Hippocampus: Anatomy and functions

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!

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.