Article:.Images:.Gross anatomy LocationThe hippocampus lies in the immediately below the floor of the temporal horn of the, and in cross section (coronal) has appearances that are reminiscent of a seahorse. It has a head (posterior to the ), a body, and a tail (which follows the upwardly curving lateral ventricle).Although there is a lack of consensus relating to terms describing the hippocampus and the adjacent, the term hippocampal formation generally applies to the, fields CA1-CA3 (CA4 is frequently called the and considered part of the dentate gyrus), and the. The hippocampus proper is made up of CA1, CA2 and CA3 fields (Figure 1).NB: CA is an abbreviation of cornu Ammonis PathwaysThe majority of its output is via the into the which breaks free to the hippocampus to form the. Information flows through the hippocampus proceeds from the dentate gyrus to CA3 to CA1 to the, with additional input information at each stage and outputs at each of the two final stages. CA2 represents only a very small portion of the hippocampus and its presence is often ignored in accounts of hippocampal function, though it is notable that this small region seems unusually resistant to conditions that usually cause large amounts of cellular damage, such as.The, which brings information primarily from (but also, among others), is generally considered the main source of input to the hippocampus. Layer II of entorhinal cortex (EC) brings input to the dentate gyrus and field CA3, while EC layer III brings input to field CA1 and the subiculum.
Its main functions involve human learning and memory. Knowing about the hippocampus has helped researchers understand how memory. However, it's been unclear which specific memory functions the hippocampus manages—recall, which is the ability to retrieve memory;.
The main output pathways of the hippocampus are the bundle and the fimbria/, which arise from field CA1 and the subiculum.Perforant path input from EC layer II enters the dentate gyrus and is relayed to region CA3 (and to mossy cells, located in the hilus of the dentate gyrus, which then sends information to distant portions of the dentate gyrus where the cycle is repeated). Region CA3 combines this input with signals from EC layer II and sends extensive connections within the region and also sends connections to region CA1 through a set of fibers called the. Region CA1 receives input from the CA3 subfield (these afferents selectively target the basal dendrites in the, and the apical dendrites in the - this is the Schaffer collateral pathway), EC layer III and the nucleus reuniens of the thalamus (which project only to the terminal apical dendritic tufts in the ). In turn, CA1 projects to the as well as sending information along the aforementioned output paths of the hippocampus.
The is the final stage in the pathway, combining information from the CA1 projection and EC layer III to also send information along the output pathways of the hippocampus.It is widely accepted that each of these regions has a unique functional role in the information processing of the hippocampus, but to date, the specific contribution of each region is poorly understood. The hippocampus also plays a vital role in the reproductive cycle. Arterial supply. collateral branches of the and 6. forming the deep intrahippocampal arteries 6Venous drainage. intrahippocampal veins, draining in the superficial hippocampal veins, forming two arches, which ultimately drain into the basilar vein 6,7.
posterior end: draining in the medial atrial vein 6,7. anterior end: inferior ventricular vein, draining in the medial atrial vein 6,7Radiographic features MRIThe hippocampus is best imaged in the coronal plane, angled perpendicular to the long axis of the hippocampal body. The three parts of the hippocampus (head, body and tail) can be identified based on morphology and by using local landmarks 3.-hippocampal head junction. landmarks. anterior lobe of the to.
the posterior amygdala is located above the anterior aspect of the hippocampal head 4. hippocampal head. morphology: hippocampal digitations a.k.a. Pes hippocampus. landmarks.to.
posterior-most extent is the first slice where the is clearly seen 4. hippocampal body. morphology: swiss roll appearance, of two interlocking U-shaped structures ( and ). landmark: to the. hippocampal tail.
morphology: smaller and harder to discern the internal structure. landmarks.
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from. from the point at which the can be seen in full profile, or is separate from the wall of the ventricle 4. Duvernoy HM, Cattin F, Risold P. The Human Hippocampus: Functional Anatomy, Vascularization and Serial Sections with MRI. Lasts Anatomy Regional and Applied. Churchill Livingstone. (2003) ISBN:B0084AQDG8.
Bronen RA, Cheung G. Relationship of hippocampus and amygdala to coronal MRI landmarks. Magn Reson Imaging. 1991;9 (3): 449-57. 4. Malykhin NV, Bouchard TP, Ogilvie CJ et-al.
