What is the hippocampus?
Humans have known about the hippocampus for more than 4 centuries. It is one of the most studied parts of the brain.
The name comes from the Greek words hippo, meaning horse, and kampo, meaning monster, as its shape resembles that of a sseahorse
Its main functions involve human learning and memory. Knowing about the hippocampus has helped researchers understand how memory works.
The hippocampus is important for learning and memory.
The hippocampus is part of the limbic system, which is associated with the functions of feeling and reacting.
The limbic system is situated on the edge of the cortex, and it includes the hypothalamus and the amygdala.
These structures help control different bodily functions, such as the endocrine system and what is commonly known as the "fight or flight" reaction.
Hippocampus and memory
The hippocampus helps humans process and retrieve two kinds of memory, declarative memories and spatial relationships.
Declarative memories are those related to facts and events. Examples include learning how to memorize speeches or lines in a play.
Spatial relationship memories involve pathways or routes. For example, when a cab driver learns a route through a city, they use spatial memory. Spatial relationship memories appear to be stored in the right hippocampus.
The hippocampus is also where short-term memories are turned into long-term memories. These are then stored elsewhere in the brain.
Research has shown that nerve cells continue to develop throughout adulthood. The hippocampus is one of the few places in the brain new nerve cells are generated.
What can go wrong?
If one or both parts of the hippocampus are damaged by illnesses such as Alzheimer's disease, or if they are hurt in an accident, the person can experience a loss of memory and a loss of the ability to make new, long-term memories.
They may be unable to remember some things that happened shortly before the hippocampal damage, but they may still remember things that happened longer ago. This is because the long-term memories are stored in another part of the brain, once they become long term.
Transient global amnesia is a specific form of memory loss that develops suddenly, seemingly on its own, and then goes away fairly quickly.
Most people with transient global amnesia eventually regain their memories, but the reasons why the problem occurs and why it resolves are unclear. It may be that damage to the hippocampus is involved.
Damage to the hippocampus can make it hard to remember how to get from one place to another. The person may be able to draw a map of the neighborhood they lived in as children, but find going to a store in a new area can be difficult.
Diseases that affect the hippocampus
The hippocampus is a sensitive part of the brain. A range of conditions can adversely affect it, including long-term exposure to high levels of stress.
Several diseases and factors are known to impair the hippocampus' ability to do its job.
The hippocampus is one of the first areas to be affected by Alzheimer's disease. An early sign of Alzheimer's is when a person begins to lose their short-term memory. They may also find it difficult to follow directions.
As the disease progresses, the hippocampus loses volume, and it becomes harder to function in daily life.
However, it is not clear whether epilepsy is the cause or the result of this damage.
Depression and stress
In people with severe depression, the hippocampus loses volume.
Scientists are unsure whether the small size is the result of depression or if it is a contributing factor. There is evidence that stress has a negative impact on the hippocampus.
What happens if the hippocampus is small?
Alzheimer's disease, depression, and stress appear to be linked to a smaller-sized hippocampus.
In Alzheimer's, the size of the hippocampus can be used to diagnose the progress of the disease.
In people with depression, the hippocampus can shrink by up to 20 percent, according to some researchers.
Reviews of studies have suggested that the hippocampus in people with severe depression may be an average of 10 percent smaller than in those without depression.
Cushing's disease features a number of symptoms that are linked to high levels of cortisol, a hormone produced when people are under stress. One of these symptoms is a reduction in the size of the hippocampus.
A study in monkeys has shown that the size of the hippocampus is 54 percent heritable. However, since the hippocampus continues to produce neurons throughout adult life, the link remains unclear.
It is also unclear whether a small hippocampus is an underlying cause of certain conditions, or whether it is a result.
In 2016, scientists published a review of studies into the effects of exercise on cognitive decline and aging.
Results suggest that exercise in old age may strengthen this structure's ability to generate new nerve cells. This would preserve and potentially improve memory. How this happens remains unclear, however. In addition, a number of variables affect the outcome. More research is needed to confirm any findings.
In August 2017, researchers in Hong Kong published findings suggesting that low-frequency activities in the hippocampus can drive functional connectivity in other parts of the brain. In other words, activity in the hippocampus can affect not only memory and pathfinding, but also functions such as vision, hearing, and touch.
In this sense, the hippocampus could be described as the "heart of the brain."