Hippocampus

The hippocampus is a curved structure located in the medial temporal lobe and forms a major component of the hippocampal formation and limbic system. It is closely associated with the temporal horn of the lateral ventricle and extends along much of its floor.

The hippocampus is especially important for the formation of new declarative and episodic memories. It also participates in spatial navigation, contextual processing, memory consolidation, and the organization of relationships among different components of an experience.

Anatomically, the hippocampus is continuous with surrounding medial temporal cortical structures and communicates extensively with the entorhinal cortex, parahippocampal region, amygdala, septal region, hypothalamus, thalamic-related memory circuits, and widespread association cortex.

Location

The hippocampus lies in the medial temporal lobe, where it forms a curved elevation along the floor of the temporal horn of the lateral ventricle.

Its long axis follows the curved course of the temporal horn. Anteriorly, it approaches the amygdala, while posteriorly it continues toward structures associated with the fornix and splenial region.

Relationship to the Temporal Horn

The hippocampus forms a prominent part of the floor of the temporal horn of the lateral ventricle.

Its ventricular surface is covered by a thin layer of white matter called the alveus. Fibers within the alveus converge medially to form the fimbria of the hippocampus.

Relationship to the Amygdala

The amygdala lies anterior to the hippocampus in the anteromedial temporal lobe.

The two structures are strongly interconnected. The hippocampus contributes memory and contextual information, while the amygdala contributes emotional and behavioral significance. Their interactions allow emotional states to influence memory formation.

Relationship to the Parahippocampal Gyrus

The hippocampus lies deep to the parahippocampal gyrus. The parahippocampal region contains cortical areas that provide an important interface between the hippocampal formation and widespread cerebral association cortex.

The entorhinal cortex, located in the anterior parahippocampal region, is particularly important for hippocampal input and output.

Hippocampal Formation

The term hippocampal formation generally includes the hippocampus proper, dentate gyrus, and subicular complex.

These structures are tightly interconnected and function together in memory and spatial processing.

Component	General Anatomical Role
Hippocampus proper	Contains CA fields and major intrinsic hippocampal circuitry
Dentate gyrus	Receives prominent entorhinal input and projects to CA3
Subiculum	Important output region linking hippocampus with cortical and subcortical targets

Hippocampus Proper

The hippocampus proper, also called the cornu ammonis, is divided histologically into fields designated CA1, CA2, CA3, and CA4.

These fields differ in cellular organization, connectivity, and physiological properties.

CA1 Region

The CA1 field receives major input from CA3 through Schaffer collateral fibers and projects toward the subiculum and entorhinal cortex.

CA1 neurons are particularly vulnerable to several forms of metabolic injury, including severe hypoxia and ischemia.

CA2 Region

The CA2 field is a relatively narrow region between CA1 and CA3.

It has distinct connectivity and cellular properties and participates in hippocampal processing, including networks associated with memory and social information.

CA3 Region

The CA3 field receives prominent mossy fiber input from granule cells of the dentate gyrus.

CA3 pyramidal neurons give rise to recurrent connections and Schaffer collateral fibers that project toward CA1.

CA4 Region

The term CA4 is commonly used for pyramidal neurons located within the hilus of the dentate gyrus.

Terminology and boundaries vary somewhat among anatomical classifications.

Dentate Gyrus

The dentate gyrus is a narrow, curved cortical structure closely associated with the hippocampus proper.

It contains densely packed granule cells and receives substantial input from the entorhinal cortex through the perforant pathway.

Granule cell axons form the mossy fibers that project toward CA3.

Subiculum

The subiculum lies between CA1 and the parahippocampal cortical region.

It is an important output structure of the hippocampal formation. Subicular neurons project toward the entorhinal cortex and contribute fibers to the alveus, fimbria, and fornix.

Entorhinal Cortex

The entorhinal cortex is located in the anterior medial temporal region and provides a major interface between the hippocampal formation and neocortical association areas.

It receives highly processed information from widespread cortical regions and transmits this information into hippocampal circuitry. Hippocampal output can return through entorhinal pathways to distributed cortical networks.

Intrinsic Hippocampal Circuitry

A commonly described sequence of information flow through the hippocampal formation is:

1. Entorhinal cortex
2. Perforant pathway
3. Dentate gyrus
4. Mossy fibers
5. CA3
6. Schaffer collaterals
7. CA1
8. Subiculum
9. Entorhinal cortex and other output pathways

This sequence is useful for understanding hippocampal anatomy, although actual connectivity includes numerous parallel, recurrent, and direct pathways.

Perforant Pathway

The perforant pathway is a major projection from the entorhinal cortex into the hippocampal formation.

Its fibers project prominently to the dentate gyrus and hippocampal fields, allowing cortical information to enter hippocampal circuits.

Mossy Fibers

Mossy fibers are the axons of dentate granule cells.

They project primarily to pyramidal neurons of CA3 and form a distinctive component of the hippocampal intrinsic circuit.

Schaffer Collaterals

Schaffer collateral fibers arise predominantly from CA3 pyramidal neurons and project to CA1 pyramidal neurons.

This pathway has been extensively studied in relation to synaptic plasticity and mechanisms associated with learning and memory.

