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Hippocampus is one of the key parts of our brain. It a horseshoe-shaped structure. It is responsible for the process of transferring the data from the short-term into long-term memory. Besides, it is related to emotions.
This brain structure is a part of the limbic system. The limbic system is the highest part of the subcortical structures responsible for emotions, motivation, instincts, vegetative functions, learning, and memory.
Except for the hippocampus, the limbic system consists of the hypothalamus, septal region, limbic bark, the limbic nuclei, including the amygdala, basal forebrain, septal nuclei, anterior thalamic nuclei, and habenula nuclei, as well as the parts of the brainstem and the limbic pathways.
Moreover, the role of the hippocampus in the limbic system and the nervous system itself is of the utmost importance. First of all, the hippocampus is responsible for the formation, organization, and storage of memories (1).
Secondly, it plays a part in the process of linking the sensations to the memories. It is also related to memory consolidation during sleep.
Position and anatomy of the hippocampus
Both sides of the brain are symmetrical, and the hippocampus is located in both hemispheres (1). Its anatomy is extremely important for its function. Namely, this part of our brain receives input and sends output through the entorhinal cortex. It is a structure positioned beneath the frontal hippocampus region.
This part of the brain consists of
the CA1-4 or the cornu ammonis segment, the subiculum, and the dentate gyrus.
Most importantly, the hippocampus subregions are linked via two neural
circuits. Those are the monosynaptic and the trisynaptic circuit.
The monosynaptic circuit transmits
information from the entorhinal cortex to the CA1 and it bypasses the CA3 and
the dentate gyrus.
On the other hand, the trisynaptic circuit sends the information to the dentate gyrus through the path which perforates through the subiculum. Next, the information travels to CA3 through the mossy fiber pathway.
Several theories aim to prove the range of the hippocampus functions. Two of them are the most prominent and the most likely to be correct. Those are the theories related to memory and space (1).
In 1971, a discovery of the hippocampus cells that burst action potential led to the reinforcement of the spatial theory hypothesis.
Namely, these cells reacted when certain locations in space were hit. These are the so-called "place fields." This discovery led us to the conclusion that the hippocampus scans the space and maps the environment.
As a result, the hippocampus is the key spatial navigation center in our brain.
These are the so-called "place fields." This discovery led us to the conclusion that the hippocampus scans the space and maps the environment. As a result, the hippocampus is the key spatial navigation center in our brain.
On the other hand, the memory theory hypothesis is somewhat older. Namely, it dates back to 1957. After thorough hippocampus removal experiments showed that the hippocampus is a center in the brain responsible for forming new memories, both event, and fact-related ones, this theory became more influential.
Today, scientists agree that the hippocampus is the key brain center for memory. However, we still don't know exactly with a 100% certainty of how memory processes happen. Some scientists believe that it links memories to experiences and that this is the mode of storing them.
Others think that the process resembles a conscious recollection of memories. Some theories say that the hippocampus controls memories through reducing similar experience interference and overlapping, which helps rapid learning processes.
According to another theory, the hippocampus is an index of our brain and experiences. You could think of it as a book index that helps us find the information we are looking for in seconds.
Interestingly, this theory says that the hippocampus stores the elements, not the whole experience. This theory is contrary to the theory that observes it as memory plus experience reinforced by sensations.
Furthermore, scientists disagree about the following issue. Do the long-term memories become independent of this part of the brain or not? Is the cortex itself able to perform the memory recall?
This is the well-known issue of systems consolidation. The leading theory today says that the hippocampus is necessary for the long-term recall of the context-rich memories. At the same time, it is not required for semantic memories.
When it comes to other functions of
the hippocampus, we can say that its orbitofrontal cortex is related to the
regulation of stress and emotions. Moreover, the posterior parietal cortices
are responsible for spatial processing, as well as cognitive processes.
Hippocampus and our Memory
As already mentioned, the hippocampus plays a key role in creating, organizing, and storing new memories, as well as connecting some of the feelings and sensations to those memories (1).
Simply put, a scent or an image, as well as a melody, can often trigger a memory. It is the hippocampus that plays a role in this situation.
