Hippocampus proper

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Hippocampus proper
Hippocampus (brain).jpg
Hippocampus location and regions in a human, coronal plane
CajalHippocampus (modified).png
Basic circuit of the hippocampus, shown using a modified drawing by Ramon y Cajal. DG: dentate gyrus. Sub: subiculum. EC: entorhinal cortex
Details
Identifiers
Latin hippocampus proprius, cornu Ammonis
NeuroNames 182
TA98 A14.1.09.327
TA2 5520
FMA 62493
Anatomical terms of neuroanatomy

The hippocampus proper refers to the actual structure of the hippocampus which is made up of four regions or subfields. The subfields CA1, CA2, CA3, and CA4 use the initials of cornu Ammonis, an earlier name of the hippocampus.

Contents

Structure

There are four hippocampal subfields, regions in the hippocampus proper which form a neural circuit called the trisynaptic circuit.

CA1

CA1 is the first region in the hippocampal circuit, from which a major output pathway goes to layer V of the entorhinal cortex. Another significant output is to the subiculum.

CA2

CA2 is a small region located between CA1 and CA3. It receives some input from layer II of the entorhinal cortex via the perforant path. Its pyramidal cells are more like those in CA3 than those in CA1. It is often ignored due to its small size.

CA3

CA3 receives input from the mossy fibers of the granule cells in the dentate gyrus, and also from cells in the entorhinal cortex via the perforant path. The mossy fiber pathway ends in the stratum lucidum. The perforant path passes through the stratum lacunosum and ends in the stratum moleculare. There are also inputs from the medial septum and from the diagonal band of Broca which terminate in the stratum radiatum, along with commisural connections from the other side of the hippocampus.

The pyramidal cells in CA3 have a unique type of dendritic spine called a thorny excrescence or thorn, only found in CA3 pyramidal cells and hilar mossy cells. The thorn has a thin single spine with a number of heads. Clusters of thorns sit on a dendrite on a broad stem. There are also longer spines called long-neck spines. These unique structures also help to demarcate CA3 from CA2. [1] [2]

The pyramidal cells in CA3 send some axons back to the dentate gyrus hilus, but they mostly project to regions CA2 and CA1 via the Schaffer collaterals. There are also a significant number of recurrent connections that terminate in CA3. Both the recurrent connections and the Schaffer collaterals terminate preferentially in the septal area in a dorsal direction from the originating cells. CA3 also sends a small set of output fibers to the lateral septum.

The region is conventionally divided into three divisions. CA3a is the part of the cell band that is most distant from the dentate (and closest to CA1). CA3b is the middle part of the band nearest to the fimbria and fornix connection. CA3c is nearest to the dentate, inserting into the hilus. CA3 overall, has been considered to be the “pacemaker” of the hippocampus. Much of the synchronous bursting activity associated with interictal epileptiform activity appears to be generated in CA3. Its excitatory collateral connectivity seems to be mostly responsible for this. CA3 uniquely, has pyramidal cell axon collaterals that ramify extensively with local regions and make excitatory contacts with them. CA3 has been implicated in a number of working theories on memory and hippocampal learning processes. Slow oscillatory rhythms (theta-band; 3–8 Hz) are cholinergically driven patterns that depend on coupling of interneurons and pyramidal cell axons via gap junctions, as well as glutaminergic (excitatory) and GABAergic (inhibitory) synapses. Sharp EEG waves seen here are also implicated in memory consolidation. [3]

A key physiological function of the CA3 is encoding heteroassociative memories using its recurrent circuitry. A seminal hypothesis by John Lisman postulated that during a single theta cycle, a defined set of CA3 principal neurons can activate each other to form a well defined sequence, and the spikes (action potentials) of these cells tend to coincide with the peaks of the superimposed gamma oscillation. [4] [5] Approximately a decade later, the existence of well-defined CA3 sequences has experimentally been shown in the laboratory of Loren Frank, [6] [7] moreover these results demonstrated that previously encoded sequential experience can be replayed by the CA3 region during episodes called "awake replay". A recent hypothesis postulates that CA3 sequences are built up pair by pair during memory encoding, relying on precisely timed, phase-precessing input from the entorhinal cortex. [8] This mechanism is based on the synapses of the CA3 recurrent axon corraterals on the dendrites of the CA3 population [9] that form a complete matrix of connections.

