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Memory and the brain
How Memory Works

People who have received electroshock therapy or a powerful blow to the head generally have little memory of any events immediately before or after the treatment or accident. Both such events seem to temporarily interrupt the normal flow of nerve impulses in the circuits of the hippocampus, making them unable to store new information and “erasing” the markers associated with information that has been acquired recently and still depends on the hippocampus.


Certain encephalopathies due to anoxias, ischemias, hypoglycemias, carbon monoxide poisoning, or prolonged epileptic attacks can cause the loss of large numbers of neurons in both hippocampi.

The pyramidal neurons of hippocampal area CA1, as well as the cortical neurons of layers 3, 5 and 6, the Purkinje cells, and the striate neurons are especially sensitive to lack of oxygen and energy.

Since these neurons are involved in various memory systems, malfunctions in their circuits inevitably lead to memory problems.

Thus, damage to the temporal lobes of the cortex can cause severe, permanent anterograde amnesia, as well as retrograde amnesia extending back from three to ten years before the accident.

When selective neuronal losses occur in area CA1 of the hippocampus, the resulting anterograde amnesia is just as severe, but the resulting retrograde amnesia generally remains slight (extending only one to two years before the accident).

Lesions in the diencephalon (Korsakoff’s syndrome) cause the same type of symptoms, because all of these regions are interconnected via the circuits of the limbic system.

In Alzheimer’s disease, beta-amyloid, an insoluble toxic substance, forms clumps known as “senile plaques” around the neurons. These plaques release free radicals that strip atoms from organic molecules that are vital to the neurons, including molecules in their cell membranes. Holes thus develop in these membranes, allowing large amounts of undesirable substances to enter and kill the neurons. The memory circuits that depend on them, especially those in the hippocampus, are thus irremediably compromised.  

Senile plaque composed of degenerated neurons
Source: University of Wisconsin Medical School

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