page_title: Hippocampus
section_title: Anatomy
hierarchical_section_title: Hippocampus / Anatomy
caption_attribution_description: English: An anatomical illustration from the 1908 edition of Sobotta's Anatomy Atlas
context_page_description: The hippocampus is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, and plays important roles in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation. The hippocampus is located under the cerebral cortex in the allocortex, and in primates it is in the medial temporal lobe. It contains two main interlocking parts: the hippocampus proper and the dentate gyrus.
  In Alzheimer's disease, the hippocampus is one of the first regions of the brain to suffer damage; short-term memory loss and disorientation are included among the early symptoms. Damage to the hippocampus can also result from oxygen starvation, encephalitis, or medial temporal lobe epilepsy. People with extensive, bilateral hippocampal damage may experience anterograde amnesia: the inability to form and retain new memories.
  Since different neuronal cell types are neatly organized into layers in the hippocampus, it has frequently been used as a model system for studying neurophysiology.
context_section_description: The hippocampus can be seen as a ridge of gray matter tissue, elevating from the floor of each lateral ventricle in the region of the inferior or temporal horn. This ridge can also be seen as an inward fold of the archicortex into the medial temporal lobe. The hippocampus can only be seen in dissections as it is concealed by the parahippocampal gyrus. The cortex thins from six layers to the three or four layers that make up the hippocampus.
  The term hippocampal formation is used to refer to the hippocampus proper and its related parts. However, there is no consensus as to what parts are included. Sometimes the hippocampus is said to include the dentate gyrus and the subiculum. Some references include the dentate gyrus and the subiculum in the hippocampal formation, and others also include the presubiculum, parasubiculum, and entorhinal cortex. The neural layout and pathways within the hippocampal formation are very similar in all mammals.
  The hippocampus, including the dentate gyrus, has the shape of a curved tube, which has been compared to a seahorse, and a ram's horn (Cornu Ammonis). Its abbreviation CA is used in naming the hippocampal subfields CA1, CA2, CA3, and CA4. It can be distinguished as an area where the cortex narrows into a single layer of densely packed pyramidal neurons, which curl into a tight U shape. One edge of the "U," – CA4, is embedded into the backward-facing, flexed dentate gyrus. The hippocampus is described as having an anterior and posterior part (in primates) or a ventral and dorsal part in other animals. Both parts are of similar composition but belong to different neural circuits. In the rat, the two hippocampi resemble a pair of bananas, joined at the stems by the commissure of fornix (also called the hippocampal commissure). In primates, the part of the hippocampus at the bottom, near the base of the temporal lobe, is much broader than the part at the top. This means that in cross-section the hippocampus can show a number of different shapes, depending on the angle and location of the cut.
  In a cross-section of the hippocampus, including the dentate gyrus, several layers will be shown. The dentate gyrus has three layers of cells (or four if the hilus is included). The layers are from the outer in - the molecular layer, the inner molecular layer, the granular layer, and the hilus. The CA3 in the hippocampus proper has the following cell layers known as strata: lacunosum-moleculare, radiatum, lucidum, pyramidal, and oriens. CA2 and CA1 also have these layers except the lucidum stratum.
  The input to the hippocampus (from varying cortical and subcortical structures) comes from the entorhinal cortex via the perforant path. The entorhinal cortex (EC) is strongly and reciprocally connected with many cortical and subcortical structures as well as with the brainstem. Different thalamic nuclei, (from the anterior and midline groups), the medial septal nucleus, the supramammillary nucleus of the hypothalamus, and the raphe nuclei and locus coeruleus of the brainstem all send axons to the EC, so that it serves as the interface between the neocortex and the other connections, and the hippocampus.
  The EC is located in the parahippocampal gyrus, a cortical region adjacent to the hippocampus. This gyrus conceals the hippocampus. The parahippocampal gyrus also includes the perirhinal cortex, which plays an important role in the visual recognition of complex objects. There is also substantial evidence that it makes a contribution to memory, which can be distinguished from the contribution of the hippocampus. It is apparent that complete amnesia occurs only when both the hippocampus and the parahippocampus are damaged.
caption: Image 2: Cross-section of cerebral hemisphere showing structure and location of hippocampus
exif: {}