Hippocampus
The hippocampus, also hippocampus proper, is a major component of the brain of humans and many other vertebrates. In the human brain the hippocampus, the dentate gyrus, and the subiculum are components of the hippocampal formation located in the limbic system.
The hippocampus plays important roles in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation. In humans and other primates the hippocampus is located in the archicortex, one of the three regions of allocortex, in each hemisphere with direct neural projections to, and reciprocal indirect projections from the neocortex. The hippocampus, as the medial pallium, is a structure found in all vertebrates.
In Alzheimer's disease, the hippocampus is one of the first regions of the brain to be damaged; 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. The form of neural plasticity known as long-term potentiation was initially discovered to occur in the hippocampus and has often been studied in this structure. LTP is widely believed to be one of the main neural mechanisms by which memories are stored in the brain.
Using rodents as model organisms, the hippocampus has been studied extensively as part of a brain system responsible for spatial memory and navigation. Many neurons in the rat and mouse hippocampi respond as place cells: that is, they fire bursts of action potentials when the animal passes through a specific part of its environment. Hippocampal place cells interact extensively with head direction cells, whose activity acts as an inertial compass, and conjecturally with grid cells in the neighboring entorhinal cortex.
Name
The earliest description of the ridge running along the floor of the inferior horn of the lateral ventricle comes from the Venetian anatomist Julius Caesar Aranzi, who likened it first to a silkworm and then to a seahorse from ἵππος, 'horse' + κάμπος. The German anatomist Johann Georg Duvernoy, the first to illustrate the structure, also wavered between "seahorse" and "silkworm". "Ram's horn" was proposed by the Danish anatomist Jacob Winsløw in 1732; and a decade later his fellow Parisian, the surgeon de Garengeot, used cornu Ammonis – horn of Amun, after the ancient Egyptian god who was often represented as having a ram's head. Ammon is the Greek name for Amun.The head region of the hippocampus is enlarged, and presents two or three rounded elevations or foot-like digitations, and hence it was named the pes hippocampi. Later this part was described as pes hippocampi major, with an adjacent bulge in the occipital horn of the lateral ventricle, described as pes hippocampi minor later renamed as the calcar avis. In 1786 Félix Vicq-d'Azyr published an authoritative description naming just the hippocampus but the term remained largely unused with no description of any function proposed until in the middle of the 20th century it was associated with memory.
Mayer mistakenly used the term hippopotamus in 1779, and was followed by some other authors until Karl Friedrich Burdach resolved this error in 1829. In 1861 the hippocampus minor became the center of a dispute over human evolution between Thomas Henry Huxley and Richard Owen, satirized as the Great Hippocampus Question. The term hippocampus minor fell from use in anatomy textbooks and was officially removed in the Nomina Anatomica of 1895. The modern term for the structure is just the hippocampus, with the term cornu Ammonis surviving in the names of the hippocampal subfields CA1–CA4.
In the limbic system
The hippocampus is one of the structures of the limbic lobe, first described by Broca in 1878, as the cortical areas that line the deep edge of the cerebral cortex. The limbic lobe is the main component of the limbic system. The cingulate gyrus, and the parahippocampal gyrus are the two main parts of the described lobe, which had been largely associated with olfaction. Many studies later culminating in work by Papez, and MacLean, the involvement of other interacting brain regions associated with emotion was recognized. The hippocampus is anatomically connected to parts of the brain that are involved with emotional behavior, including the septal area, the hypothalamic mammillary bodies, and the anterior nuclear complex in the thalamus. MacLean proposed that the associated structures of the limbic lobe be included in what he termed as the limbic system.Anatomy
The hippocampus is a five centimeter long ridge of gray matter tissue within the parahippocampal gyrus that can only be seen when the gyrus is opened. The hippocampus is an inward fold of three-layered archicortex into the medial temporal lobe of the brain, where it elevates into the floor of each lateral ventricle inferior horn. The hippocampus stretches along its anterior-posterior axis, from the amygdala to the splenium of the corpus callosum, with the head, body, and tail as subregions of this axis. The dentate gyrus, CA subfields, fimbria, and subiculum are divisions across the short axis, the proximal-distal axis.The hippocampal formation refers to the hippocampus, and its related adjoining parts to include the dentate gyrus, the subiculum, the presubiculum, parasubiculum, and the entorhinal cortex. Sometimes the subiculum, presubiculum, and parasubiculum are grouped together as the subicular complex, but the regions are neuroanatomically distinct. Some sources may only include the hippocampus, dentate gyrus, and subiculum, being regions of the hippocampal three-layered archicortex. But the six regions are linked together serially by almost unidirectional neural pathways. Other sources include the indusium griseum, gyrus fasciolaris, the medial and longitudinal striae, and uncus, and exclude subicular regions. The neural layout and pathways within the hippocampal formation are very similar in all mammals.
The hippocampus has a generally similar appearance across the range of mammals, from egg-laying mammals such as the echidna, to humans and other primates. The hippocampal-size-to-body-size ratio broadly increases, being about twice as large for primates as for the echidna. It does not, however, increase at anywhere close to the rate of the neocortex-to-body-size ratio. Therefore, the hippocampus takes up a much larger fraction of the cortical mantle in rodents than in primates. In adult humans the volume of the hippocampus on each side of the brain is about 3.0 to 3.5 cm3 as compared to 320 to 420 cm3 for the volume of the neocortex. There is also a general relationship between the size of the hippocampus and spatial memory. When comparisons are made between similar species, those that have a greater capacity for spatial memory tend to have larger hippocampal volumes.
Neuroanatomy
The hippocampus and dentate gyrus that is folded into the hippocampal archicortex have the shape of a curved, rolled-up tube. The curve of the hippocampus uses the initial letters CA to name the hippocampal subfields CA1-CA4. CA4 is in fact the polymorphic layer or hilus of the dentate gyrus, but CA4 is still sometimes in use to describe the part of CA3 that inserts between the dentate gyrus regions or blades. 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" is CA4, the hilus of the dentate gyrus. This is embedded into the backward-facing, flexed dentate gyrus.In humans the hippocampus is described as having an anterior and posterior part; in other primates they are termed rostral and caudal, and in rodents they are termed ventral and dorsal. Both parts are of similar composition but belong to different neural circuits. The dentate gyrus combined with other hippocampal regions form a banana-like structure, with the two hippocampi joined at the stems by the commissure of fornix.
In humans and other 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 – the outer molecular layer, the middle granular layer, and the inner polymorphic layer also known as the hilus. The CA3 subfield 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 comes from the entorhinal cortex via the perforant path. The entorhinal cortex is strongly and reciprocally connected with many cortical and subcortical structures as well as with the brainstem. Different thalamic nuclei,, 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 is adjacent to 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.