Three-dimensional volumetric analysis and reconstruction of amygdala and hippocampal head, body and tail. Psychiatry Res. 2007;155 (2): 155-65. Holodny AI, George AE, Golomb J et-al. The perihippocampal fissures: normal anatomy and disease states. 1998;18 (3): 653-65. Tatu L, Vuillier F.
Structure and vascularization of the human hippocampus. (2014) Frontiers of neurology and neuroscience. Destrieux C, Bourry D, Velut S. Surgical anatomy of the hippocampus. (2013) Neuro-Chirurgie. 59 (4-5): 149-58.
Hippocampus is a complex brain structure embedded deep into temporal lobe. It has a major role in learning and memory. It is a plastic and vulnerable structure that gets damaged by a variety of stimuli. Studies have shown that it also gets affected in a variety of neurological and psychiatric disorders. In last decade or so, lot has been learnt about conditions that affect hippocampus and produce changes ranging from molecules to morphology. Progresses in radiological delineation, electrophysiology, and histochemical characterization have made it possible to study this archicerebral structure in greater detail. Present paper attempts to give an overview of hippocampus, both in health and diseases.
Relationship between limbic system and hippocampus. Note that hippocampus is a C-shaped structure having amygdaloid body at its anterior end, culminating deep into mammillary bodies ( Figure: courtesy AITBS publishers India)This part of brain has been one of the most extensively studied, and atrophy– of this region has clinical consequence; both potential and real.– It is the earliest and most severely affected structure in several neuropsychiatric disorders such as Alzheimer's disease (AD), epilepsy etc.Limbic system is considered to be a “primitive brain,” deep with-in brain. It is concerned with hunger, motivation, sex drive, mood, pain, pleasure, appetite, and memory etc.
Hippocampus is the posterior part of limbic lobe while frontal part is amygdala. Hippocampal Anatomy–Hippocampus contains two parts: Cornu ammonis (hippocampus proper) and dentate gyrus. Both of these parts are separated by hippocampal sulcus and curve into each other.
Below the sulcus lies subiculum. Since hippocampus is a part of allocortex (archicortex), there is a zone that separates it from neocortex. Some anatomists divide it into hippocampus proper (Cornu ammonis; CA), dentate gyrus (DG), subiculum, and entorhinal area (EC). Entire set is called as hippocampal formation.Hippocampus is divided into head, body, and tail; head being expanded part while tail is a thin curved part. Based upon histology, hippocampus proper (Cornu ammonis) is divided into CA1, CA2, CA3, and CA4 Figures and. Just opposite CA1, subiculum is located that connects hippocampus with entorhinal cortex in the ventricle; hippocampus is covered by choroid plexus. Hippocampus is supplied by posterior cerebral artery, which has three branches: Anterior, middle, and posterior.
Supply is also reinforced by anterior choroidal artery (uncal branch). Veins of hippocampus drain into basal vein. Coruna Ammonia. After the Greek God who had horn and this folded structure resembles to that of hippocampal layers. Historically, it has been linked to silkworm or sea horse. A Danish Anatomist divided it into Cornu ammonis, also in short called CA ( Figure: courtesy AITBS publishers India)Functionally, anatomically, and cytoarchitecturally, hippocampus is quite different from cerebral cortex. For example: Hippocampus is a part of archicortex while cerebral cortex is a part of neocortex.
Cortex is outermost structure while hippocampus is a small extension of brain; 5 centimeters in length lying in floor of lateral ventricle. Outer structure of cerebral cortex is highly folded by gyri and sulci while upper surface of hippocampus is convex (which include dentate gyrus, subiculum, and entorhinal cortex). Anterior surface is expanded to form a paw-like structure called pes hippocampi. Importantly, hippocampus is a three-layered structure made up of single pyramidal layer with plexiform layer on both sides.Hippocampus is an ancient structure in brain that differs both anatomically and physiologically from cerebral cortex. Different cells are organized in layers hippocampus and thus has most extensively been studied part of brain for neurophysiology.
