Alzheimer’s disease damages the nervous system through multiple simultaneous attacks: it destroys the connections between brain cells, collapses the internal transport system that keeps neurons alive, depletes critical chemical messengers, and triggers chronic inflammation that kills healthy tissue. The damage starts in memory centers and gradually spreads across the brain, eventually affecting parts of the nervous system beyond the brain itself.
Two Proteins That Drive the Damage
The hallmark of Alzheimer’s is the buildup of two abnormal proteins in the brain. The first, amyloid beta, accumulates between neurons and forms sticky plaques. These plaques impair the membranes of surrounding nerve cells and disrupt the signaling pathways they use to communicate. Interestingly, the soluble forms of amyloid beta floating freely between cells appear to cause more harm than the hardened plaques themselves.
The second protein, tau, does its damage from inside the neuron. In a healthy brain, tau helps stabilize microtubules, the tiny structural tubes that run through each nerve cell like railroad tracks. Nutrients, mitochondria (the cell’s energy sources), and chemical signals all travel along these tracks to reach the far ends of the neuron. When tau becomes chemically altered in Alzheimer’s, it detaches from the microtubules and clumps into tangles. Without tau holding them together, the microtubules collapse. The internal transport system fails, and cargo that should flow outward toward the synapse gets stuck, shifting back toward the cell body. The neuron can no longer deliver energy or materials where they’re needed, and it eventually dies.
Synapse Loss and the Breakdown of Communication
Synapses are the tiny gaps where one neuron passes a signal to the next. They are the physical basis of every thought, memory, and sensation. In Alzheimer’s, synapses are lost early and extensively, particularly across the cortex (the brain’s outer layer responsible for reasoning and perception) and the hippocampus (the region critical for forming new memories). Brain imaging studies using tracers that bind to synaptic proteins show significantly lower synaptic density in people with Alzheimer’s compared to healthy adults, spanning regions involved in planning, spatial awareness, language, and memory.
This synapse loss correlates directly with cognitive decline. It’s not simply that neurons die; the connections between surviving neurons deteriorate first, meaning the brain loses its ability to relay information even before large numbers of cells are gone.
A Chemical Messenger Goes Missing
Alzheimer’s devastates the brain’s supply of acetylcholine, a chemical messenger essential for memory and attention. The neurons that produce acetylcholine are concentrated in a structure deep in the brain called the nucleus basalis of Meynert. These are among the first and hardest-hit cells in the disease. A healthy adult has roughly 500,000 of these neurons. In advanced Alzheimer’s, fewer than 100,000 remain.
The loss is not subtle. With acetylcholine production gutted by 80% or more, the brain’s circuits for encoding new memories and sustaining attention become profoundly impaired. This is why memory loss and difficulty concentrating are among the earliest and most recognizable symptoms of the disease.
Inflammation Turns Against the Brain
The brain has its own immune cells, called microglia, which normally act as cleanup crews. They clear debris, release protective compounds, and help repair minor damage. In Alzheimer’s, however, the constant presence of amyloid plaques and dying neurons pushes microglia into a chronically activated state. Instead of protecting the brain, they begin releasing a flood of inflammatory molecules and toxic free radicals.
This creates a destructive feedback loop. Inflammation damages nearby healthy neurons, which produces more debris, which activates more microglia, which release more inflammatory signals. Over time, this chronic neuroinflammation becomes a major driver of brain tissue loss, compounding the damage already caused by amyloid and tau.
Glutamate Overload and Calcium Flooding
Glutamate is the brain’s primary excitatory chemical messenger, responsible for most of the signaling that drives learning and neural activity. In Alzheimer’s, the system that regulates glutamate breaks down. Damaged neurons release too much of it, and the receptors on neighboring cells become overly sensitive. The result is a phenomenon called excitotoxicity: neurons are stimulated so intensely and for so long that they essentially burn out.
The killing mechanism involves calcium. When glutamate floods a synapse, it forces open channels that allow massive amounts of calcium to rush into the receiving neuron. Calcium at normal levels is a vital signaling molecule, but in excess it triggers a cascade of destructive processes inside the cell, ultimately leading to its death. This glutamate-calcium cycle is one of the reasons neuronal loss in Alzheimer’s accelerates over time.
The Hippocampus Shrinks Dramatically
As neurons and synapses are lost, the brain physically shrinks. The hippocampus, which is ground zero for Alzheimer’s pathology, shows the most dramatic volume loss. In mild stages of the disease, hippocampal volume is reduced by about 5% compared to healthy adults. By moderate stages, that figure reaches 12%. In severe Alzheimer’s, the hippocampus loses roughly a third of its volume.
Overall, Alzheimer’s brains show a 25% reduction in hippocampal volume compared to age-matched controls. This shrinkage is visible on brain scans and is one of the key markers used in diagnosis. But the damage extends well beyond the hippocampus. As the disease progresses, the cortex thins across wide areas, affecting regions responsible for language, decision-making, and spatial navigation.
The Brain’s Protective Barrier Breaks Down
The blood-brain barrier is a tightly sealed lining of cells that controls what can pass from the bloodstream into brain tissue. It keeps out toxins, pathogens, and inflammatory molecules while allowing nutrients through. In Alzheimer’s, the tight junction proteins that hold this barrier together weaken, making it more permeable. Studies in Alzheimer’s models show that this increased leakiness correlates with amyloid and tau buildup and worsens over time.
A compromised blood-brain barrier also impairs the brain’s waste-clearance system. Normally, fluid flowing through spaces around blood vessels helps flush out metabolic waste, including amyloid beta. When the barrier and its associated water-transport channels are disrupted, waste removal slows, allowing toxic proteins to accumulate faster than the brain can clear them.
Effects Beyond the Brain
Alzheimer’s is primarily a brain disease, but it doesn’t stop at the central nervous system. The autonomic nervous system, which controls unconscious functions like blood pressure regulation, bladder control, and digestion, can also be affected. People with Alzheimer’s experience higher rates of orthostatic hypotension (a sudden drop in blood pressure upon standing), urinary symptoms, and constipation compared to healthy older adults. Orthostatic symptoms tend to be worse in the morning, after meals, and during physical activity, and they may go unrecognized because the person cannot reliably report dizziness or lightheadedness.
There is also evidence linking Alzheimer’s to changes in the peripheral nervous system. Studies have found that people with the disease tend to have worse vibration detection in their feet, reduced tactile discrimination, and impaired fine motor control. Neurofibrillary tangles, the same tau-based structures found in the brain, have been identified in dorsal root ganglia, the clusters of sensory nerve cells along the spinal cord. Research following older adults over time found that sensory nerve impairments in the lower extremities were associated with up to 73% higher risk of developing dementia, and people with multiple nerve impairments had up to 2.4 times the risk compared to those with none. Whether peripheral nerve damage is an early sign of the same degenerative process or a parallel vulnerability remains an open question, but it underscores that Alzheimer’s affects the nervous system more broadly than many people realize.