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Hippocampus anatomy

Hippocampus anatomy describes the physical aspects and properties of the hippocampus, a neural structure in the medial temporal lobe of the brain. It has a distinctive, curved shape that has been likened to the sea-horse monster of Greek mythology and the ram's horns of Amun in Egyptian mythology. This general layout holds across the full range of mammalian species, from hedgehog to human, although the details vary. For example, in the rat, the two hippocampi look similar to a pair of bananas, joined at the stems. In primate brains, including humans, the portion of the hippocampus near the base of the temporal lobe is much broader than the part at the top. Due to the three-dimensional curvature of this structure, two-dimensional sections such as shown are commonly seen. Neuroimaging pictures can show a number of different shapes, depending on the angle and location of the cut.

"Alveus" redirects here. For the nonprofit, see Alveus Sanctuary.

Topologically, the surface of a cerebral hemisphere can be regarded as a sphere with an indentation where it attaches to the midbrain. The structures that line the edge of the hole collectively make up the so-called limbic system (Latin limbus = border), with the hippocampus lining the posterior edge of this hole. These limbic structures include the hippocampus, cingulate cortex, olfactory cortex, and amygdala. Paul MacLean once suggested, as part of his triune brain theory, that the limbic structures constitute the neural basis of emotion. While most neuroscientists no longer believe in the concept of a unified "limbic system", these regions are highly interconnected and do interact with one another.

Major fiber systems in the rat[edit]

Angular bundle[edit]

These fibers start from the ventral part of entorhinal cortex (EC) and contain commissural (EC◀▶Hippocampus) and Perforant path (excitatory EC▶CA1, and inhibitory EC◀▶CA2[8]) fibers. They travel along the septotemporal axis of the hippocampus. Perforant path fibers, as the name suggests, perforate subiculum before going to the hippocampus (CA fields) and dentate gyrus.[9]

Stratum oriens (str. oriens) is the next layer superficial to the alveus. The cell bodies of inhibitory and horizontal trilaminar cells, named for their axons innervating three layers—the oriens, Pyramidal, and radiatum are located in this stratum. The basal dendrites of Pyramidal neurons are also found here, where they receive input from other Pyramidal cells, septal fibers and commissural fibers from the contralateral hippocampus (usually recurrent connections, especially in CA3 and CA2.) In rodents the two hippocampi are highly connected, but in primates this commissural connection is much sparser.

basket cells

Stratum pyramidale (str. pyr.) contains the cell bodies of the Pyramidal neurons, which are the principal excitatory neurons of the hippocampus. This stratum tends to be one of the more visible strata to the naked eye. In region CA3, this stratum contains synapses from the mossy fibers that course through stratum lucidum. This stratum also contains the cell bodies of many , including axo-axonic cells, bistratified cells, and radial trilaminar cells.

interneurons

Stratum lucidum (str. luc.) is one of the thinnest strata in the hippocampus and only found in the CA3 region. Mossy fibers from the dentate gyrus course through this stratum in CA3, though synapses from these fibers can be found in str. pyr.

granule cells

Stratum radiatum (str. rad.), like str. oriens, contains septal and commissural fibers. It also contains fibers, which are the projection forward from CA3 to CA1. Some interneurons that can be found in more superficial layers can also be found here, including basket cells, bistratified cells, and radial trilaminar cells.

Schaffer collateral

Stratum lacunosum (str. lac.) is a thin stratum that too contains Schaffer collateral fibers, but it also contains fibers from the superficial layers of entorhinal cortex. Due to its small size, it is often grouped together with stratum moleculare into a single stratum called stratum lacunosum-moleculare (str. l-m.).

perforant path

Stratum moleculare (str. mol.) is the most superficial stratum in the hippocampus. Here the perforant path fibers form synapses onto the distal, apical dendrites of Pyramidal cells.

(sulc.) or fissure is a cell-free region that separates the CA1 field from the dentate gyrus. Because the phase of recorded theta rhythm varies systematically through the strata, the sulcus is often used as a fixed reference point for recording EEG as it is easily identifiable.[33]

Hippocampal sulcus

Schematic Diagram of a Hippocampal Brain Slice

at the U.S. National Library of Medicine Medical Subject Headings (MeSH)

Hippocampus

at the BrainMaps project

Stained brain slice images which include the "hippocampus"

Hippocampus anatomy and connectivity