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Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less.
In this installment I will discuss the hippocampus.
There is a hippocampus in the temporal lobe of each cerebral hemisphere.
The name “hippocampus” comes from the Greek for "seahorse" because when it is removed
from the brain, it vaguely resembles a seahorse.
Although it has many functions, the hippocampus is best known for its role in memory.
The hippocampus is part of a larger structure in the temporal lobe called the hippocampal
formation.
Definitions of what structures are included in the hippocampal formation vary, but generally
it is considered to at least include the hippocampus, the adjacent cortex which is called the hippocampal
or parahippocampal gyrus, and a strip of grey matter in between the two called the dentate
gyrus.
The hippocampal gyrus contains areas called the entorhinal cortex and subiculum, which
are both involved in the flow of information through the hippocampus.
In addition to being compared to a seahorse, the hippocampus has also been likened to the
curved horn of a ram or the horns of the ancient Egyptian god ammon and thus has been called
Ammon’s horn or cornu ammonis.
Accordingly, the hippocampus has been subdivided anatomically into 4 regions designated CA1
through CA4; the CA stands for cornu ammonis.
The hippocampus receives information from the rest of the cerebral cortex primarily
via the perforant pathway, which originates in the entorhinal cortex and projects to the
dentate gyrus.
Fibers then leave the dentate gyrus and project to neurons in the CA3 region of the hippocampus;
neurons in CA3 then send axons to neurons in the CA1 region, which projects to neurons
in the subiculum.
The subiculum can be considered the main output region of the hippocampal formation; fibers
from the subiculum project back upon neurons in the entorhinal cortex and then fibers from
the entorhinal cortex travel out to a variety of areas in the cerebrum.
Output fibers also leave the subiculum and hippocampus and enter the fornix, a fiber
bundle that connects the hippocampus with a variety of subcortical areas like the thalamus
and hypothalamus.