Hippocampus

The hippocampus is a major component of the vertebrate brain and an essential structure within the human limbic system. Together with the dentate gyrus and the subiculum, it forms the hippocampal formation, a region crucial for memory consolidation and spatial navigation. Its location in the medial temporal lobe and its distinctive archicortical architecture underpin its role in storing and retrieving information. The hippocampus is among the first brain regions affected in neurodegenerative diseases such as Alzheimer’s disease, contributing to early symptoms of memory loss and spatial disorientation.
The structure is evolutionarily ancient, appearing as the medial pallium in all vertebrates. Despite differences in size across species, the hippocampus maintains a remarkably conserved neural organisation, making it a fundamental model for studying learning, memory and synaptic plasticity.

Historical Background and Nomenclature

The term hippocampus derives from the Ancient Greek words for “horse” (hippos) and “sea monster” (kampos), reflecting its curved, seahorse-like shape when observed in coronal brain sections. Anatomists in the sixteenth and seventeenth centuries offered varied descriptions: Julius Caesar Aranzi compared it to a silkworm and later to a seahorse; Duvernoy illustrated the structure; and Winslow proposed “ram’s horn”, a name echoed in the enduring term cornu Ammonis (Amun’s horn), referencing the Egyptian deity depicted with ram’s horns.
The head of the hippocampus, containing rounded elevations known as digitations, was named the pes hippocampi, meaning “the foot of the seahorse”. Subsequent terms such as hippocampus minor featured in nineteenth-century debates on human evolution, but these were later removed from anatomical nomenclature. Modern usage retains “hippocampus” and the cornu Ammonis subfield terminology (CA1–CA4), now standard in neuroanatomy.

Location and Role in the Limbic System

The hippocampus forms part of the limbic lobe identified by Paul Broca in 1878. This lobe, comprising structures such as the cingulate and parahippocampal gyri, was originally associated with olfaction but later recognised as integral to emotion, memory and motivated behaviours. Through extensive connections with the septal area, hypothalamus, mammillary bodies and anterior thalamic nuclei, the hippocampus participates in circuits that regulate emotional responses as well as learning.
Hippocampal afferent and efferent pathways link the archicortex with the neocortex via direct and indirect projections, allowing information from multiple sensory modalities to be processed, integrated and stored. Its involvement in the Papez circuit and other limbic pathways underpins both declarative memory functions and the emotional context of memories.

Neuroanatomy and Structural Organisation

The hippocampus is a curved ridge of grey matter embedded within the parahippocampal gyrus. It protrudes into the inferior horn of each lateral ventricle and extends from the amygdala anteriorly to the splenium of the corpus callosum posteriorly. Its principal subdivisions include:

• Head, body and tail, arranged along the anterior–posterior axis.
• Dentate gyrus, a C-shaped structure providing granule cell input.
• Cornu Ammonis (CA) fields, comprising CA1, CA2, CA3 and CA4.
• Subicular complex, including the subiculum, presubiculum and parasubiculum.
• Fimbria, a fibre tract leading to the fornix.
• Entorhinal cortex, which serves as the major cortical gateway to the hippocampal formation.

Across species, the hippocampus maintains a three-layered archicortical structure, distinct from the six-layered neocortex. The dentate gyrus contributes to this architecture by forming a rolled-up configuration with densely packed granule cells leading into the pyramidal cell layers of the CA fields. The CA4 region, sometimes described as part of CA3, corresponds to the hilus of the dentate gyrus.
In humans and primates, the hippocampus is typically divided into anterior and posterior portions; rodents use the terms ventral and dorsal, while other mammals use caudal and rostral. Despite differences in terminology, these regions participate in distinct neural circuits that support different aspects of cognition.

Functional Roles in Memory and Cognition

The hippocampus is central to memory consolidation, converting short-term memories into long-term storage. It also enables spatial memory and navigation, allowing individuals to form cognitive maps of their environment. The processes underlying these functions depend heavily on synaptic plasticity, particularly long-term potentiation (LTP), first discovered in the hippocampus. LTP remains a core model for understanding how experience alters synaptic strength to support learning.
Rodent studies have revealed place cells, neurons that fire when an animal enters a specific location. These cells interact with:

• Head direction cells, forming a directional reference network.
• Grid cells in the entorhinal cortex, providing a coordinate-like internal metric.

Together, these networks help orchestrate spatial navigation and episodic memory, offering insights into broader mechanisms of cognitive mapping in mammals.

Vulnerability and Clinical Significance

The hippocampus is particularly vulnerable to damage. In Alzheimer’s disease, its neurons are among the earliest to degenerate, contributing to hallmark symptoms such as short-term memory loss and confusion. Other causes of hippocampal injury include hypoxia, viral encephalitis and temporal lobe epilepsy. Extensive bilateral damage can result in anterograde amnesia, the profound inability to form new memories.
The structured layers and readily identifiable pathways within the hippocampus make it a valuable model for neurophysiological studies. Research on hippocampal circuitry has informed understanding of memory disorders, epileptic activity and age-related cognitive decline.

Comparative Anatomy and Species Variation

The hippocampus appears across all vertebrates, reflecting its deep evolutionary origins. Although its proportion relative to total brain size varies, the basic layout is preserved. Primates have a hippocampus roughly twice the size of that in monotremes such as the echidna, but its growth does not match the dramatic expansion of the neocortex. As a result, rodents possess a larger hippocampal-to-neocortical ratio than primates.
Across species, there is a general correlation between hippocampal volume and spatial memory capabilities. Animals that rely strongly on navigation, such as food-caching species, tend to have larger hippocampi, illustrating the adaptive value of enhanced spatial memory.