CA4

CA4 is a misleading term introduced by Lorente de Nó. [10] He observed that the pyramidal layer of the CA3 was continuous with polymorphic layer of the dentate gyrus and that the "modified pyramids" (later known as mossy cells [11] ) had Schaffer collaterals similar to CA3 pyramidal cells. Amaral showed that the mossy cells in the CA4 of Lorente de Nó did not have schaffer collaterals and that, in contrast to pyramidal cells, they project to the inner molecular layer of the DG and not to CA1. [11] The same author thus concluded that the term CA4 should be abandoned and that the zone should be regarded as the polymorphic layer of the dentate gyrus [11] (the area dentata of Blackstad (1956)). The polymorphic layer is often called the hilus or hilar region. [12] The neurons in the polymorphic layer, including mossy cells and GABAergic interneurons, primarily receive inputs from the granule cells in the dentate gyrus in the form of mossy fibers and project to the inner molecular layer of the dentate gyrus via the associational/commissural projection. [10] [11] They also receive a small number of connections from pyramidal cells in CA3. They, in turn, project back into the dentate gyrus at distant septotemporal levels.

Additional images

Related Research Articles

<span class="mw-page-title-main">Entorhinal cortex</span> Area of the temporal lobe of the brain

The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time. The EC is the main interface between the hippocampus and neocortex. The EC-hippocampus system plays an important role in declarative (autobiographical/episodic/semantic) memories and in particular spatial memories including memory formation, memory consolidation, and memory optimization in sleep. The EC is also responsible for the pre-processing (familiarity) of the input signals in the reflex nictitating membrane response of classical trace conditioning; the association of impulses from the eye and the ear occurs in the entorhinal cortex.

<span class="mw-page-title-main">Hippocampus</span> Vertebrate brain region involved in memory consolidation

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 in the allocortex, with neural projections into the neocortex, in humans as well as other primates. The hippocampus, as the medial pallium, is a structure found in all vertebrates. In humans, it contains two main interlocking parts: the hippocampus proper, and the dentate gyrus.

<span class="mw-page-title-main">Dentate gyrus</span> Region of the hippocampus in the brain

The dentate gyrus (DG) is part of the hippocampal formation in the temporal lobe of the brain, which also includes the hippocampus and the subiculum. The dentate gyrus is part of the hippocampal trisynaptic circuit and is thought to contribute to the formation of new episodic memories, the spontaneous exploration of novel environments and other functions.

<span class="mw-page-title-main">Neural pathway</span> Connection formed between neurons that allows neurotransmission

In neuroanatomy, a neural pathway is the connection formed by axons that project from neurons to make synapses onto neurons in another location, to enable neurotransmission. Neurons are connected by a single axon, or by a bundle of axons known as a nerve tract, or fasciculus. Shorter neural pathways are found within grey matter in the brain, whereas longer projections, made up of myelinated axons, constitute white matter.

<span class="mw-page-title-main">Pyramidal cell</span> Projection neurons in the cerebral cortex and hippocampus

Pyramidal cells, or pyramidal neurons, are a type of multipolar neuron found in areas of the brain including the cerebral cortex, the hippocampus, and the amygdala. Pyramidal cells are the primary excitation units of the mammalian prefrontal cortex and the corticospinal tract. One of the main structural features of the pyramidal neuron is the conic shaped soma, or cell body, after which the neuron is named. Other key structural features of the pyramidal cell are a single axon, a large apical dendrite, multiple basal dendrites, and the presence of dendritic spines.

Schaffer collaterals are axon collaterals given off by CA3 pyramidal cells in the hippocampus. These collaterals project to area CA1 of the hippocampus and are an integral part of memory formation and the emotional network of the Papez circuit, and of the hippocampal trisynaptic loop. It is one of the most studied synapses in the world and named after the Hungarian anatomist-neurologist Károly Schaffer.

An apical dendrite is a dendrite that emerges from the apex of a pyramidal cell. Apical dendrites are one of two primary categories of dendrites, and they distinguish the pyramidal cells from spiny stellate cells in the cortices. Pyramidal cells are found in the prefrontal cortex, the hippocampus, the entorhinal cortex, the olfactory cortex, and other areas. Dendrite arbors formed by apical dendrites are the means by which synaptic inputs into a cell are integrated. The apical dendrites in these regions contribute significantly to memory, learning, and sensory associations by modulating the excitatory and inhibitory signals received by the pyramidal cells.

