The hippocampus is part of thecerebral cortex. It is closely associated with a limited number of related cortical areas collectively referred to as the parahippocampal region. Both the hippocampus and the parahippocampal region are connected to a variety of higher-order association cortices that represent all sensory domains. From this recently emerged perspective, it is not surprising that scientific conjectures about the functions of the (para)hippocampal network emphasize an important role in higher-order cognitive functions, particularly in learning and memory processes. Structures in the (para-)hippocampus also have strong functional connections to subcortical structures, which the researchers suggest play a role in selecting behavior for individual or species survival.
General features of the (para)hippocampus
To describe any feature of a structure, be it morphological or connective, one needs a nomenclature. Opinions on nomenclature will inevitably differ, so for the purposes of this essay differences in the number of major strata have been chosen as the main criterion. All hippocampal fields have a characteristic three-layered appearance. The parahippocampal region, unlike the hippocampus, includes cortical regions that have more than three, usually five or six layers. The hippocampus consists of three main divisions: the dentate gyrus, the horn of ammon (fields CA1, CA2, and CA3), and the subiculum. Note that some authors distinguish a prosubiculum from the actual subiculum. In addition, the reader will find texts referring to the subicular complex, which consists of (pro-)subiculum, presubiculum and parasubiculum. Based on connection arguments as well as the criteria chosen for this discussion (number of layers), it is preferred to consider the presubiculum and parasubiculum as functionally distinct from the subiculum.
In all species, the parahippocampal region includes five major regions: the presubiculum and parasubiculum, the perirhinal and entorhinal cortices, and a fifth area commonly referred to in primates as the parahippocampal cortex. In non-primates, the most likely homologue of this last region is what is known as the post-rhinal cortex. Different researchers have divided each of these five regions in different ways.
All fields of the (para-)hippocampus can be easily perceived as longitudinal strips of cortex perfectly aligned next to each other, starting at the dentate gyrus at one end and the outermost part of the parahippocampal region, in fact the perirhinal field. on the other end. The exact orientation and curvature of all of these cortical structures, and hence their general position in the brain, can vary between different species. In species with a prominent temporal lobe (eg, humans and monkeys), the hippocampus is more ventral and anterior; in contrast, the rat hippocampus more closely resembles a c-shaped structure located in the caudal third of the hemisphere. However, such a positional difference does not alter the main features and topological relationships between the hippocampal and parahippocampal structures. The fact that the most anterior/ventral part of the hippocampus has a close spatial relationship with the amygdala is also irrelevant.
Wiring of the (para)hippocampus
The most detailed information about the connectivity of the system comes from anatomical tracing studies in the rat, although detailed information relevant to some signaling pathways has also been collected in other species. In general, connectivity is quite conservative, so a non-species-specific overview may suffice. In addition, this "conserved" connection organization allows conclusions to be drawn about the general organization of the human (para-)hippocampus. The hippocampus has two main pathways that connect it to the rest of the brain. The first pathway uses parahippocampal connectivity and primarily mediates connections to the cortex. The second pathway, which primarily but not exclusively connects the hippocampus to subcortical structures, uses the fornix.
The parahippocampal-cortical system
The famous Spanish anatomist Ramón y Cajal, who provided one of the earliest and most detailed descriptions of the (para)hippocampal system, emphasized the so-called trisynaptic circuit, which comprises a unique unidirectional pathway from the entorhinal cortex to the distal dendrites of the hippocampal granular system. (simple pyramidal) in the dentate gyrus, which in turn give rise to the mossy fibrous pathway to the proximal part of the apical dendrites of CA3 pyramidal cells. These CA3 neurons eventually transmit their output via Schaffer's axon collaterals to the apical and basal dendrites of pyramidal cells in CA1. The trisinatic circuits were previously thought to be organized in finite planes perpendicular to the long axis of the hippocampus, so the structure as a whole was viewed as a series of repeating circuits called lamellae stacked along the long axis. This proposal has stimulated functional analysis of the network using isolated brain slices containing these lamellar circuits. However, he also emphasized the relative isolation of the trisynaptic circuits, which is contrary to the notion that the hippocampal trisynaptic circuits must be part of a larger neural system in order to function. The added details emphasize general longitudinal connectivity as an integral feature of trisynaptic organization. Furthermore, the entorhinal cortex is itself a complex cortical structure that also has strong reciprocal connections with generalized association cortices, largely mediated through their neighboring parahippocampal fields. Finally, the subiculum was added at a crucial position within this circuit.
