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Hippocampus: Anatomy and functions

Recommended video: Hippocampus and fornix [12:00]
Hippocampus, fornix and neighbouring structures.

The hippocampus is a paired structure present in each temporal lobe of the brain. It takes its name from the Greek word for the seahorse, because it resembles this small upright-swimming fish.

The hippocampus is part of a larger structure of the temporal lobe called the hippocampal formation. The hippocampal formation extends from the amygdala anteriorly, to the splenium of the corpus callosum posteriorly. The hippocampal formation is a major part of the limbic system and it is composed of three main parts:

  • The hippocampus (also known as hippocampus proper or Ammon’s horn),
  • The dentate gyrus,
  • The subiculum.

The elongated hippocampal structures lie along the longitudinal axis of the brain and form the floor and part of the medial wall of the inferior horns of the lateral ventricles. The hippocampus has many functions, but it is best known for its role in learning and memory

Key facts about the hippocampus
Layers Molecular, pyramidal and polymorphic layers
Fields CA1, CA2, CA3, CA4 
Afferent pathways Entorhinal cortex, septal area, prefrontal cortex, anterior cingulate gyrus, premammillary region, reticular formation
Efferent pathways Septal area (precommissural fornix), anterior thalamic nucleus, hypothalamic mammillary bodies (postcommissural fornix), entorhinal cortex, cingulate cortex, prefrontal cortex, contralateral hippocampus
Functions Learning, memory, aggression, rage, hormone regulation
Contents
  1. Anatomy
  2. Internal structure
  3. Dentate gyrus
  4. Subicular cortex 
  5. Hippocampal connections: Ins and outs!
    1. Input to the hippocampus
    2. Output from the hippocampus
  6. Functions 
  7. Clinical correlations
    1. Temporal lobe epilepsy
    2. Rabies encephalitis
    3. Global cerebral ischemia 
    4. Alzheimer’s dementia
    5. Korsakoff syndrome 
  8. Sources
+ Show all

Anatomy

The hippocampus is an elongated convex structure bulging in the floor and medial wall of the temporal horn of the lateral ventricle. It is about 5 cm long and widest at its anterior extent where it bends toward the medial aspect of the brain. The hippocampus proper is also called 'Cornu ammonis' which means the horn of Ammon, the ancient Egyptian god.

The hippocampus extends from the amygdala anteriorly, and then tapers as it courses posteriorly. Some authors divide its surface into three parts: head, body and tail. The head of the hippocampus containts several grooves that resemble a paw, and is thus referred to as the pes hippocampus (L. pes, “foot”). 

The ventricular surface of the hippocampus is called the alveus. The alveus is a thin sheet of white matter formed by the axons of the hippocampal pyramidal cells. It is covered by the layer of ependymal cells. The fibers of the alveus converge at the medial margin of the hippocampus to form fimbria hippocampi which in turn gives rise to the the fornix.  

When viewed in coronal section, the hippocampus is subdivided into four zones designated CA1-CA3 (CA - Cornu ammonis). The initial division also included the field CA4, but nowdays this field is considered a part of the dentate gyrus. 

The CA1 field, also known as Sommer’s sector, contains the pyramidal cells located closest to the subiculum, whereas the two other fields CA2 and CA3 are closer to the surface. One special feature of the CA3 field to note is that the collaterals of the axonal processes extending from the CA3 pyramidal cells are known as recurrent or Schaffer collaterals, and these fibers actually project back to the CA1 field.

Internal structure

The internal hippocampus consists of archicortex. Note that archicortex has fewer cortical layers than both the neocortex (which has six layers), and the paleocortex (which has either four or five).

The archicortex of hippocampus is comprised primarily of pyramidal cells. Like all cells, pyramidal cells have afferent processes (dendrites) and efferent processes (axons). It should be noted that the dendrites of a pyramidal cell extend from both the apex and base. The basal dendrites extend toward the surface of the lateral ventricles; while the apical dendrites extend away from the lateral ventricles and toward the dentate gyrus.

