Hippocampus: Definition, Functions, Importance and Problems

Hippocampus is a brain area that is mostly linked with memory. Hippocampus is a portion of the brain that is located in the inner folds of the temporal lobe, which is the bottom middle section of the brain. The hippocampus was discovered by humans four centuries ago i.e. in the 16th century and is today one of the most studied sections of the brain. Julius Caesar Aranzi, a Venetian anatomist, described it as a ridge along the floor of the temporal horn of the lateral ventricle and likened it first to a silkworm and later, to a seahorse. The hippocampus has a form similar to that of a seahorse. The discovery of the hippocampus has aided researchers in their understanding of how memory works.

Key Terms: Hippocampus, Brain, Anatomy of Hippocampus, memory, Functions of Hippocampus, Hippocampal Impairment

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Functions of Hippocampus

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The following are some of the most important functions of hippocampus:

• The limbic system includes the hippocampus, which is linked to the functions of sensation, expression, and response.
• Space and memory are the two most prominent hypotheses for hippocampus function. The seminal discovery in 1971 of cells in the hippocampus that fired bursts of action potentials when a rat passed certain locations in space, or "place fields," backed up the spatial hypothesis. This suggested that the hippocampus was a component in the brain that helped it map out the layouts of its surroundings.
• These components aid in the regulation of various body activities, including the endocrine system. This is generally referred to as the ‘fight or flight’ response.
• The limbic system is located on the cortex's border. It primarily consists of the hypothalamus (a small gland located near the brain's base) and amygdala.

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Importance of Hippocampus in Memory

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Hippocampus aids in the processing and retrieval of two types of memory i.e. spatial connections and declarative memories. Pathways or routes are stored in the spatial memory. When a cab driver learns a route through a metropolis, for example, they use spatial memory. The right hippocampus appears to be where spatial relationship memories are stored and maintained. Memories of facts and events are better known as declarative memories. Learning how to memorise stories or lines in a play is an example.

The hippocampus, according to psychologists and neuroscientists, is essential for the creation of new memories about previously experienced events. Hippocampal participation in the identification of new places and stimuli is a part of this function. The emotional context from the amygdala is also encoded in the hippocampus, which is why revisiting to a place where an emotional event occurred might trigger that emotional feeling. Between episodic memories and human locations, there is a strong emotional bond.

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Anatomy of Hippocampus

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The anatomy of the hippocampus is crucial for it to operate properly. The entorhinal cortex is a region that receives input and immediately sends output to the brain. The entorhinal cortex is a subregion of the hippocampus that includes the cornu ammonis (CA1–4), the dentate gyrus, and the subiculum. It is positioned beneath the anterior region of the hippocampus.

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Input Sources to Hippocampus

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Modulatory neurotransmitter system, which includes the serotonin, norepinephrine, and dopamine systems, provides input to the hippocampus. The medial septum provides cholinergic input to the hippocampus, which modulates its physiological state. The medial septum is linked to the theta rhythms, which are essential oscillatory rhythms in the hippocampus. The hippocampus function is linked to this rhythm. 

Brain possesses a hippocampus in each of the cerebral hemispheres due to bilateral symmetry; if the hippocampus in one hemisphere is injured, the structure in the other hemisphere remains intact. Impairment to the hippocampus in both hemispheres causes significant difficulty in creating new memories, as well as affecting memories created prior to the damage. 

Only in rare situations do older memories of the brain survive the retrograde effect, which usually lasts several years before the brain is harmed. The assumption that older memories are transferred out of the hippocampus to other sections of the brain over time is based on the retention of older memories in the brain.

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Hippocampal Impairment

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The case of patient H.M. (Henry Molaison), who underwent surgical removal of parts of the medial temporal lobes in 1953, including the hippocampus and adjacent regions, in an effort to revive him of epileptic seizures, was one of the early studies of hippocampal dysfunction. His cognitive functions were essentially intact after the surgery, but he lost the ability to form new memories for facts and experiences, a condition called anterograde amnesia. 

Amnesia like this is common in illnesses involving hippocampus dysfunction. One of the very first parts of the brain to be impacted by Alzheimer's disease is the hippocampus. When a person's short-term memory begins to fail, this may be an early symptom of Alzheimer's disease. The hippocampus suffers significant cell death in Alzheimer's disease, which is linked to memory loss in the early stages of Alzheimer. Stress and depression are connected to a decrease in the ability to create new cells in the dentate gyrus. It reduces dendritic spines and branching throughout the hippocampus.

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What can go Wrong?

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A person's memory and ability to generate new, long-term memories can be affected if one or both sections of the hippocampus are injured by illnesses such as Alzheimer's disease or injuries suffered in an accident. Patients may be unable to recall events that occurred just before the hippocampus loss, but they may recall events that occurred more recently. 

This is because, after long-term memories are formed, they are stored in a different area of the brain. Transient global amnesia is a type of memory loss that occurs unexpectedly, presumably on its own, and then recovers swiftly. Most patients with transient global amnesia regain their memories eventually, but the causes of the problem and how it resolves are unknown. It's possible that hippocampal damage is involved.

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Things to Remember

• The word hippocampus comes from the Greek hippokampus (hippos, which means "horse," and kampos, which means "sea monster"). Julius Caesar Aranzi, a Venetian anatomist, first mentioned the hippocampus in 1587. He described it as a ridge along the floor of the temporal horn of the lateral ventricle and likened it first to a silkworm and later, to a seahorse.
• The hippocampus is regarded to be primarily responsible for storing long-term memories and making those memories difficult to forget. It also plays a function in navigation and spatial processing.
• The hippocampus aids in the processing and retrieval of two types of memory i.e. spatial connections and declarative memories. Pathways or routes are stored in the spatial memory. Memories of facts and events are better known as declarative memories.
• The entorhinal cortex is a region that receives input and immediately sends output to the brain. The entorhinal cortex is a subregion of the hippocampus that includes the cornu ammonis (CA1–4), the dentate gyrus, and the subiculum. It is positioned beneath the anterior region of the hippocampus.
• The modulatory neurotransmitter system, which includes the serotonin, norepinephrine, and dopamine systems, provides input to the hippocampus. The medial septum provides cholinergic input to the hippocampus, which modulates its physiological state.
• The case of patient H.M. (Henry Molaison), who underwent surgical removal of parts of the medial temporal lobes in 1953, including the hippocampus and adjacent regions, in an effort to revive him of epileptic seizures, was one of the early studies of hippocampal dysfunction.

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