Motor neurone disease (MND) has no single cause. In most people, the disease results from a combination of genetic vulnerability and environmental exposures that together trigger the progressive death of motor neurons, the nerve cells that control voluntary movement. About 10% of cases run in families, but a 2024 analysis from King’s College London found that 25% of all patients, regardless of family history, carry a genetic change related to their disease. For the remaining cases, the triggers are still not fully understood.
Genetics Play a Larger Role Than Expected
MND has traditionally been split into two categories: familial (inherited) and sporadic (no known family connection). Around 90% of diagnoses fall into the sporadic category. But that label is somewhat misleading. The King’s College London study, the largest rare-variant analysis of MND to date, established that one in four patients carries a disease-related genetic change, even when no relative has been diagnosed. This suggests that genetics contribute to MND far more broadly than the simple familial/sporadic divide implies.
The most common genetic culprit is a mutation in a gene called C9orf72. In healthy people, a short DNA sequence in this gene repeats a handful of times. In people with the mutation, that sequence repeats hundreds or even thousands of times. This expansion causes the gene to produce abnormal RNA molecules and toxic proteins that accumulate in the brain and spinal cord. The mutation also reduces the gene’s normal protective function, creating a double hit of gained toxicity and lost protection.
A Rogue Protein Found in Nearly All Cases
Regardless of genetic background, 97% of MND patients share the same hallmark in their affected tissues: clumps of a protein called TDP-43 in places it shouldn’t be. TDP-43 normally works inside the nucleus of a cell, helping manage RNA. In MND, it gets displaced into the surrounding fluid of the cell (the cytoplasm), where it misfolds, fragments, and accumulates into insoluble clumps. These aggregates are toxic to motor neurons and are also seen in a related condition, frontotemporal dementia.
What triggers this mislocalization varies. Certain gene mutations directly cause it. Environmental stressors can promote it too. And as you’ll see below, excess calcium flooding into a neuron activates an enzyme called calpain, which chops TDP-43 into fragments that are especially prone to clumping. This means several of the disease’s pathways converge on TDP-43, making it a central player in the destruction of motor neurons.
How Motor Neurons Become Overstimulated
Glutamate is the brain’s main excitatory chemical messenger. After it delivers a signal between neurons, surrounding cells called astrocytes normally mop it up. In MND, this cleanup system breaks down. Excess glutamate lingers in the gap between neurons and keeps firing signals, forcing motor neurons to absorb more and more calcium.
Motor neurons are uniquely vulnerable to this calcium overload because they produce very low levels of calcium-buffering proteins. They simply lack the machinery to handle the surge. The rising calcium triggers a destructive chain reaction: it damages mitochondria (the cell’s energy factories), which then churn out reactive oxygen species, a form of molecular damage commonly called oxidative stress. Those reactive molecules leak out and further disable the glutamate cleanup transporters on nearby astrocytes, which allows even more glutamate to accumulate. The result is a self-reinforcing loop where each round of damage makes the next round worse.
At the same time, the excess calcium activates calpain, the enzyme that fragments TDP-43. The fragments migrate to the cell’s protein-processing center (the endoplasmic reticulum), disrupting its function and releasing yet more calcium into the cell. This second feedback loop ties glutamate toxicity directly to the protein clumping seen in nearly all MND patients.
Mitochondrial Damage and Energy Failure
Mitochondria sit at the crossroads of several MND pathways. They depend on intact membranes to function, and they carry their own DNA and RNA that are especially susceptible to oxidative damage. When calcium floods in and reactive oxygen species build up, mitochondria lose their ability to produce energy efficiently. Calpain also degrades a protein essential for mitochondrial repair, preventing damaged mitochondria from fusing with healthy ones to restore function.
Because motor neurons are among the largest and most energy-hungry cells in the body, with axons stretching up to a meter long, even a modest drop in energy production has outsized consequences. The cell can no longer maintain its electrical signaling, transport proteins along its length, or clear out toxic waste. This energy crisis accelerates degeneration in a way that smaller, less metabolically demanding neurons can better withstand.
Inflammation Speeds the Damage
As motor neurons begin dying, they release internal contents like DNA, RNA, and proteins into the surrounding tissue. These act as danger signals that activate the brain’s immune-like cells, particularly astrocytes. Once activated, astrocytes release a flood of inflammatory molecules, including one called TNF-alpha that can directly trigger cell death through programmed self-destruction pathways. They also produce additional reactive oxygen species, compounding the oxidative stress already underway.
This inflammatory response is intended to clean up damage, but in MND it becomes part of the problem. The inflammation injures neighboring neurons that were still functional, expanding the zone of degeneration beyond the cells initially affected. It creates another vicious cycle: more neuron death triggers more inflammation, which causes more neuron death.
Who Is Most at Risk
MND incidence rises with age and peaks between 70 and 80 years old, then declines. Men are diagnosed more often than women across all age groups, though the gap is largest in younger adults: between ages 20 and 49, the male-to-female ratio is roughly 2.3 to 1. Between 50 and 84, it narrows to about 1.4 to 1.
Occupational and environmental exposures also appear to matter. Lead exposure has been linked to increased risk in case-control studies, particularly through occupational contact rather than residential or lifestyle sources. Pesticides, mercury, and industrial solvents have been investigated as well, though the evidence is strongest for lead.
Professional Athletes
Professional contact-sport athletes face a notably elevated risk. American football players have roughly four times the expected rate of MND diagnosis compared to the general population. Among professional soccer players, studies have produced varying results, but the most striking found a hazard ratio of 4.33, meaning former players were more than four times as likely to be diagnosed. Whether the mechanism involves repeated head trauma, extreme physical exertion, or some combination remains an open question.
Military Veterans
Military service is an established risk factor. The ALS Association notes that veterans face a greater risk of both diagnosis and death from the disease regardless of which branch they served in, which war or era they served during, and whether they saw combat. The specific reason is unknown, but exposures to environmental toxins, physical trauma, and extreme exertion during service are all hypothesized contributors. In the United States, the Department of Veterans Affairs recognizes ALS as a service-connected disease for all veterans with 90 or more days of active duty.
Why No Single Cause Has Been Found
MND is best understood as a disease of converging insults rather than a single broken mechanism. Genetic susceptibility lowers the threshold. Environmental exposures, whether chemical, physical, or occupational, push cells closer to failure. Once a tipping point is reached, the cellular processes described above, including glutamate toxicity, calcium overload, mitochondrial damage, protein misfolding, and inflammation, feed into each other through multiple self-reinforcing loops. This interconnected web of pathology explains why the disease progresses relentlessly once it begins and why blocking any single pathway has so far been insufficient to stop it.