The axons of pyramidal cells take information received by the hippocampus and send it to other structures in the brain; these efferent processes extend from the pyramidal cell body, travel through a structure called the alveus fiber layer next to the inferior horn of the lateral ventricle and then enter into either the entorhinal cortex or the fimbria-fornix.

The archicortex of hippocampus has four main layers:

  • The lacunar-molecular layer is the deepest layer of the hippocampal archicortex. This layer is mainly composed of interneurons. 
  • The radiate layer is mainly composed of the dendrites of the pyramidal cells and stellate cells.
  • The pyramidal layer is the thickest and the most important layer of the hippocampus. It is composed of densly packed pyramidal neurons. The pyramidal layer merges with the internal pyramidal layer of the neocortex.
  • The oriens layer is the most superficial layer of the hippocampus, situated just inferior to the alveus. It is composed mainly of basket-cell interneurons and shares many structural characteristics with the deepest layer of the neocortex. 

Note that some anatomy textbooks use older classification of the hippocampal layers, which divide the hippocampal structure into three following layers: molecular, pyramidal and polymorphic. 

Dentate gyrus

The dentate gyrus is a band of cortex situated between the upper aspect of the parahippocampal gyrus and
the fimbria hippocampi. The dentate gyrus gets its name from its tooth-like configuration. This configuration is created by numerous blood vessels that pierce the ventricular surface of the hippocampus and dentate gyrus.

Like the hippocampus, the dentate gyrus is also multilayered; but, unlike the hippocampus whose primary cell is the pyramidal cell, the primary cell of the dentate gyrus is the granule cell. Granule cell axons are called mossy fibers, and they synapse with the pyramidal cells in the CA3 field of the hippocampus.

The three layers that comprise the dentate gyrus are (superficial to deep):

  • The molecular layer composed mainly of nerve cell bodies and granule
    cell dendrites; 
  • The intermediate granular layer composed of granule cells, the main cells of the dentate gyrus; 
  • The multiform layer is composed mainly of interneurons. 

Subicular cortex 

The subicular cortex or simply the subiculum is a transitional area between the hippocampus and the entorhinal cortex. The main differentiating feature between the hippocampus and the subicular cortex is that the pyramidal cell layer in the subicular cortex is significantly thicker than it is in the hippocampus.

The main function of the subiculum is to convey information from the pyramidal cells of the hippocampus to the mammillary nuclei of the hypothalamus and the anterior nuclei of the thalamus.

Feeling a bit confused? Why not try testing your understanding with some quiz questions? You can use them to learn the anatomy of the hippocampus from scratch, or to identify holes in your knowledge. 

Hippocampal connections: Ins and outs!

Input to the hippocampus

The entorhinal cortex is an important source of two different groups of afferent fibers delivering information to the hippocampal formation. The lateral perforant pathway arises from the lateral entorhinal cortex and extends into the molecular layer of the hippocampus. The medial perforant pathway arises from the medial entorhinal cortex, extends through the white matter from the subicular cortex, and enters the alveus of the hippocampus. Many of these fibers carry olfactory, visual, and auditory information to the hippocampus. 

The diagonal band of Broca originates from the septal area, and acts as part of a feedback circuit to the hippocampus from the septal area. The other part of this feedback circuit is the precommissural fornix, which allows the septal area to receive feedback from the hippocampus.

The hippocampus also receives afferent input from the prefrontal cortex, anterior cingulate gyrus, and premammillary region of the brain, as well as from monoamine neuronal projections from the reticular formation in the brainstem (specifically from the locus coeruleus, raphe nucleus, and ventral tegmental area). The monoamine pathway in particular plays an essential role in regulating mood. 

Because of these connections, the hippocampal formation is able to respond to changes in activity in the cortex and brainstem and relay this information to the hypothalamus, ultimately adding an emotional or visceral quality to these changes in brain activity.

Output from the hippocampus

The efferent fibers of the hippocampal formation, sending signals from the hippocampus to other parts of the brain, come from the pyramidal cells of the hippocampus and subicular cortex. Fibers from the amygdala, situated anteriorly to the hippocampus, also travel largely in tandem with the hippocampal fibers. These fiber bundles arising from the hippocampus and amygdala pass postero-dorsally along the body of the lateral ventricle, around the posterior part of the thalamus, and then anteriorly along the inferior horn of the lateral ventricle.

