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The Papez Circuit
The Papez circuit: Overview
Papez Circuit Physiology
- It is the fundamental extra-hippocampal circuitry, which Papez introduced in 1937.
- Originally believed to play a fundamental role in emotional processing/regulation but is now understood to be the cornerstone of memory processing/consolidation, instead.
Steps
The entorhinal cortex -> hippocampus -> mammillary nuclei -> anterior thalamic nuclei -> cingulate gyrus -> back to the entorhinal cortex*
The perforant pathway: Overview
Perforant Pathway
- The key intra-hippocampal circuitry (so-named because the entorhinal cortex projects through (perforates) the subiculum).
- Whereas most cortical projections are bidirectional, the perforant pathway follows a specific unidirectional progression.
Steps
- Hippocampus: The dentate gyrus -> CA3 -> CA1 (via Schaffer collaterals) -> subiculum
The Papez circuit: Anatomy
Key structures: the parahippocampal gyrus, the entorhinal cortex, the hippocampus, the anterior thalamic nucleus, mammillary nucleus, and the cingulate gyrus and specify the neocortex (external to it).
- The entorhinal cortex projects to the hippocampus.
- The hippocampus projects via the fornix to the mammillary nuclei.
- The mammillary nuclei project to the anterior thalamic nuclei via the mammillothalamic fasciculus (aka the Vicq d'Azyr bundle).
- The anterior thalamic nuclei project to the cingulate gyrus.
- The cingulate gyrus projects back to the entorhinal cortex via the cingulum to close the Papez circuit loop.
The fornix divides into:
- Crus — vertical ascent.
- Body — anterior projection underneath the corpus callosum.
- Column — descent.
Note that the fornix descends both anterior and posterior to the anterior commissure.
Key extra-hippocampal projections.
- The cingulate gyrus has reciprocal connections with the neocortex.
- Neuronal input reaches the hippocampus through neocortical, limbic, diencephalic, and brainstem neurobehavioral areas.
- Neocortical areas project to the posterior parahippocampal gyrus, which then projects to the perirhinal cortex, and subsequently to the entorhinal cortex.
Note that although not shown as such, here, many areas skip the posterior parahippocampal gyrus and directly synapse in the perirhinal or entorhinal cortices or even directly in the hippocampus, itself.
The perforant pathway: Anatomy
Key anatomy: Hippocampus, the dentate gyrus, cornu ammonis, subiculum, entorhinal cortex, alveus, and fimbria (which becomes the crus of the fornix), the cornu ammonis - CA 4, CA 3, CA 2, and CA1.
- The entorhinal cortex projects through the subiculum (it perforates it) to synapse in the dentate gyrus.
- The dentate gyrus projects to CA3, which projects to CA1 via Schaffer collaterals.
- CA1 then projects to the subiculum, which projects along the alveus to the fimbria, which passes posteriorly and becomes the crus of the fornix.
- The fornix ultimately projects, most notably, to the mammillary nuclei (as shown in the Papez circuit portion of the diagram).
- The subiculum also projects back to the entorhinal cortex.
- CA3 and CA1 also send direct projections to the fimbria that skip the intervening steps in this pathway, and also note that many other intrahippocampal projections also exist, including subiculum projections to the other components of the subicular complex (ie, the presubiculum and parasubiculum).
Because of the anatomic relationship between the entorhinal cortex, dentate gyrus, cornu ammonis, and subiculum, these structures are often collectively referred to as the hippocampal formation. This bit of nomenclature can be a helpful simplification of this anatomical region.