The aging brain that doesn't fail: how neural resilience masks neurodegeneration.

Neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's disease, and Parkinson's disease are usually framed as consequences of aging-related pathogenic processes, including impaired proteostasis with protein aggregation, mitochondrial dysfunction, oxidative stress, and neuroinflammation. Yet, most individuals, even into advanced age, do not develop clinically significant neurodegenerative disease. This discrepancy suggests that the nervous system possesses robust and redundant protective mechanisms that maintain neural integrity despite cumulative molecular and cellular stress. In this perspective, we propose that neurodegenerative diseases arise not simply from the presence of pathogenic processes, but when integrated resilience systems fail to maintain homeostasis or when reserve mechanisms no longer compensate for accumulated pathology. We have synthesized a threshold-based model of disease emergence based on evidence across proteostasis, mitochondrial function, neuroimmune regulation, glial biology, network-level compensation, and barrier integrity, while integrating genetic, environmental, developmental, and stochastic modifiers. We distinguish biological resilience, which actively limits or repairs pathology, from reserve, which permits function despite pathology. We further propose that clinical disease emerges only when age-related cumulative stress exceeds the combined capacity of resilience and reserve. Reframing neurodegeneration as a failure of preservation systems offers new directions for prevention and therapeutic development.