Neural substrate of memory, also known as long term potentiation (LTP), was first discovered in this region.Hippocampus has connections both to and fro from various parts of brain,–:.Entorhinal Cortex: It has been observed that entorhinal cortex input to dentate gyrus (layer II of perforant path) plays an important role in pattern recognition and encoding of memories.Perforant Path: Axons present in perforant path arise in layer II and III of entorhinal cortex with little contribution from deeper layers IV and V. Axons of layer II and IV project to granule cells and pyramidal cells of CA3 while those from III and IV project to pyramidal cells of CA1 and subiculum. Importantly, retrieval of information is achieved by entorhinal cortex input to CA3 (layer III of perforant path).
A distinct pathway also goes from entorhinal cortex (layer III of perforant path) to CA1 neurons. Pyramidal cells of CA1 send their axons to the subiculum and deep layers of the entorhinal cortex. Subicular neurons send their axons mainly to the EC. Perforant pathway is noteworthy for severe degeneration seen in patients with AD.Shaffer's Collaterals: These are axon collaterals given off by CA3 cells of hippocampus coruna and project to CA1 region. Apart from being important part of hippocampal tri-synaptic loop; this part plays an important role in memory formation and also in emotional network of Papez circuit.
Conduction in these circuits is excitatory (glutaminergic), and these are involved in activity-dependent neuronal plasticity.Recurrent Collaterals: These send excitatory input to CA3 and are called recurrent collaterals (RC). These play a vital role in “holding memory.” For example, if someone is teaching students and a phone bell rings then CA3 region will remember the information that one was giving to the students temporarily, and this is the function of collaterals.Dentate Gyrus: It is a deep region within hippocampus and is surrounded by cornu ammonis (CAs). This plays a crucial role as a processor of information from EC to CA3. Cells lying in DG are discharging at a low frequency and provide low intensity stimulation of CA3, which is believed to be an economical storage process.
It has been shown to play an important role in pattern separation and associate memory.Cornu Ammonis (CA1, CA2, CA3, CA4): CA1 contains pyramidal cells principally, and it plays an important role in matching and mismatching of information obtained from CA3. CA2 is a small area and is not considered to be very important hippocampal subfield, except for its high resistance to hippocampal sclerosis due to epileptic damage. Pyramidal cells are also seen in dentate gyrus of hippomcampus. Hippocampal PhysiologySince hippocampus receives direct inputs from olfactory bulb, it is important that it was implicated in olfaction for a long time. Memory started dominating in early 1970s, with the description of LTP. Anterograde and partial retrograde amnesia developed in a patient called Henry Gustav Molaison (called HM) following removal of hippocampus due to refractory epilepsy. HM was unable to form new episodic memories following this surgery.
In medical science, HM has been perhaps the most studied medical patient. Later studies have shown that damage to hippocampus causes anterograde amnesia and often retrograde amnesia also. Implicit memory is spared in hippocampal damage.,Hippocampus is one of the unique regions in brain where the neurogenesis continues even in adult life. Though, described initially, as “too little,” neurogenesis in brain is now thought to be functionally important.
It has been seen that neurons, hence, produced integrate into the mainstream neurons. They have also, hence, shown to be functionally important. However, a recent review agreed that neurogenesis per se may be less attractive drug target than hippocampal atrophy as a whole.Hippocampus is now known not just to be important in learning and memory but also in:.Spatial navigation.Emotional behavior.Regulation of hypothalamic functions.Learning and Memory: Hippocampus is vital for learning, memory, and spatial navigation.
Connections between hippocampus and neocortex are important for awareness about conscious knowledge. An intricate balance is maintained during encoding of memories in hippocampus and retrieval of experiences from frontal lobe. For learning and memory loop, there are two prominent pathways: polysynaptic and direct pathway. In polysynaptic pathway, hippocampus gets afferent connections from parietal, temporal, and occipital areas via entorhinal cortex and then to dentate gyrus→CA3→ CA1→ subiculum→ alveus→ fimbria→ fornix→ mammillothalamic tract→ anterior thalamus→ posterior cingulated→ retrosplenial cortex.
In the direct intra-hippocampal pathway, it gets its input from temporal association cortex through perirhinal and entorhinal area to CA1. From there, projections move via subiculum and entorhinal crtex to inferior temporal cortex, temporal pole, and prefrontal cortex. It is important to remember that polysynaptic pathway is important in semantic memory while direct intra-hippocampal pathway is important in episodic and spatial memory.Other Roles: Hippocampus is a part of ventral striatal loop, hence can affect motor behavior. Though emotional behavior is regulated mainly by amygdala, hippocampus and amygdala both have reciprocal connections, thus can influence each other (latter affects more than former).
