The hippocampus is the cortical area concerned with formation and recall of memories; it also has a role in emotional behaviour.
The adjacent entorhinal (parahippocampal) cortex and subiculum communicate with the hippocampus and the closely associated dentate gyrus. It has a role to play is spatial navigation.
The hippocampus and dentate gyrus are shaped like interlocking 'C's and, unlike most areas of cerebral cortex, both have only three layers.
Axons from the superficial layers of the entorhinal cortex project through a three-neurone circuit that returns information from the hippocampus back to the deeper layers of the entorhinal cortex. The cells in this 3-neurone circuit are
granule cells in the dentate gyrus, whose axons, 'mossy fibres', project to
pyramidal cells in area CA3 of Ammon's Horn (Cornus Ammonis), that connect with
pyramidal cells in area CA1, which send signals back to the deeper layers of the entorhinal cortex.
The pyramidal cells of CA1 have an ability to alter synaptic efficacy by trafficking more receptors into the post-synaptic membrane during repetitive activity, a process called long term potentiation (LTP), believed to be associated with short term memory.
The subgranular layer of neurones in the dentate gyrus is the site of production of new neurones in adults; it has been suggested that adult neurogenesis may be involved in the laying down of long term memories.
The limbic system: Connections between the Cingulate Cortex, Hippocampus and the Hypothalamus.
Connections of the Hippocampus
The hippocampus is a specialised area of cerebral cortex within the temporal lobe that is unlike other cortical areas in that it has three layers of cells rather than six.
The dentate gyrus and entorhinal cortex are adjacent to the hippocampus, and are folded together when seen in cross section. This formation is sometimes called Ammon's horn or cornu ammonis (CA), which is the origin of the term CA3, used later.
Electrical stimulation of the hippocampus in conscious humans causes the recall of memories, while bilateral lesions result in an inability to convert short term memories into long term ones.
The entorhinal (parahippocampal) cortex is closely connected with the cingulate cortex via the cingulum.
The entorhinal cortex is an important input to the hippocampus; but it also receives the output of the hippocampus, via the subiculum. The latter links the hippocampus to the entorhinal cortex, and its three layers gradually merge into the six layers found in the entorhinal cortex.
The fornix arises in the mamillary bodies of the hypothalamus and curves into the temporal lobe and connects with the hippocampus. This pathway allows emotions to be accompanied by the autonomic and other changes associaated with emotion.
Another adjacent structure conneccted to the hippocampus is the Amygdala; electrical stimulation of the amygdala gives rise to emotional behaviours. The amygdala is connected to the anterior hypothalamus by a band of axons called the striae terminalis.
Internal Structure of the Hippocampus
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Neuronal Circuits in the Temporal Lobe
Unlike much of the cerebral cortex, the hippocampal cortex has only three layers and these interlock with another three-layered structure - the dentate gyrus.
In cross section, the hippocampus and dentate gyrus are shaped like two interlocking C's. The inner surface of the dentate gyrus that interlocks with the hippocampus is called the subgranular zone (SGZ).
The structure of the hippocampus can be reduced to a three neurone circuit including pyramidal neurones that have a high capacity for long term potentiation, the basis of synaptic short term memory; and granule cells in the subgranular zone, where new neurones can be formed - a rare event in the central nervous system. Adult neurogenesis may play a role in laying down long term memories.
The entorhinal cortex projects strongly to the granule cells of the dentate gyrus, and these project to the pyramidal neurones of area CA3 of the hippocampus. These connect with pyramidal neurones in area CA1. (CA stands for cornu ammonis)
The basis of short term memory is believed to involve changes in synaptic efficacy in these pyramidal cells, called long term potentiation (LTP); in other words memories of recent events may occur because of changes in the size of the post-synaptic repsonse to repeated inputs. In LTP additional post-synaptic receptors are expressed in the post-synaptic memebrane of the pyramidal cells.
The Hippocampus and Dentate Gyrus, showing the Sub-Granular Zone, where neural stem cells develop.
Adult Neurogenesis and Long Term Memory
Unlike much of the cerebral cortex, the hippocampal cortex has only three layers and these interlock with a similar three-layered structure in the dentate gyrus.
In cross section, the hippocampus and dentate gyrus are shaped like two interlocking C's. The inner surface of the dentate gyrus that interlocks with the hippocampus is called the subgranular zone (SGZ), a site where new neurones are generated in adults.
Long term memories depend on LTP in circuits of neurones that have been stimulated sufficiently often to induce new protein production that maintains the strength of the synapse.
New neurone formation in the subgranular zone is interesting and has been linked to the ofrmation of long term memories; however for the moment this is largely hypothesis.