Alveus

The alveus is a thin white matter layer covering the ventricular surface of the hippocampus.

It contains hippocampal efferent fibers that converge medially into the fimbria.

Fimbria

The fimbria of the hippocampus is a white matter bundle along the medial border of the hippocampus.

Posteriorly, the fimbria continues as the crus of the fornix, providing a major pathway from the hippocampal formation toward septal and hypothalamic structures.

Fornix

The fornix is one of the principal efferent pathways of the hippocampal formation.

Fibers pass from the alveus into the fimbria, continue into the crura and body of the fornix, and then descend through the columns toward septal, basal forebrain, and hypothalamic targets.

Hippocampal Commissure

Some fibers cross between the right and left hippocampal-related regions through the hippocampal commissure, also called the commissure of the fornix.

These fibers provide interhemispheric communication between components of the hippocampal system.

Afferent Connections

The hippocampal formation receives information from numerous cortical and subcortical sources.

Important inputs include:

• Entorhinal cortex
• Parahippocampal cortical regions
• Septal and basal forebrain structures
• Amygdala
• Thalamic-related systems
• Brainstem neuromodulatory nuclei

Much of the cortical information reaching the hippocampus is funneled through the entorhinal cortex.

Efferent Connections

Hippocampal output reaches cortical and subcortical structures through several routes.

Important pathways include projections through the subiculum and entorhinal cortex back toward association cortex and fibers traveling through the alveus, fimbria, and fornix toward septal and hypothalamic structures.

Connections with the Mammillary Bodies

Postcommissural fibers of the fornix connect the hippocampal system with the mammillary bodies of the hypothalamus.

The mammillary bodies subsequently project to the anterior thalamic nuclei through the mammillothalamic tract.

Connections with the Septal Region

The hippocampus has reciprocal relationships with the septal region and basal forebrain.

These pathways influence hippocampal activity and participate in memory-related processing and rhythmic neural activity.

Connections with the Amygdala

Connections between the hippocampus and amygdala allow contextual and memory information to interact with emotional significance.

This interaction is important when emotional responses depend on the environment or previous experience.

Connections with the Prefrontal Cortex

The hippocampal system interacts with the prefrontal cortex through direct and indirect pathways.

These networks contribute to memory retrieval, contextual decision-making, planning, and the use of previous experience to guide behavior.

Papez Circuit

The hippocampal formation forms a major component of the classical Papez circuit.

A simplified sequence is:

1. Hippocampal formation
2. Fornix
3. Mammillary bodies
4. Mammillothalamic tract
5. Anterior thalamic nuclei
6. Cingulate cortex
7. Cingulum
8. Parahippocampal and entorhinal regions
9. Hippocampal formation

Memory Formation

The hippocampus is particularly important for establishing new declarative memories.

It helps bind information concerning people, places, events, objects, and context into representations that can later be retrieved.

Episodic Memory

Episodic memory concerns personally experienced events and their temporal and spatial context.

The hippocampus is especially important for organizing the relationships among different components of an episode.

Semantic Memory

Semantic memory concerns factual and conceptual knowledge.

The hippocampal system contributes to acquiring new semantic information, while established knowledge becomes represented across distributed cortical networks.

Memory Consolidation

The hippocampus participates in memory consolidation, the processes through which newly acquired memories become stabilized and reorganized over time.

Interactions between hippocampal and distributed cortical networks are particularly important during this process.

Spatial Memory

The hippocampus is strongly associated with spatial memory and navigation.

It interacts with entorhinal, parahippocampal, retrosplenial, thalamic, and parietal systems that represent spatial relationships and environmental context.

Place-Related Neural Activity

Neurons within hippocampal circuits can show activity related to an individual's location within an environment.

These spatial representations interact with entorhinal and other navigation-related systems to support memory for places and routes.

Contextual Processing

The hippocampus contributes to representing the context in which an event occurs.

This contextual information can influence memory retrieval, emotional responses, and behavioral decisions.

Pattern Separation

Hippocampal circuitry, particularly networks involving the dentate gyrus, contributes to distinguishing between similar patterns of incoming information.

This process helps reduce interference between experiences that share overlapping features.

Pattern Completion

Recurrent circuitry, particularly within CA3-related networks, is associated with the ability to reconstruct a broader memory representation from incomplete or partial information.

Pattern separation and pattern completion provide useful functional concepts for understanding how hippocampal circuits may distinguish and retrieve related experiences.

Synaptic Plasticity

The hippocampus has been extensively studied as a model for synaptic plasticity.

Activity-dependent changes in synaptic strength within hippocampal pathways provide important experimental mechanisms for understanding how neural circuits can be modified by experience.

Long-Term Potentiation

Long-term potentiation is a persistent increase in synaptic efficacy following particular patterns of neural activity.

It has been studied extensively in hippocampal pathways, including the Schaffer collateral connection between CA3 and CA1.

Hippocampus and Emotion

The hippocampus is not primarily an emotional response center, but it strongly influences emotional processing through its representation of memory and context.

Its interactions with the amygdala and prefrontal cortex help determine whether an emotional response is appropriate to the current environment.