Since hippocampus has projections to hypothalamus, thus can affect release of adrenocorticotropic hormones. That is why, in patients with atrophied hippocampus, there is rise of cortisol.
Hippocampal PharmacologyHippocampus receives projections that contain serotonin, norepinephrine, and dopaminergic neurons. An important projection, cholinergic and GABAergic in nature, comes from medial septal area and innervates all parts of hippocampus. This projection may play a special role in maintaining physiological state, and destruction of this projection can disrupt hippocampal theta rhythm leading to severe impairment of memory. Entorhinal area plays an important role in visual recognition of objects. Complete amnesia occurs when both hippocampus and entorhinal area are destroyed.
Complete amnesia may occur with bilateral hippocampal destruction. However, memory may be preserved with unilateral damage (e.g., stroke) or with hippocampal resection (e.g., temporal lobectomy for epilepsy). Granule cells of dentate gyrus are principally excitatory and contain glutaminergic projections. Some cells also show projections that are immune-reactive for opiates such as dynorphin and endorphins.
These have opposing effects on neuronal excitability.Hippocampal n-methyl d-aspartate receptors have been long known important in learning, memory, and spatial navigation. These are also vitally important in LTP in hippocampus. Similarly, glucocorticoids and mineralocorticoid receptors have assumed important role of late due to description of stress and its role in hippocampal atrophy.Hippocampus is richly innervated with muscarinic and nicotinic receptors of acetylcholine.
Medial septal nucleus is major source of cholinergic output to hippocampus. It represents a direct input both to principle neurons and inter-neurons. Other than this projection, some sparse cholinergic projections are also there. Activity of septo-hippocampal projection has been considered important in hippocampal pacemaker function and also in generating hippocampal theta rhythm. Muscarininc M1 and M3 receptors are expressed in principal cells while M2 and M4 are present on inter-neurons.
Nicotinic receptors are also activated by septo-hippocampal pathways. Acetylcholine classically activates pyramidal cells tonically.Adenosine, via activation of inhibitory A1 receptors, contributes to neuro-modulatory influence. Similarly, by activating A2 A receptors, adenosine can facilitate excitatory neurotransmitter system.Two different types of GABAergic mechanisms have been identified in hippocampus: Tonic nd phasic. Though in general GABA is an inhibitory neurotransmitter in brain, its role in hippocampus remains to be fully known. Recent reports are appearing that GABA A subunits plays an important role in learning and memory; core functions of hippocampus. GABA receptors are present on principle cells (pyramidal cells) of hippocampus and are important in neural connections between inter-neuronal circuits. Basic pathways of hippocampus.
Note that entorhinal cortex (EC) sends projections to dentate gyrus, which in turn sends to CA3. Fibers go from here to CA1 region and are called as Schaffers collaterals (SC). Note that, as Mann and Eichenbaum (2005) have suggested, hippocampus operates in the form of a tricynaptic loop. That means, fibers go from EC to DG (1), CA3 to CA1 (2), and from CA1 to subiculum (3), which project to EC again. Lesion or damage to any component along the trisynaptic loop may result into hippocampal dysfunction. Flow of hippocampus is largely unidirectional, and it flows from EC to dentate gyrus and then to CA3 and onwards to CA1.
Importantly, main input to hippocampus is from entorhinal cortex, which receives inputs from multiple cortical areas and all sensory modalities. Cortical input that terminates on the layer I, II, III of entorhinal cortex goes to hippocampus. Eventually, this information goes to entorhinal cortex and then to subiculum.
It is important to know that each layer of CA has its own complex circuitry and longitudinal connections ( Figure: courtesy AITBS publishers India)Hippocampal neurons generate a characteristic theta rhythm that has a slow frequency. Perhaps, it is generated by a small group of pyramidal cells.