The fiber bundle that arises from the hippocampus is called the fornix; it runs just inferiorly to the corpus callosum. The fiber bundle that arises from the amygdala is called the stria terminalis, and it runs parallel and ventromedial to the tail of the caudate nucleus. The fornix and stria terminalis fiber networks eventually terminate in different parts of the hypothalamus and septal area.

Fibers from the fornix that travel rostrally to the anterior commissure are called the precommissural fornix. These originate from both the hippocampus and subicular cortex and terminate in the septal area. The fibers of the precommissural fornix are topographically organized: this means that fibers near the anterior pole of the hippocampal formation project to the lateral region of the lateral septal nucleus, whereas fibers near the posterior part of the hippocampal formation project to more medial parts of the lateral septal nucleus.

Fibers from the fornix that travel ventrally behind the anterior commissure are called the postcommissural fornix. These fibers originate in the subicular cortex and terminate either in the anterior thalamic nucleus or in the mamillary bodies of the hypothalamus. The postcommissural fornix innervates parts of the diencephalon, including the anterior thalamic nucleus, mammillary bodies, and parts of the medial hypothalamus. 

Neurons from the subiculum also send axons to the entorhinal cortex, cingulate cortex, and regions of the prefrontal cortex. The entorhinal cortex subsequently sends axons to the amygdala and parts of the temporal cortex. These networks of connections allow the hippocampal formation to send signals throughout the cerebral cortex, including to regions which receive and process different types of sensory information.

There is also a commissural component of the fornix: the purpose of this part of the fornix is to connect the hippocampi on either side of the brain to each other. The axons that do this arise predominantly from the CA3-CA4 parts of the hippocampus and synapse in the contralateral hippocampus. It is theorized that these connective fibers may provide the pathway by which seizures spread from their primary epileptogenic focus in one hippocampus to the contralateral hippocampus, ultimately allowing for a secondary epileptogenic focus to form in the contralateral hippocampus.

Functions 

Although it’s quite a small structure in the brain, size can be deceptive! The hippocampus plays a number of crucial roles, including regulating emotions, motivation, hormonal activity, autonomic activity, and memory formation.

Probably the most well recognized function of the hippocampus is its role in learning and memory. Although the exact mechanisms remain somewhat mysterious, it is believed that the hippocampus receives and consolidates information, allowing for establishment of long-term memories in a process known as long-term potentiation (LTP). It also plays a role in spatial memory, allowing us to keep track of where things are, as well as where they are in relation to each other; as such, it is instrumental in the formation of cognitive maps.

There have been numerous reports linking tumors, lesions, and epileptogenic activity in humans within the hippocampus to aggressive reactions, ranging from minor hostility to explosive acts of violence. The hippocampus’ role in mediating aggression and rage appears to be dependant on the region of the structure that is stimulated: activation of the temporal pole, i.e. the region closest to the amygdala, stimulates predatory or fight behavior; while activation of the region closest to the septal pole instead suppresses those impulses. 

Given the numerous synaptic connections between the hypothalamus and hippocampus, it is not surprising that the hippocampus is also involved in hormone regulation and contributes to various endocrine functions. Ventral aspects of the hippocampus have been found to house dense regions of estradiol-concentrating neurons, as well as high concentrations of corticosterone which inhibits those estradiol-concentrating neurons. 

It has been postulated that the hippocampus may be selectively sensitive to varying hormone levels, playing a role in providing feedback to the pituitary gland via its hypothalamic connections. This is believed to occur indirectly via synaptic relay within the septal area, as well as directly via a  pathway called the medial cortico-hypothalamic tract. The medial cortico-hypothalamic tract arises near the temporal pole of the hippocampus and projects to the ventromedial hypothalamus; it terminates between the suprachiasmatic and arcuate nuclei in a region containing hypophysiotrophic hormones that mediate functions of the anterior pituitary gland. 

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