Hippocampus

The hippocampus is a complex brain structure embedded deep into the temporal lobe, playing a major role in learning and memory^[1]. Being a part of the limbic system, it's involved in many of our emotions and motivations, particularly those associated with survival such as fear and anger^[2]. It is one of the few areas of the brain capable of creating new neurons throughout life, a process known as neurogenesis^[3].

The term "hippocampus" comes from the Greek words "hippos," which means "horse," and "kampos," which means "sea monster." This is because the hippocampus, especially when seen in cross-section, resembles a seahorse.

The hippocampus was named by the anatomist Julius Caesar Aranzi in the late sixteenth century. When he observed it in a dissected brain, he thought its curved shape and two 'horns' (the fimbria and fornix) looked like a seahorse or a silkworm.

The name has stuck and is still used today to refer to this critical structure in the brain.

The hippocampus is primarily located in the medial temporal lobe of the brain, underneath the cortical surface. It forms a C-shape that stretches from near the base of the brain up and around to the center, with one in each hemisphere of the brain^[5].

In terms of connections, the hippocampus receives input from and sends output to various regions of the brain:

1. Input: Major sources of input include the entorhinal cortex (via the perforant path), the septal nuclei, the hypothalamus, and the amygdala. These inputs carry information about the context (including spatial location), emotional state, and physiological state, which contribute to the formation of memories^[8].

2. Output: The major output paths from the hippocampus are to the entorhinal cortex, the hypothalamus, and the septal nuclei. From the entorhinal cortex, signals are sent back to many of the same areas that provided input to the hippocampus. In this way, the hippocampus contributes to the consolidation of information in other brain areas^[9].

The key functions of the hippocampus include:

1. Memory Formation: The hippocampus plays a vital role in transforming short-term memories into long-term memories. This process is known as memory consolidation^[10]. The hippocampus, particularly the area known as CA1, helps in the formation and storage of new memories related to experiences and events, a type of memory known as declarative or explicit memory^[11].

2. Spatial Navigation: The hippocampus plays an essential role in our ability to navigate through space. It is home to "place cells," which activate when we are in specific locations, helping to form cognitive maps of our environment^[12]. Damage to the hippocampus can lead to severe impairments in spatial navigation^[13].

3. Pattern Separation: The dentate gyrus area of the hippocampus is especially important for a process known as pattern separation. This is the ability to separate similar experiences or memories into distinct entities, preventing confusion and promoting the formation of discrete memories^[14].

4. Memory Retrieval: The hippocampus is also instrumental in memory retrieval, particularly for episodic memories - memories of specific events or experiences^[15]. It helps in retrieving and reconstructing these memories when we need them^[16].

5. Spatial and Contextual Memory: The hippocampus helps form memories about the contexts in which events occur, known as contextual memory^[17]. This includes information about the spatial environment (where an event happened), as well as other context cues like time and emotional state^[18].

6. Neurogenesis: The hippocampus, particularly the dentate gyrus, is one of the few areas in the brain where new neurons can be generated throughout adulthood^[19]. This process of neurogenesis is thought to contribute to learning and memory processes and to the regulation of mood^[20]. Certain factors, like stress and aging, can decrease hippocampal neurogenesis, which can lead to memory impairments and increased susceptibility to mood disorders^[21].