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
|WHEN THE HIPPOCAMPAL
CIRCUITS ARE DISTURBED
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
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
plaque composed of degenerated neurons
Source: University of Wisconsin Medical School