The neurons of entorhinal layers II and III, which are the main receptors of the extensive cortical inputs, lead to the above-mentioned inputs to the hippocampus, the so-called perforating pathway. This name derives from traditional descriptions by Ramón y Cajal, who found that fibers from the entorhinal cortex penetrate the pyramidal cell layer of the subiculum to access the molecular layer of the dentate gyrus. The perforation path accommodates two different projection systems. Layer II cells distribute their axons to most if not all of the dentate gyrus as originally described, but also to CA3. Cells in a particular subdivision of the entorhinal cortex, commonly referred to as the lateral entorhinal cortex, distribute their axons exclusively to the distal-most portions of the CA3 and dentate cell dendrites. The other subdivision, called the medial entorhinal cortex, sends its projection to the middle parts of the apical dendrite. This routing results in a specific laminar termination such that each neuron receives both pathways but on a different segment of its apical dendrite. The second pathway, which only attracted more experimental attention in the late 20th century, originates from layer III cells and is distributed between CA1 and the subiculum. In contrast to the laminar pattern described for the lateral and medial layer II components, axons from layer III cells target only limited groups of the available neurons in CA1 and the subiculum. This targeting results in an organization where the lateral entorhinal cortex distributes its output only to neurons clustered around the edge of the CA1 subiculum, while the median entorhinal cortex selectively assigns CA1 neurons near the edge with CA2/CA3 and subiculum -Neurons are innervated to the border with the presubiculum.
CA1 neurons project to the subiculum, adding a fourth synapse to the originally defined trisynaptic circuit. These projections, again almost entirely unidirectional, show an interesting topographical organization along the transverse axis reminiscent of the terminal entorhinal distribution. Neurons in CA1, near the subiculum border, are connected to subicular neurons also near this border; These two connected populations are likely innervated through entorhinal inputs from the lateral entorhinal cortex. Conversely, CA1 neurons closest to the CA3 border distribute their axons to subicular neurons near the presubicular border; these two connected populations are therefore likely to receive medial entorhinal input.
The lateral and medial entorhinal cortices can functionally process different types of information. The main cortical input to the lateral entorhinal cortex comes from the perirhinal and olfactory cortices, while the medial entorhinal cortex receives strong inputs from the presubiculum and the parahippocampal or postrhinal cortex. Given the anatomical organization mentioned above, researchers have proposed that these two functionally distinct input streams converge in the hippocampus at the dentate gyrus and CA3 levels, while remaining more or less separate at the CA1 and subiculum levels. This connection differentiation together with the general differences in intrinsic wiring between, say, CA3 and CA1 suggests that the (para)hippocampal system harbors two systems that may be involved in different memory processes.
Both CA1 and the subiculum form the main exit structures of the hippocampus. They distribute strong projections back to the entorhinal cortex, mainly to its deep layers. The general topographical organization of this projection is consistent with that of the perforant pathway, such that a particular part of the entorhinal cortex that projects to limited populations of neurons in CA1 and the subiculum receives a backprojection, projected from these same neuronal groups in CA1 and 2 comes the subiculum. This remarkable reciprocity, along with the general intrinsic hippocampal organization mentioned above, may have important functional implications that are not yet fully understood by researchers.
the fornix
The fornix is an important bundle of fibers that connects the hippocampus to the hypothalamus, particularly the mammary bodies. The fornix emanates from CA1 and the subiculum, although the parahippocampal cortices, particularly the pre- and parasubiculum, and to a much lesser extent the entorhinal cortex, contribute fibers. On its way to the mammillary bodies, the fornix also gives off fibers to the septal complex, ventral striatum, and amygdala. The fornix also carries CA1 and subiculum fibers leading to parts of the prefrontal cortex. The fornix is not a pure exit route as protrusions from the septal complex to the hippocampus and partially to the entorhinal cortex migrate through the fornix. These septal afferents provide the (para-)hippocampus with most of its cholinergic inputs. The fornix also forms one of the entry routes for noradrenergic, serotonergic, and dopaminergic innervation. Additional aminergic fibers enter the parahippocampal region via a ventral pathway. Finally, the commissural connections between the left and right hippocampus also pass partially through the fornix.
The connection of the hippocampus to the nipple bodies is part of the traditionally described limbic or Papez circuit, which includes the mamillo-thalamic tract, which connects the nipple bodies to the anterior thalamic complex, which in turn projects to large portions of the cortex. including anterior and posterior cingulate cortex and pre- and parasubiculum. All of these structures, in turn, provide information to the hippocampus, either directly or indirectly via the entorhinal cortex.
Diploma
The (para)hippocampus can be viewed as a cortical system with bidirectional connections to almost all multimodal associative domains of the brain.cerebral cortexas well as a number of subcortical structures purportedly involved in the motivation and selection of appropriate behaviors. Thus, the (para)hippocampus may lie at the crossroads of the cognitive and affective sides of behavior. Unfortunately, most function-oriented research addresses both sides, while a combined analysis of both is rare.
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Ramon y Cajal, S. (1911).Histology of the nervous system of humans and vertebrates.Malone, Paris
Witter, M. P., Wouterlood, F. G., Naber, P. A. & van Haeften, T. (2000). Anatomical Organization of the Parahippocampals Hippocampus-Netzwerks.annals ofNYAcademy of Sciences 911, 1-24.
larry wSchwanson
reviewed byMenno P.Witter