Damage to the hippocampus can also result from oxygen starvation ( anoxia), encephalitis or mesial temporal lobe epilepsy. In Alzheimer's disease, the hippocampus is one of the first regions of the brain to suffer damage memory problems and disorientation appear among the first symptoms. The name, in fact, derives from the Greek word for seahorse ( Greek: hippos = horse, campos = sea). In humans, it has a curved and convoluted shape that reminded early anatomists of a seahorse. In rodents, where it has been studied most extensively, the hippocampus is shaped something like a banana. Humans and other mammals have two hippocampi, one in each side of the brain. It belongs to the limbic system and plays major roles in long term memory and spatial navigation. The hippocampus is a part of the forebrain, located in the medial temporal lobe. For the journal of the same name, see Hippocampus (journal) Because the phase of recorded theta rhythm varies systematically through the strata, the fissure is often used as a fixed reference point for recording EEG as it is easily identifiable.The location of the hippocampus in the human brain.
The hippocampal sulcus (sulc.) or fissure is a cell-free region that separates the CA1 field from the dentate gyrus. Here the perforant path fibers form synapses onto the distal, apical dendrites of pyramidal cells. mol.) is the most superficial stratum in the hippocampus. Due to its small size, it is often grouped together with stratum moleculare into a single stratum called stratum lacunosum-moleculare (str. lac.) is a thin stratum that too contains Schaffer collateral fibers, but it also contains perforant path fibers from the superficial layers of entorhinal cortex. Some interneurons that can be found in more superficial layers can also be found here, including basket cells, bistratified cells, and radial trilaminar cells. It also contains Schaffer collateral fibers, which are the projection forward from CA3 to CA1. oriens, contains septal and commissural fibers. Mossy fibers from the dentate gyrus granule cells course through this stratum in CA3, though synapses from these fibers can be found in str. luc.) is one of the thinnest strata in the hippocampus and only found in the CA3 region. This stratum also contains the cell bodies of many interneurons, including axo-axonic cells, bistratified cells, and radial trilaminar cells. In region CA3, this stratum contains synapses from the mossy fibers that course through stratum lucidum. This stratum tends to be one of the more visible strata to the naked eye. pyr.) contains the cell bodies of the pyramidal neurons, which are the principal excitatory neurons of the hippocampus. 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. The cell bodies of inhibitory basket cells and horizontal trilaminar cells, named for their axons innervating three layers- the oriens, pyramidal, and radiatum are located in this stratum. oriens) is the next layer superficial to the alveus. The alveus is the deepest layer and contains the axons from pyramidal neurons, passing on toward the fimbria/fornix, one of the major outputs of the hippocampus. The CA regions are also structured depthwise in clearly defined strata (or layers): As shown in the figure, the structure itself is curved and subfields or regions are defined along the curve, from CA4 through CA1 (only CA3 and CA1 are labeled). In rodents, the hippocampus is positioned so that, roughly, one end is near the top of the head (the dorsal or septal end) and one end near the bottom of the head (the ventral or temporal end). The horned appearance of the hippocampus is caused by cell density differentials and the existence of varying degrees of neuronal fibers. The name cornu ammonis refers to the Egyptian deity Amun, who has the head of a ram. However, the region known as CA4 is in fact the 'deep, polymorphic layer of the dentate gyrus' (as clarified by Theodor Blackstad (1956) and by David Amaral (1978)Ĭut in cross section, the hippocampus is a C-shaped structure that resembles a ram's horns. The dentate gyrus contains the fascia dentata and the hilus, while CA is differentiated into fields CA1, CA2, CA3, and CA4. Though terminology varies among authors, the terms most frequently used are dentate gyrus and the cornu ammonis (literally "Amun's horns", abbreviated CA). The hippocampus is composed of multiple subfields.
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