protein (APP) is a normal protein sitting in every neuron's outer membrane; ordinarily an enzyme called alpha-secretase cuts it up harmlessly. In Alzheimer's disease, two different enzymes — beta-secretase and gamma-secretase — cut APP instead, and the fragment they produce, beta-amyloid, is sticky. It clumps together outside neurons into plaques, and for example, imagine glue slowly filling the gaps between the wires of a circuit board: signals between neurons start misfiring.
Inside the neuron, a second protein called tau normally acts like scaffolding, holding open the internal tracks (microtubules) that transport nutrients up and down the cell. In Alzheimer's disease, tau becomes hyperphosphorylated (loaded with extra phosphate groups), detaches from the tracks, and tangles up with itself instead — a neurofibrillary tangle. The tracks collapse, and the neuron can no longer feed itself properly.
Both processes converge on one especially vulnerable population: cholinergic neurons in the nucleus basalis of Meynert, a small cluster deep in the forebrain that supplies most of the cortex's acetylcholine (the chemical messenger neurons use to communicate for attention and memory). As plaques and tangles kill these cells, acetylcholine supply collapses, which is exactly why drugs that block its breakdown (cholinesterase inhibitors) are the first-line treatment you will meet again below.
The spread of tangles follows a predictable route named after the pathologist who mapped it, Braak. Consider a patient in the earliest stage: tangles are confined to the entorhinal cortex and , the brain's memory-recording structures, so the only symptom is trouble remembering recent events, such as a shopping list or a name just heard. As tangles spread outward into the surrounding and then the wider neocortex over several years, language, judgement and personality progressively join memory loss.
This spread also explains the genetics. The APP gene sits on chromosome 21. A person with Down syndrome (trisomy 21, meaning three copies of chromosome 21 instead of the usual two) therefore makes roughly 50% more APP than usual, for life — which is why almost everyone with Down syndrome develops Alzheimer's disease pathology by their forties, decades earlier than the general population, from the same amyloid mechanism running on an extra dose of raw material. You can think of the APOE e4 allele differently: it does not cause extra APP, but it makes amyloid less efficient, and it is the single strongest genetic risk factor for the common, late-onset form of the disease.
Several other diseases produce a failing memory, and one clinical clue usually separates each from Alzheimer's disease:
| Looks like Alzheimer's disease | How to tell it apart |
|---|---|
| Vascular dementia | Sudden STEPS after a vascular event, with focal signs, not a smooth slide |
| Dementia with Lewy bodies | Fluctuating alertness, visual and spontaneous parkinsonism together, with antipsychotic |
| Parkinson's disease dementia | Motor disease for years FIRST; cognition falls only later |
| Frontotemporal dementia | Personality or language change first; memory stays intact early |
| Creutzfeldt-Jakob disease | Rapid decline over WEEKS, often with |
| Normal pressure hydrocephalus | Gait apraxia plus incontinence; reversible by draining fluid |
| Wernicke- | Alcohol excess plus points to thiamine deficiency |
There is no single blood test for Alzheimer's disease; diagnosis rests on a clinical history of progressive cognitive decline, commonly screened with a bedside tool such as the or the , alongside structural imaging (MRI showing hippocampal and medial temporal atrophy, and used to rule out a vascular or structural cause) and, where available, cerebrospinal fluid or PET biomarkers. A reduced level of amyloid-beta 42 alongside a raised level of phosphorylated tau in the CSF mirrors the two proteins building up in the brain itself. A firm pathological diagnosis, counting plaques and tangles at post-mortem, is only ever made after death; everything made in life is a clinical-plus-biomarker diagnosis, however confident it looks.
NICE NG97 (Dementia: assessment, management and support) sets out the real, currently-relevant plan, and it is not simply that there is no cure. For mild-to-moderate Alzheimer's disease, a cholinesterase inhibitor — donepezil, rivastigmine, or galantamine — is recommended first-line, directly targeting the acetylcholine shortage this page has already explained. For moderate disease where a cholinesterase inhibitor is unsuitable, or for severe disease, memantine (a drug that blocks excess NMDA-receptor signalling) is recommended instead. Neither drug slows the underlying amyloid or tau build-up; both treat the downstream chemical shortage, which is why non-drug support — structured activity, carer education, and planning ahead for and future care — sits alongside medication as an equally real part of the guideline, not an afterthought to it.
Whenever a stem hands you an older adult with cognitive decline, resist naming Alzheimer's disease on reflex. Ask yourself: is the course smooth or stepwise? Did memory fail first, or did personality, movement, or continence fail first? Before you move on, see if you can explain to a friend why a patient with eleven years of tremor and rigidity who only develops memory problems in the last year is not the same diagnosis as a patient whose and memory loss arrived together.
Alzheimer's disease is beta-amyloid plaques plus tau tangles plus a collapsing acetylcholine supply, spreading smoothly outward from the over years — a specific signature the exam expects you to tell apart from every other cause of a failing memory.