Perhaps, due to its densely packed structure of neurons, hippocampus generates some of the biggest waves in EEG of any brain structure. Hippocampal AtrophyEver since, HM developed severe anterograde amnesia following his surgery; hippocampus has been linked with memory. It is particularly vulnerable to stress and contains rich number of glucocoritcoids, estrogen, and progesterone receptors. A number of mechanisms have been linked to hippocampal atrophy.Rates of hippocampal atrophy have been used as both diagnostic and prognostic marker in clinical trials of AD. It has been seen that patients with minimal cognitive impairment (MCI), have 10-15% of volume loss of hippocampus while those with early AD, this loss is about 15-30%.
In those with moderate AD, it may reach to the extent of 50%.Although we do not know why hippocampus is so very vulnerable and undergoes atrophy in diverse conditions, a number of neurochemical and vascular alternations have been identified contributing to hippocampal atrophy. These include glucocorticoids, serotonin, excitatory amino acid, e.g. Glutamate and its transporters etc.
Importantly, several drugs of unrelated pharmacological classes have been shown to prevent this atrophy implying multi-model manner in which atrophy is produced.–. MRI scans showing hippocampal atrophy of both sides in an-85-year-old female with advanced ADOne of the clear yet not complete mechanisms described for hippocampal atrophy is stress-induced hippocampal atrophy. Circulating glucocortioids are involved in the mechanism that produces atrophy, along with excitatory amino acids (EAA) and serotonin.
Atrophy in Cushing's disease and during normal aging involve the entire hippocampal formation and are linked to deficits in short-term verbal memory. This suggests that hippocampal atrophy may have functional consequences as well.Deposition of neuritic plaques and neurofibrillary tangles appear to be an obvious mechanism. Some regions of hippocampus (e.g. CA1) are particularly vulnerable to global ischemia.
Alzheimer's disease,Atrophy of hippocampal region in brains is one of the most consistent features of AD. It is the earliest brain region and is most severely affected. A popular hypothesis called ‘hippocampo-cortical-dissociation’ has proposed that early damage to hippocampus causes a ‘dissociation’ between hippocampus and cerebral cortex, leading to failure of registration of information emanating from hippocampus. Some amount of hippocampal atrophy is seen in all patients with AD. A number of neurotransmitter alterations also occur in brains of AD such as noradrenergic, serotonergic, and glutaminergic regions corresponding to neuron loss in hippocampal region. Depression,Ever since the biological basis of depression is getting unfolded, evidence is accumulating that prolonged depression can cause volume loss of hippocampus. Moreover, duration of depression has been correlated with severity of hippocampal atrophy.
Evidence suggests that atrophy thus produced may be permanent and persist long even though depression has undergone remission. It has been hypothesized that it could be a consequence of affective disturbance seen in depression. It is believed that this could result from prolonged stress generated as a result of depression. Retraction of cell volume and/or suppression of hippocampal neurogenesis could be responsible in this case. Robert, 1963Hippocampal volume reduction is one of the most consistent findings found in MRI of schizophrenic patients.
Functional and biochemical abnormalities have also been identified there. Though initially patho-physiology of schizophrenia focused mainly upon prefrontal cortex, but now hippocampus is being considered for last 20 years or so. Now there is compelling data to suggest that there are anatomical and functional aberrations due to neuronal disturbances in hippocampus of schizophrenic patients. Evidence has been gathered from MRI, PET, and MRS studies of disturbances with in hippocampus of schizophrenia. Volume reduction in hippocampus of schizophrenia is modest and not as marked as that seen in AD. Still, biochemical and functional disturbances provide a reliable evidence of involvement of hippocampus in patho-physiology of schizophrenia. Epilepsy,Up to 50% to 75% of patients with epilepsy may have hippocampal sclerosis upon postmortem analysis, in case they died and had medically refractory temporal lobe epilepsy.
It is, however, not clearly known if epilepsy is generated as a result of hippocampal sclerosis or repeated seizures damage hippocampus. ConclusionsHippocampus is an extension of cerebral cortex situated deep into temporal lobe. It is a vulnerable and plastic structure. It gets damaged by a variety of stimuli, and hence is important clinically both diagnostically and therapeutically. Ragnarok clicker trainer. Currently, it is one of the marker of cognitive decline and diagnosis of AD. It is also a prognostic marker in research setting. Drugs that are able to cause slow down of atrophy or reversal are actively being sought.
These could then potentially have disease-modifying effects.,.