Hippocampus and Stress

The hippocampal formation interacts with hypothalamic and limbic systems involved in physiological responses to stress.

Prolonged or severe disturbances affecting these networks can influence hippocampal function and memory processing.

Hippocampus Compared with Amygdala

Feature	Hippocampus	Amygdala
Location	Medial temporal lobe along temporal horn	Anteromedial temporal lobe
Major association	Declarative memory and context	Emotional salience and learning
Major output pathway	Fornix	Stria terminalis and ventral amygdalofugal pathway
Important cortical interface	Entorhinal cortex	Prefrontal and sensory association cortex
Shared function	Interaction of memory, context, emotion and behavior

Blood Supply

The hippocampus receives arterial supply predominantly from branches of the posterior cerebral artery, including hippocampal branches.

The anterior choroidal artery may also contribute, particularly to portions of the anterior hippocampal region. Vascular patterns vary among individuals.

Venous Drainage

Venous drainage from the hippocampal and medial temporal region occurs through deep and basal cerebral venous channels.

These veins ultimately communicate with larger deep venous systems and basal venous pathways.

MRI Appearance

The hippocampus is well visualized with high-resolution magnetic resonance imaging, especially on coronal images obtained perpendicular to its long axis.

MRI assessment commonly evaluates hippocampal size, internal architecture, signal characteristics, symmetry, and relationships with the temporal horn and surrounding medial temporal structures.

Clinical Significance

Hippocampal Lesions

Bilateral hippocampal damage can severely impair the ability to form new declarative memories.

Other forms of learning, including some procedural skills, may remain relatively preserved because they depend on different neural systems.

Anterograde Amnesia

Anterograde amnesia is difficulty forming new memories after the onset of neurological injury or disease.

It is a characteristic consequence of significant bilateral dysfunction involving the hippocampal formation or connected medial temporal and diencephalic memory systems.

Retrograde Amnesia

Retrograde amnesia refers to loss of memories formed before an injury.

Hippocampal lesions can produce temporally graded retrograde memory impairment, although the pattern depends on lesion extent and involvement of broader cortical memory networks.

Hypoxic-Ischemic Injury

Hippocampal neurons, particularly neurons within CA1, are vulnerable to severe hypoxic and ischemic injury.

Bilateral hippocampal injury following oxygen deprivation can produce prominent memory impairment.

Temporal Lobe Epilepsy

The hippocampus is frequently involved in mesial temporal lobe epilepsy.

Seizures may originate from or propagate through hippocampal and neighboring medial temporal circuits.

Hippocampal Sclerosis

Hippocampal sclerosis is characterized by neuronal loss and gliosis within the hippocampal formation and is strongly associated with mesial temporal lobe epilepsy.

MRI may demonstrate hippocampal volume loss, altered signal intensity, and loss of normal internal architecture.

Temporal Lobectomy and Amygdalohippocampectomy

Surgical treatment for selected cases of medically refractory temporal lobe epilepsy may involve resection of portions of the anterior temporal lobe or targeted removal of amygdalar and hippocampal structures.

Preoperative evaluation of memory function and hemispheric specialization is important because medial temporal surgery can affect memory.

Neurodegenerative Disease

The hippocampal formation and entorhinal region can be affected early in neurodegenerative disorders associated with progressive memory impairment.

As pathology extends through broader cortical networks, additional cognitive functions become involved.

Transient Global Amnesia

Transient global amnesia is a clinical syndrome characterized by sudden temporary impairment of new memory formation with repetitive questioning and relative preservation of other neurological functions.

Small transient abnormalities involving hippocampal regions, particularly CA1-related areas, may be demonstrated on diffusion-weighted MRI in some patients.

Surgical Anatomy

The hippocampus is an important landmark in medial temporal and ventricular neurosurgery.

Its relationships with the temporal horn, choroidal fissure, fimbria, amygdala, parahippocampal region, cerebral vessels, and optic pathways are particularly important during surgical approaches to the medial temporal lobe.

Key Anatomical Features

Feature	Key Point
Location	Medial temporal lobe
Ventricular relationship	Forms part of floor of temporal horn
Main subdivisions	CA1, CA2, CA3 and CA4 fields
Associated structures	Dentate gyrus and subiculum
Major cortical interface	Entorhinal cortex
Major input pathway	Perforant pathway
Major output pathway	Alveus, fimbria and fornix
Major function	Formation of declarative and episodic memories
Spatial function	Navigation and contextual representation
Clinically vulnerable field	CA1

Anatomical Importance

The hippocampus is a central structure within the neural systems responsible for memory, context, and spatial representation. Its position in the medial temporal lobe allows it to receive highly processed information from widespread association cortex through the entorhinal region.

Within the hippocampal formation, information passes through interconnected dentate, CA, and subicular circuits. Outputs return toward cortical networks and also travel through the alveus, fimbria, and fornix toward septal and hypothalamic structures.

This organization allows the hippocampus to bind distributed components of experience into coherent memories and relate those memories to spatial and contextual information. Its anatomy also explains why bilateral hippocampal injury can cause profound memory impairment even when perception, movement, and many other cognitive abilities remain relatively preserved.