Alzheimer’s disease strikes the hippocampus earlier and harder than almost any other brain region, shrinking it by roughly 22% compared to healthy brains. This small, curved structure deep in the temporal lobe is the brain’s primary hub for forming new memories and navigating space, which is why memory loss and disorientation are the hallmark early symptoms of the disease. The damage unfolds through a specific sequence of protein buildup, synapse destruction, and neuron death that progressively dismantles the hippocampus from the inside out.
Where the Damage Starts
The hippocampus is not a single uniform structure. It contains distinct subregions, and Alzheimer’s does not attack them all equally. The entorhinal cortex, a gateway that funnels information into the hippocampus, is the first area hit. From there, the disease spreads to two connected zones called CA1 and the subiculum, which show the most severe volume loss of any hippocampal subfield: 23% and 24% respectively. These areas carry the highest burden of tangled tau protein and lose the most tissue on brain scans.
This pattern follows what’s known as Braak staging, a system that maps how tau tangles spread through the brain over years. Tau begins accumulating in the medial temporal lobe, including the hippocampus, as early as a person’s fifties or sixties. In many people this happens as a normal part of aging. What distinguishes Alzheimer’s is the interaction between tau and amyloid plaques, which accelerates the destruction. In the most common subtype, amyloid plaques spread across the outer brain first, then trigger a catastrophic worsening of tau pathology in the hippocampus and entorhinal cortex. In a less common subtype, mild tau appears in the hippocampus and nearby regions before amyloid arrives, but the destructive interaction between the two proteins still occurs.
How Synapses Are Destroyed
Synapse loss is the strongest predictor of cognitive decline in Alzheimer’s, more closely linked to symptoms than either plaques or tangles alone. Synapses are the tiny connection points between neurons where signals pass from one cell to the next, and the hippocampus depends on dense networks of them to encode and retrieve memories.
Amyloid pathology destroys these connections through a cascade that begins with calcium. Neurons normally keep calcium levels tightly controlled within each synaptic spine, the small protrusion where a connection forms. Under amyloid pathology, this compartmentalization breaks down. Calcium floods freely between spines and the main branches of the neuron, and the spines closest to amyloid plaques show the highest calcium levels. Within about 24 hours of this calcium disruption, affected spines begin to shrink and disappear.
Tau pathology contributes its own form of synapse destruction through a different mechanism. Rather than flooding neurons with calcium, abnormal tau reduces the formation of new synaptic spines without equally reducing the rate at which old ones are eliminated. The result is a steady net loss. Healthy neurons constantly form and prune connections in a balanced cycle. In the presence of pathological tau, the balance tips toward elimination.
The brain’s own immune cells, called microglia, make things worse. During normal development, microglia prune excess synapses using a tagging system based on complement proteins. In Alzheimer’s, this same pruning pathway reactivates and begins mistakenly devouring healthy synapses. This misdirected cleanup starts before plaques are even visible, suggesting the immune response goes haywire very early in the disease process. Under amyloid pathology, neurons keep trying to form new connections, but those connections fail to stabilize, creating a futile cycle of synapse formation and loss.
New Neuron Production Shuts Down
The hippocampus is one of the only brain regions where new neurons are born throughout adult life. This process, called neurogenesis, happens in a specific zone within the dentate gyrus, one of the hippocampal subregions. These newly born neurons are excitatory cells that integrate into existing memory circuits and are thought to help with learning and pattern separation, the ability to distinguish between similar memories.
Alzheimer’s dramatically impairs this process. A study of 45 patients across a range of disease stages found that the number of immature neurons was reduced at every stage compared to healthy aging, with deterioration visible across the entire developmental pipeline from stem cell to mature neuron. Strikingly, this decline appeared even in people at early disease stages with low levels of plaques and tangles, suggesting that neurogenesis falters before widespread damage is obvious.
Multiple mechanisms drive this shutdown. Phosphorylated tau building up in nearby inhibitory neurons disrupts the signals that activate neural stem cells. Increased inflammation from overactive microglia suppresses both stem cell multiplication and the maturation of new neurons. Fat droplets accumulate and block stem cell proliferation. And the chemical signaling environment shifts: the balance between excitatory and inhibitory input that new neurons need to wire into circuits becomes disrupted, preventing successful integration. Importantly, tau alone can impair neurogenesis even without amyloid present, which may explain why the decline begins so early.
Acetylcholine Loss in the Hippocampus
The hippocampus is densely packed with receptors for acetylcholine, a chemical messenger essential for learning and memory. Alzheimer’s causes severe degeneration of the neurons that produce acetylcholine, leading to a sharp drop in its availability. The enzyme that manufactures acetylcholine becomes significantly less active, while the enzyme that breaks it down becomes more active, creating a double hit that depletes the hippocampus of this critical signal.
Amyloid plaques directly worsen this cholinergic dysfunction. In animal studies, amyloid deposits in the hippocampus reduce both acetylcholine production and the activity of its manufacturing enzyme, while simultaneously increasing cell death in the region. This chemical deficit reduces the hippocampus’s ability to strengthen connections between neurons, a process called synaptic plasticity that underlies memory formation. The loss of cholinergic signaling also further suppresses neurogenesis in the dentate gyrus, compounding the damage.
Memory Loss and What It Feels Like
The hippocampus is essential for encoding new episodic memories: the richly detailed recollections of personal experiences tied to a specific time and place. This was first demonstrated dramatically through patient Henry Molaison, who lost the ability to form new memories after surgical removal of his hippocampal structures while retaining older memories and general knowledge. Alzheimer’s produces a similar pattern. The earliest cognitive symptom is typically difficulty forming new episodic memories, which is why a person might forget a recent conversation but vividly recall events from decades ago.
Different parts of the hippocampus contribute to different aspects of memory and cognition. The posterior (back) portion is primarily involved in spatial memory and navigation. The anterior (front) portion connects strongly to the amygdala and hypothalamus and plays a larger role in the emotional dimensions of memory. Because Alzheimer’s damages both regions, patients lose not only factual recall but also the emotional context that makes memories feel personal and meaningful.
Why People Get Lost
Spatial disorientation is one of the earliest functional signs of Alzheimer’s, sometimes appearing before a formal diagnosis. The hippocampus supports allocentric navigation, the ability to build a mental map of your environment and orient yourself within it regardless of which direction you’re facing. This is distinct from the simpler route-based navigation (turn left at the store, then right at the light) that relies on other brain regions.
The right hippocampus plays a particularly important role. Research in people with cognitive impairment found that smaller right hippocampal volume predicted poorer navigation performance in both real-world and virtual environments, independent of overall brain shrinkage, age, sex, or education. The relationship was direct: less right hippocampal tissue meant worse ability to navigate using a mental map. This explains why people in early Alzheimer’s may get lost in once-familiar places, struggle to find their car in a parking lot, or have difficulty orienting themselves in new environments well before other cognitive symptoms become obvious.