Progressive loss of structure or function of neurons, often leading to dementia.Neurodegeneration is the gradual, irreversible loss of neurons — the cells that carry electrical signals through the brain, spinal cord, and peripheral nerves — along with the connections between them. Unlike a stroke, which damages tissue suddenly, neurodegeneration unfolds over years: individual cells shrink, lose their synapses, and eventually die, and the tissue they occupied is not replaced. Because the adult human brain produces very few new neurons, each lost cell represents a permanent subtraction from a circuit.
The consequences depend on which populations of neurons are affected. Degeneration in the hippocampus and cortex erodes memory and reasoning; degeneration of dopamine-producing cells in the midbrain disrupts movement control; degeneration of motor neurons in the spinal cord weakens muscles. This is why a single underlying process can appear clinically as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, or Huntington's disease — different names largely reflect which cells fail first.
Neurodegeneration matters because age is its strongest risk factor, and populations are aging. It also serves as a meeting point for several fields: protein misfolding, mitochondrial dysfunction, chronic inflammation, and impaired waste clearance inside cells are all implicated, and researchers debate how these mechanisms interact rather than which one is primary.
In many neurodegenerative conditions, a normal protein adopts an abnormal shape and then induces the same change in neighbouring copies, so the misfolded protein accumulates into clumps inside or between cells. These aggregates interfere with the cell's internal transport, overwhelm the machinery that degrades damaged proteins, and trigger stress responses. Energy supply falters as mitochondria — the cell's power plants — are damaged by the same toxic environment, and the resulting shortage of ATP leaves the neuron unable to maintain its ion gradients. A neuron that cannot sustain its gradients cannot fire normally, and if the stress persists, programmed cell death pathways are activated.
A second layer is the failure of clearance. Cells normally dispose of worn-out proteins and whole organelles through autophagy and through the proteasome; when these systems slow with age, damaged material accumulates faster than it can be removed. Because neurons are long-lived and largely non-dividing, they cannot dilute damage by dividing, so errors compound over decades. Glial cells, which normally support and protect neurons, can shift into a chronically inflamed state and release signals that further harm the tissue.
Research on neurodegeneration underpins the development of biomarkers — such as imaging of protein deposits or blood tests for neuronal injury — and of therapies that aim to slow protein aggregation or boost cellular clearance, though most such treatments remain experimental.
Some neurons in the human brain are as old as the person: they are formed before birth and must survive, without dividing, for eight or nine decades.