Causes
Researchers are finding specific biologic factors involved with Alzheimer's disease. Various environmental and genetic players appear to contribute to or trigger the process by which these factors destroy nerve cells leading to this disease.
Biologic Factors in the Brain
Imaging techniques in patients with Alzheimer's disease have found significant loss of cells and volume in the regions of the brain devoted to memory and higher mental functioning. Important abnormalities have specifically been observed during biopsies:
- Twisted nerve cell fibers, known as neurofibrillary tangles
- A sticky protein called beta amyloid
Other factors also play a role.
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The Effects of Neurofibrillary Tangles and Beta Amyloid in Alzheimer's Disease. These biologic factors appear to be involved in the development Alzheimer's disease in the following ways:
- Neurofibrillary tangles are the damaged remains of microtubules, the support structure that allows the flow of nutrients through the neurons (nerve cells). A key component in these tangled fibers is an abnormal form of the tau protein, which in its healthy version helps in the assembly of the microtubule structure. The defective tau, however, appears to block the actions of the normal version.
- Beta Amyloid (also called A beta) is the second significant finding. This insoluble protein accumulates and forms sticky patches called neuritic plaque, which are found surrounded by the debris of dying nerve cells in the brains of Alzheimer's victims.
- Amyloid precursor protein (APP) is a large nerve-protecting protein that is the source of beta amyloid. In Alzheimer's certain enzymes, particularly those called gamma-secretases, snip APP into beta amyloid pieces. This process is controlled by factors called presenilin proteins. (Genetic abnormalities that affect either APP or presenilin proteins occur in some inherited cases of early-onset Alzheimer's.)
- High levels of beta amyloid are associated with reduced levels of the neurotransmitter acetylcholine. (Neurotransmitters are chemical messengers in the brain.) Acetylcholine is part of the cholinergic system, which is essential for memory and learning and is progressively destroyed in Alzheimer's disease.
- Beta amyloid may also disrupt channels that carry sodium, potassium, and calcium. These elements serve the brain as ions, producing electric charges that must fire regularly in order for signals to pass from one nerve cell to another. If the channels that carry ions are damaged, an imbalance can interfere with nerve function and signal transmission.
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