<span class="mw-page-title-main">Subiculum</span> Most inferior part of the hippocampal formation

The subiculum is the most inferior component of the hippocampal formation. It lies between the entorhinal cortex and the CA1 subfield of the hippocampus proper.

Theta waves generate the theta rhythm, a neural oscillation in the brain that underlies various aspects of cognition and behavior, including learning, memory, and spatial navigation in many animals. It can be recorded using various electrophysiological methods, such as electroencephalogram (EEG), recorded either from inside the brain or from electrodes attached to the scalp.

<span class="mw-page-title-main">Perforant path</span>

In the brain, the perforant path or perforant pathway provides a connectional route from the entorhinal cortex to all fields of the hippocampal formation, including the dentate gyrus, all CA fields, and the subiculum.

The stratum lucidum of the hippocampus is a layer of the hippocampus between the stratum pyramidale and the stratum radiatum. It is the tract of the mossy fiber projections, both inhibitory and excitatory from the granule cells of the dentate gyrus. One mossy fiber may make up to 37 connections to a single pyramidal cell, and innervate around 12 pyramidal cells on top of that. Any given pyramidal cell in the stratum lucidum may get input from as many as 50 granule cells.

<span class="mw-page-title-main">Mossy fiber (hippocampus)</span> Pathway in the hippocampus

In the hippocampus, the mossy fiber pathway consists of unmyelinated axons projecting from granule cells in the dentate gyrus that terminate on modulatory hilar mossy cells and in Cornu Ammonis area 3 (CA3), a region involved in encoding short-term memory. These axons were first described as mossy fibers by Santiago Ramón y Cajal as they displayed varicosities along their lengths that gave them a mossy appearance.

The trisynaptic circuit or trisynaptic loop is a relay of synaptic transmission in the hippocampus. The trisynaptic circuit is a neural circuit in the hippocampus, which is made up of three major cell groups: granule cells in the dentate gyrus, pyramidal neurons in CA3, and pyramidal neurons in CA1. The hippocampal relay involves 3 main regions within the hippocampus which are classified according to their cell type and projection fibers. The first projection of the hippocampus occurs between the entorhinal cortex (EC) and the dentate gyrus (DG). The entorhinal cortex transmits its signals from the parahippocampal gyrus to the dentate gyrus via granule cell fibers known collectively as the perforant path. The dentate gyrus then synapses on pyramidal cells in CA3 via mossy cell fibers. CA3 then fires to CA1 via Schaffer collaterals which synapse in the subiculum and are carried out through the fornix. Collectively the dentate gyrus, CA1 and CA3 of the hippocampus compose the trisynaptic loop.

<span class="mw-page-title-main">Hippocampus anatomy</span> Component of brain 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 the human and other primates, 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.

<span class="mw-page-title-main">Fascia dentata</span>

The fascia dentata is the earliest stage of the hippocampal circuit. Its primary input is the perforant path from the superficial layers of entorhinal cortex. Its principal neurons are tiny granule cells which give rise to unmyelinated axons called the mossy fibers which project to the hilus and CA3. The fascia dentata of the rat contains approximately 1,000,000 granule cells. It receives feedback connections from mossy cells in the hilus at distant levels in the septal and temporal directions. The fascia dentata and the hilus together make up the dentate gyrus. As with all regions of the hippocampus, the dentate gyrus also receives GABAergic and cholinergic input from the medial septum and the diagonal band of Broca.

<span class="mw-page-title-main">Granule cell</span> Type of neuron with a very small cell body

The name granule cell has been used for a number of different types of neurons whose only common feature is that they all have very small cell bodies. Granule cells are found within the granular layer of the cerebellum, the dentate gyrus of the hippocampus, the superficial layer of the dorsal cochlear nucleus, the olfactory bulb, and the cerebral cortex.

Sharp waves and ripples (SWRs) are oscillatory patterns produced by extremely synchronised activity of neurons in the mammalian hippocampus and neighbouring regions which occur spontaneously in idle waking states or during NREM sleep. They can be observed with a variety of imaging methods, such as EEG. They are composed of large amplitude sharp waves in local field potential and produced by tens of thousands of neurons firing together within 30–100 ms window. They are some of the most synchronous oscillations patterns in the brain, making them susceptible to pathological patterns such as epilepsy.They have been extensively characterised and described by György Buzsáki and have been shown to be involved in memory consolidation in NREM sleep and the replay of memories acquired during wakefulness.

