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Malattia di Alzheimer II: Fisiopatologia
La malattia di Alzheimer comporta alterazioni strutturali cerebrali che iniziano molto prima della comparsa dei sintomi. Le caratteristiche più distin…
La malattia di Alzheimer provoca cambiamenti strutturali progressivi nel cervello, caratterizzati da placche neuritiche extracellulari e grovigli neurofibrillari intracellulari.
Le placche neuritiche derivano da un trattamento anomalo della proteina precursore amiloide, che porta all'accumulo di beta-amiloide, un peptide tossico che si aggrega all'esterno dei neuroni.
Questi depositi formano placche a nucleo denso nella corteccia cerebrale e intorno ai vasi sanguigni, circondate da neuriti distrofiche, astrociti e microglia, che interrompono la comunicazione sinaptica e causano danni neuronali.
All'interno dei neuroni, la tau, una proteina che stabilizza i microtubuli, si iperfosforila, si stacca da essi e forma filamenti contorti chiamati grovigli neurofibrillari. Questi nodi interrompono il trasporto intracellulare e favoriscono la morte neuronale.
Con il progredire della malattia, neuroni e sinapsi degenerano nell'ippocampo, nella corteccia entorhinale e nei lobi frontale e temporale, causando l'atrofia cerebrale.
La degenerazione dei neuroni colinergici nel cervello anteriore basale riduce i livelli di acetilcolina, contribuendo ai sintomi cognitivi e comportamentali.
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Q1: What are neuritic plaques and how do they form in Alzheimer disease?
Neuritic plaques are extracellular deposits that form in the cerebral cortex and around blood vessels. They result from abnormal processing of amyloid precursor protein, leading to accumulation of beta-amyloid, a toxic peptide that aggregates outside neurons. Each plaque has a dense core surrounded by damaged neuronal extensions, astrocytes, and microglia, disrupting synaptic communication and causing neuronal damage.
Q2: How do neurofibrillary tangles contribute to neuronal death in Alzheimer disease?
Neurofibrillary tangles form when tau protein, which normally stabilizes microtubules, becomes hyperphosphorylated and detaches from them. These twisted strands accumulate inside neurons, disrupting intracellular transport and promoting neuronal death. As tangles accumulate throughout the brain, they contribute to progressive neuronal degeneration and cognitive decline.
Q3: Which brain regions are most affected by neuronal degeneration in Alzheimer disease?
The hippocampus, entorhinal cortex, and frontal and temporal lobes experience the most significant neuronal and synaptic degeneration in Alzheimer disease. These regions are responsible for memory and thinking, and their progressive loss leads to brain atrophy, including widened sulci, thinned gyri, and enlarged ventricles. This structural damage correlates with the cognitive symptoms observed in patients.
Q4: What role does acetylcholine deficiency play in Alzheimer disease symptoms?
Degeneration of cholinergic neurons in the basal forebrain reduces acetylcholine levels, a key neurotransmitter for learning and memory. This neurotransmitter depletion, combined with synaptic loss and neural atrophy, contributes significantly to the cognitive and behavioral symptoms characteristic of Alzheimer disease. The loss of cholinergic function directly impairs memory formation and retention.
Q5: How does beta-amyloid accumulation damage neurons in Alzheimer disease?
Beta-amyloid is a toxic peptide fragment that results from abnormal processing of amyloid precursor protein. It aggregates extracellularly to form dense-core plaques surrounded by dystrophic neurites, astrocytes, and microglia. These plaques disrupt synaptic communication between neurons and eventually cause their death, leading to progressive cognitive decline.
Q6: What structural changes occur in the brain as Alzheimer disease progresses?
As Alzheimer disease progresses, neurons and synapses degenerate, causing overall brain atrophy. The brain exhibits widened sulci, thinned gyri, and enlarged ventricles. These structural changes result from the combined effects of neuritic plaques, neurofibrillary tangles, synaptic loss, and cholinergic neuron degeneration, ultimately leading to increased intracranial pressure and further neurological complications.
Q7: How are amyloid precursor protein and tau protein involved in Alzheimer pathology?
Amyloid precursor protein undergoes abnormal processing, generating beta-amyloid that accumulates extracellularly in neuritic plaques. Tau protein normally stabilizes microtubules but becomes hyperphosphorylated in Alzheimer disease, detaching from microtubules and forming intracellular neurofibrillary tangles. Both pathological processes disrupt neuronal function and promote cell death through distinct mechanisms.