Clinical meaning
Neurodegenerative diseases share a common pathological theme of progressive, selective neuronal loss driven by accumulation of misfolded proteins that form toxic aggregates, but each disease affects distinct neuronal populations and involves different pathological proteins. In Parkinson disease, dopaminergic neurons in the substantia nigra pars compacta progressively degenerate, reducing dopamine input to the striatum (caudate and putamen) via the nigrostriatal pathway. The basal ganglia circuitry normally modulates motor output through a balance between the direct pathway (D1 receptors, facilitating movement) and indirect pathway (D2 receptors, suppressing unwanted movement). Dopamine depletion shifts this balance toward excessive indirect pathway activity, producing the cardinal motor features: bradykinesia (slowness of movement initiation), rigidity (cogwheel or lead-pipe resistance to passive motion), resting tremor (4-6 Hz pill-rolling), and postural instability. Intraneuronal Lewy bodies composed of misfolded alpha-synuclein are the pathological hallmark. In Alzheimer disease, two proteinopathies co-occur: extracellular amyloid-beta plaques form when amyloid precursor protein (APP) is cleaved by beta-secretase and gamma-secretase, producing amyloid-beta 42 peptides that misfold, aggregate into oligomers and fibrils, and deposit as neuritic plaques. These oligomers are directly neurotoxic, disrupting synaptic function and triggering microglial activation and neuroinflammation. Intracellular neurofibrillary tangles (NFTs) consist of hyperphosphorylated tau protein: normally, tau stabilizes microtubules for axonal transport; hyperphosphorylation causes tau to dissociate from microtubules, aggregate into paired helical filaments, and form NFTs that disrupt axonal transport and eventually kill the neuron. The cholinergic hypothesis explains early cognitive symptoms: degeneration of cholinergic neurons in the nucleus basalis of Meynert reduces acetylcholine in the cortex and hippocampus, impairing memory formation and attention. In Lewy body dementia, alpha-synuclein deposits are distributed diffusely throughout the cortex (unlike Parkinson disease where they begin in the brainstem), producing a distinctive clinical triad: fluctuating cognition, visual hallucinations, and parkinsonism, with extreme sensitivity to dopamine-blocking antipsychotics that can precipitate life-threatening rigidity.