Different subregions of the hippocampus play an important role in certain types of memory. For example, the back of the hippocampus is involved in processing spatial memories.
Interestingly, managing complex mazes of a big city, its streets in particular, as well as other similar maze-like, complex patterns, is linked to the growth of the hippocampus posterior region.
The hippocampus also plays a role in consolidating memory during sleep. Studies show that a more active hippocampal movement during sleep after some type of training or learning experience leads to a better memory the next day.
This does not mean that the memories themselves are stored in the hippocampus in the long term. Instead, the hippocampus is believed to act as a form of a delivery center.
Simply put, it receives the information, registers it, and temporarily stores it before delivering it to the long-term memory.
Damage of the hippocampus
If one side of the hippocampus is damaged and destroyed, the memory function will remain almost normal as long as the other side is intact. Interestingly, damage to both sides of the hippocampus can interfere with the ability to form new memories, which is known as anterograde amnesia.
As the hippocampus plays such an important role in the formation of new memories, damage to this part of the brain can have a serious long-term effect on certain types of memory (2).
Hippocampal damage was observed after post-mortem brain analysis of patients who suffered from amnesia. Such damage is related to problems with the formation of memories such as dates, events, or names.
The exact effect of the hippocampus damage may vary depending on the part of the hippocampus that is affected. Research shows that damage to the left hippocampus affects the recall of verbal information, while damage to the right hippocampus leads to problems with visual information.
Also, the function of the hippocampus can decline with age. By the age of 80, people can lose nearly 20 percent of nerve connections in the hippocampus. Not all elderly people show this loss, but those who do, have a worse memory test result.
MRI scans of the human brain revealed that the human hippocampus was reduced by about 13 percent between the ages of 30 and 80. Cell degeneration in the hippocampus has also been linked to the onset of Alzheimer's disease (2).
Finally, it is interesting to address a case dating back to 1953. Namely, this is the year of one of the first hippocampal dysfunction studies which were performed on a patient who suffered from epilepsy.
This patient underwent surgical removal of the hippocampus, as well as the surrounding structures. The result of the surgery was surprising at the time. Namely, the patient kept all of his cognitive functions.
They were almost intact. Unfortunately, he was not able to make new memories. This state is called anterograde amnesia. We now know that this type of amnesia occurs as a result of hippocampal dysfunction.
Let's take a look at what happens in the case of Alzheimer's disease. A significant cell loss in the hippocampus takes place. As a result, the patient suffers from memory deficits.
These issues occur at the very first stages of the disease. Furthermore, the patient loses the ability to produce new cells in the hippocampus which is related to depression. Moreover, it increases stress in the patient and this condition is linked to schizophrenia too.
Consequently, these findings bring the hippocampus and neuropsychiatric disease into relation. It reinforces the vulnerability and sensitivity of the structures of this region of our brain. Moreover, it emphasizes the dangers of lesions, trauma, and injuries of the hippocampus and its segments.
Finally, patients who suffered oxygen deprivation in the brain often suffer from hippocampus damage too. This usually leads to anterograde amnesia. Besides, hippocampus trauma is often related to epileptic seizures, and it is this part of the brain that houses them.
As a result, a condition called hippocampal sclerosis (2) may occur. It is a state of loss of the cells of the hippocampus.
Hippocampus a part of our brain that resembles a horseshoe. It has many important functions. However, the single most important one is a memory. More precisely, it transfers the data from the short-term into long-term memory.
As a result, damage to the hippocampus often results in memory loss, inability to form new memories, and Alzheimer's disease.
- Anand KS, Dhikav V. Hippocampus in health and disease: An overview. Ann Indian Acad Neurol. 2012 Oct;15(4):239-46. doi: 10.4103/0972-2327.104323. PMID: 23349586; PMCID: PMC3548359. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3548359/
- Wible CG. Hippocampal physiology, structure and function and the neuroscience of schizophrenia: a unified account of declarative memory deficits, working memory deficits and schizophrenic symptoms. Behav Sci (Basel). 2013 Jun 21;3(2):298-315. doi: 10.3390/bs3020298. PMID: 25379240; PMCID: PMC4217628. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4217628/