Alzheimer's disease symptoms result from physical changes in the brain. What causes these changes is still somewhat of a mystery, however researchers have a leading theory of how the Alzheimer's disease progresses.
In a healthy brain, certain chemical processes ensure the proper functioning of neurons. One is the processing of amyloid precursor protein (APP) that is attached to the outer membrane of nerve cells. Certain enzymes cut off a section of the protein; while another enzyme snips a second portion and releases APP from the cell’s membrane.
These APP fragments are then broken down and removed from the brain. Another process, which we won’t get into here, carries nutrients through the nerve cells to keep them functioning normally.
When the processes that keep a healthy brain functioning go awry, a different enzyme, cuts shorter APP fragments from the nerve cell membrane. These smaller pieces are more resistant to breakdown and tend to clump together in toxic clusters and eventually they collect into plaques that interfere with nerve cell functioning. Within neurons, the transport of nutrients is crippled and nerve cells are destroyed. Loose threads join together to form knotted strands inside neurons. These cause further neuron destruction.
In the early stages of Alzheimer’s, plaques and tangles form in brain areas responsible for learning, thinking, and planning -- in particular, the hippocampus. This is why forgetfulness, disorientation, and verbal repetition are often among the earliest signs of Alzheimer’s. As nerve cell destruction spreads more brain areas are affected, especially the cerebral cortex, responsible for language, reasoning, and judgment. Speaking skills become impaired and emotional outbursts grow more frequent.
When large areas of nerve cells die off in the advanced Alzheimer’s stage, brain sections atrophy and the whole brain shrinks to as much as three quarters of its original size. People with Alzheimer’s lose most of their ability to communicate, walk, and care for themselves.