A Shrinking Hippocampus & Alzheimer's

Alzheimer's disease is a disorder of progressive damage that can affect many regions of the brain. Among the symptoms of Alzheimer's disease, memory problems are linked to damage of the hippocampus, whose name comes from the Latin for seahorse, as structurally it resembles this creature with a curvy tail.

Hippocampus Definition

The main role of the hippocampus is in memories of events you have personally experienced, known as episodic memory. It is also crucial for spatial memory, that is, for knowing where you are geographically.
The hippocampus is divided into sections named the subiculum and the cornu ammonis sectors, or CA1 to CA4, which are mainly composed of single layers of brain cells--called neurons--five to eight cells across. Information from other brain regions comes to the hippocampus via an area called the dentate gyrus; it then travels down the CA4 to CA1 areas and into another region greatly affected in Alzheimer's disease called the entorhinal cortex, according to "The Human Brain Coloring Book."

The Hippocampus in Alzheimer's Disease

The shrinking of the hippocampus in Alzheimer's disease is distinctive of this disorder compared to normal aging. In the very early stages, there is a reduction in hippocampal volume of 10 to 12 percent, which goes up to 20 to 30 percent in mild Alzheimer's disease, and 30 to 40 percent in later stages.
In normal brains the rate of hippocampal shrinking is between 0.24 to 1.73 percent per year. In people with Alzheimer's disease it is between 2.2 and 5.9 percent, notes the book "European Radiology."

Hippocampal Shrinkage

The reduction in hippocampal volume is due to both complete cell loss and cell damage. The damage is principally to regions at the end of a neuron, or synapses, which house mechanisms for signaling between neurons.
Stephen Scheff and colleagues showed in the May 2007 edition of "Neurology" that even people with mild Alzheimer's disease had 55 percent fewer synapses in the CA1 region compared to normal controls. Neuronal loss in this region has been shown in many studies, including one by Jillian J. Kril and colleagues in the April 2002 edition of "Acta Neuropathologica," showing cell loss linked to the disease stage.

Cell Damage Causes

It is not clear what causes neuronal damage and loss in the hippocampus. In "Acta Neuropathologica," Kril and others found that one hallmark of Alzheimer's disease is called neurofibrillary tangles, or NFTs. But she did not see a direct correlation between the number of NFTs and neuronal loss.
Kril proposed that while at least some of the damage was due to these structures, further damage may be linked to another molecule found in Alzheimer's disease--beta-amyloid--along with chemicals released as part of the immune system inflammation process.