hippocampi

Hippocampi, the plural form of hippocampus, are bilateral, curved structures located in the medial temporal lobes of the brain. Named for their seahorse-like shape, they are a central component of the limbic system and play a key role in learning and memory.

Anatomically, the hippocampus comprises the hippocampus proper (the cornu ammonis CA1–CA4), the dentate gyrus, and the subiculum. It begins at the anterior part of the medial temporal lobe and extends posteriorly in a curved arc, corresponding to the hippocampal formation.

Information reaches the hippocampus primarily from the entorhinal cortex via the perforant pathway. Within the circuitry, the dentate gyrus projects to CA3, which connects to CA1; these regions together form the trisynaptic circuit involved in encoding. Output is transmitted mainly through the subiculum and the fornix to mammillary bodies and other limbic and cortical targets, enabling communication with widespread brain networks.

Functions include encoding and consolidation of declarative (episodic and semantic) memories, spatial navigation, and contextual processing. The dentate gyrus is thought to support pattern separation, while CA3 and CA1 contribute to pattern completion and rapid memory formation. The hippocampus also participates in imagining future events and flexible cognition.

Clinical relevance includes hippocampal atrophy in Alzheimer’s disease, temporal lobe epilepsy, and post-traumatic stress disorder. It is sensitive to stress hormones, and chronic stress can affect its structure and function. Adult neurogenesis persists in the dentate gyrus and is a focus of research on learning and mood regulation.