After death, molecular and cellular changes can be observed in the brain of someone with Alzheimer's disease. Researchers are trying to determine which changes cause the disease and which changes are a result of the disease.
In 1906, German psychiatrist and neuropathologist Alois Alzheimer discovered what have come to be the hallmark brain characteristics in Alzheimer's — amyloid plaques and neurofibrillary tangles.
The beta-amyloid protein involved with the disease is evident in several molecular forms that collect between nerve cells or neurons. Abnormal levels of this naturally occurring protein clump together to form the plaques that collect between the neurons and cause a disruption of cell functions.
The neurofibrillary tangles are abnormal accumulations of a protein that collect inside the neurons. This tau protein ordinarily binds to and stabilizes the microtubules (internal structures in healthy neurons). However, in Alzheimer's disease, abnormal chemical changes cause this tau protein to detach from the microtubules and stick to other tau molecules. These then form threads that eventually join to form tangles inside the neurons, and the tangles block the neuron's whole transport system in the brain, breaking down the synaptic communication between the neurons.
Researchers suggest that brain changes in a person with Alzheimer's are a result from a complex interchange among the abnormal tau and beta-amyloid proteins and several other factors.
The majority of information processing occurs in the part of the brain known as the cerebral cortex, and as the cortex shrivels, it damages areas of the brain involved in thinking, planning and remembering. Shrinkage is especially apparent in the hippocampus, the area of the cortex that plays a key role in formation of new memories.
Additionally, brain ventricles (fluid-filled spaces within the brain) expand as the surrounding brain tissues dies. The enlargement of these ventricles occurs during mild cognitive impairment and continues as Alzheimer's disease develops and advances.
As Alzheimer's disease progresses, neurons are injured and cell death occurs throughout the brain as nutrients and other essential supplies can no longer move through the cells. Networks of neurons may break down and various areas of the brain begin to shrink. In the final stages of Alzheimer's, this shrinkage, called brain atrophy, becomes widespread causing significant loss of brain volume.