<span class="mw-page-title-main">Glucocorticoids in hippocampal development</span> HippoCampus

The hippocampus is an area of the brain integral to learning and memory. Removal of this structure can result in the inability to form new memories as most famously demonstrated in a patient referred to as HM. The unique morphology of the hippocampus can be appreciated without the use of special stains and this distinct circuitry has helped further the understanding of neuronal signal potentiation. The following will provide an introduction to hippocampal development with particular focus on the role of glucocorticoid signaling.

Early long-term potentiation (E-LTP) is the first phase of long-term potentiation (LTP), a well-studied form of synaptic plasticity, and consists of an increase in synaptic strength. LTP could be produced by repetitive stimulation of the presynaptic terminals, and it is believed to play a role in memory function in the hippocampus, amygdala and other cortical brain structures in mammals.

The supramammillary nucleus (SuM), or supramammillary area, is a thin layer of cells in the brain that lies above the mammillary bodies. It can be considered part of the hypothalamus and diencephalon. The nucleus can be divided into medial and lateral sections. The medial SuM, or SuMM, is made of smaller cells which release dopamine and give input to the lateral septal nucleus. The lateral SuM, or SuML, is made of larger cells that project to the hippocampus.

References

  1. ↑ Lauer, M; Senitz, D (October 2006). "Dendritic excrescences seem to characterize hippocampal CA3 pyramidal neurons in humans". Journal of Neural Transmission. 113 (10): 1469–75. doi:10.1007/s00702-005-0428-8. PMID   16465457. S2CID   21440177.
  2. ↑ Tsamis, IK; Mytilinaios, GD; Njau, NS; Fotiou, FD; Glaftsi, S; Costa, V; Baloyannis, JS (February 2010). "Properties of CA3 dendritic excrescences in Alzheimer's disease". Current Alzheimer Research. 7 (1): 84–90. doi:10.2174/156720510790274482. PMID   20205674.
  3. ↑ Jerome Engel TAP, ed. Epilepsy: A Comprehensive Textbook in Three Volumes. Philadelphia, PA: Lippincott Williams & Wilkins; 2008
  4. ↑ Jensen O, Lisman JE (September 1996). "Hippocampal CA3 region predicts memory sequences: accounting for the phase precession of place cells". Learning and Memory. 3 (2): 279. doi:10.1101/lm.3.2-3.279. PMID   10456097.
  5. ↑ Lisman JE (February 1999). "Relating hippocampal circuitry to function: recall of memory sequences by reciprocal dentate-CA3 interactions". Neuron. 22 (2): 233. doi:10.1016/s0896-6273(00)81085-5. PMID   10069330.
  6. ↑ Karlsson MP, Frank LM (July 2009). "Awake replay of remote experiences in the hippocampus". Nature Neuroscience. 12 (7): 913. doi:10.1038/nn.2344. PMC   2750914 . PMID   19525943.
  7. ↑ Carr MF, Karlsson MP, Frank LM (August 2012). "Transient slow gamma synchrony underlies hippocampal memory replay". Neuron. 75 (4): 700. doi:10.1016/j.neuron. PMID   22920260.
  8. ↑ Kovács KA (September 2020). "Episodic Memories: How do the Hippocampus and the Entorhinal Ring Attractors Cooperate to Create Them?". Frontiers in Systems Neuroscience. 14: 68. doi: 10.3389/fnsys.2020.559186 . PMC   7511719 . PMID   33013334.
  9. ↑ Bains JS, Longacher JM, Staley KJ (August 1999). "Reciprocal interactions between CA3 network activity and strength of recurrent collateral synapses". Nature Neuroscience. 2 (8): 720. doi:10.1038/11184. PMID   10412061.
  10. 1 2 Lorente de Nó, R (1934). "Studies on the structure of the cerebral cortex. Continuation of the study of the ammonic system". J. Psychol. Neurol. 46: 113–177.
  11. 1 2 3 4 Amaral, DG (1978). "A Golgi study of cell types in the hilar region of the hippocampus in the rat". J. Comp. Neurol. 182 (5): 851–914. doi:10.1002/cne.901820508. PMID   730852. S2CID   44257239.
  12. ↑ Anderson P, Morris R, Amaral, Bliss T, O'Keefe J (2007). "The hippocampal formation". In Anderson P, Morris R, Amaral, Bliss T, O'Keefe J (eds.). The hippocampus book (first ed.). New York: Oxford University Press. ISBN   978-0195100273. Archived from the original on 2020-03-15. Retrieved 2016-12-15.
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