Dementia is not a specific disease. It’s an overall term that describes a wide range of symptoms associated with a decline in memory or other thinking skills severe enough to reduce a person’s ability to perform everyday activities. Alzheimer’s disease accounts for 60 to 80 percent of cases. Vascular dementia, which occurs after a stroke, is the second most common dementia type. But there are many other conditions that can cause symptoms of dementia, including some that are reversible, such as thyroid problems and vitamin deficiencies.
Dementia is often incorrectly referred to as “senility” or “senile dementia,” which reflects the formerly widespread but incorrect belief that serious mental decline is a normal part of aging.
Alzheimer’s disease is a progressive brain disorder that damages and eventually destroys brain cells, leading to memory loss and changes in thinking and other brain functions. It usually develops slowly and gradually gets worse as more brain cells wither and die. Ultimately, Alzheimer’s is fatal, and currently, there is no cure.
Scientists have identified several hallmark Alzheimer’s brain abnormalities, including:
- Plaques, microscopic clumps of a protein called beta-amyloid peptide
- Tangles, twisted microscopic strands of the protein tau (rhymes with “wow”)
- Loss of connections among brain cells responsible for memory, learning and communication. These connections, or synapses, transmit information from cell to cell.
- Inflammation resulting from the brain’s effort to fend off the lethal effects of the other changes under way
- Eventual death of brain cells and severe tissue shrinkage
All these processes have a devastating impact on the brain, and over time, the brain shrinks dramatically, affecting nearly all its functions.
The real work of your brain goes on in individual cells. An adult brain contains about 100 billion nerve cells, or neurons, with branchesthat connect at more than 100 trillion points. Scientists call this dense, branching network a “neuron forest.”
Signals traveling through the neuron forest form the basis of memories, thoughts, and feelings.
Neurons are the chief type of cell destroyed by Alzheimer’s disease.
Signals that form memories and thoughts move through an individual nerve cell as a tiny electrical charge.
Nerve cells connect to one another at synapses. When a charge reaches a synapse, it may trigger release of tiny bursts of chemicals called neurotransmitters. The neurotransmitters travel across the synapse, carrying signals to other cells. Scientists have identified dozens of neurotransmitters.
Alzheimer’s disease disrupts both the way electrical charges travel within cells and the activity of neurotransmitters.
Alzheimer’s disease leads to nerve cell death and tissue loss throughout the brain. Over time, the brain shrinks dramatically, affecting nearly all its functions.
Scientists can also see the terrible effects of Alzheimer’s disease when they look at brain tissue under the microscope:
- Alzheimer’s tissue has many fewer nerve cells and synapses than a healthy brain.
- Plaques, abnormal clusters of protein fragments, build up between nerve cells.
- Dead and dying nerve cells contain tangles, which are made up of twisted strands of another protein.
Scientists are not absolutely sure what causes cell death and tissue loss in the Alzheimer’s brain, but plaques and tangles are prime suspects.
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