He hippocampus Is one of the brain structures whose functions are most important for humans. It is a highly strategic region because of its location in the brain.
The cerebral hippocampus is located in the Temporal lobe (One of the upper cerebral structures) but also forms part of the limbic system And is involved in functions of the lower structures.
Nowadays it is well documented that the main functions that it performs are related to the cognitive processes.
In fact, the hippocampus is globally recognized as the main structure of the memory .
However, it has been shown how this region performs two more activities apart from the memorization processes: inhibition of behavior and spatial orientation.
History of the hippocampus
The hippocampus, from the Latin hippocampus, was discovered in the XVII century by the anatomist Giulio Cesare Aranzio.
It owes its name to the appearance of its structure, which resembles the shape of the seahorse, hippocampus.
At first there was some controversy about the anatomy of this region of the brain And were given different names like"silkworm"or"sheep's antler".
Likewise, two distinct regions of the hippocampus were proposed:"major hippocampus"and"minor hippocampus".
At present, this subdivision of the hippocampus has been discarded and this is cataloged as a unique structure.
On the other hand, in its discovery, the hippocampus was related to the sense of smell, and it was defended that this brain structure was in charge of processing and recording the olfactory stimuli.
In fact, it was not until the year 1900 that, with the help of Vladimir Béjterev, the actual functioning of the structure was demonstrated and the memory functions performed by the hippocampus began to be investigated.
Anatomy of the hippocampus
The hippocampus is a brain region that is located at the end of the cortex.
Specifically, it deals with an area where the cortex narrows into a single layer of densely packed neurons.
Thus, the hippocampus is a small S-shaped region located at the lower border of the cerebral cortex , And comprising ventral and dorsal portions.
Due to its location, it is part of the limbic system, that is, of the group of regions that are in the region that borders the cerebral cortex, and exchanges information with different brain regions.
On the one hand, the major source of hippocampal afferents is the entorhinal cortex and is strongly connected to a large number of regions of the cerebral cortex.
Specifically, it appears that the hippocampus has a close relationship with the prefrontal cortex and the lateral septal area.
The connection of the hippocampus to these areas of the cortex explains much of the cognitive processes and memory functions of the structure.
On the other hand, the hippocampus is also connected to the lower regions of the brain.
In this sense, it has been shown how this region receives modulating inputs from the serotonergic, dopaminergic and norepinephrine systems, and is strongly connected to the thalamus.
Physiology of the hippocampus
The hippocampus works by two modes of activity, each with a different pattern of functioning and involving a specific group of neurons.
These two modes of activity are theta waves and major patterns of irregular activity (LIA).
Theta waves appear during alert and activity states, as well as during REM sleep phase .
During this time, ie when awake or in the REM sleep phase, the hippocampus works by long, irregular waves produced by pyramidal neurons and granulosa cells.
For its part, the LIA appears during sleep (except in the REM phase) and in moments of immobility (when we eat and rest).
Likewise, it seems that angular slow waves are the ones that memory .
In this way, the moments of rest would be keys so that the hippocampus can store and retain the information in its cerebral structures.
Functions of the hippocampus
As we have commented, the initial hypothesis that the hippocampus performed functions related to the sense of smell has been out of place.
In fact, the falsehood of this possible function of the hippocampus was demonstrated and has been evidenced as, although this region receives direct afferences of the Olfactory bulb , Does not participate in the sensitive operation.
Over the years, the functioning of the hippocampus was related to the performance of cognitive functions.
At present, the functionality of this region focuses on three main aspects: inhibition, memory and space.
The first of these emerged in the 1960s through O'keefe and Nadel's theory of inhibition of behavior.
In this sense, hyperactivity and the difficulty of inhibition observed in animals with lesions in the hippocampus developed this theoretical line and related hippocampal functioning and behavioral inhibition.
As regards memory, it began to be related to the famous article by Scoville and Brenda Milner, which described how the surgical destruction of the hippocampus in a patient with epilepsy caused anterograde amnesia and a very serious Retrograde amnesia .
The third and final function of the hippocampus was initiated by Tolman's theories of"cognitive maps"and O'Keefe's discovery that neurons in the hippocampus of rats appeared to exhibit an activity related to location and spatial location.
Hippocampus and inhibition
The discovery of the role of the hippocampus in behavioral inhibition is fairly recent. In fact, this function is still under investigation.
In this regard, recent studies have focused on examining a specific region of the hippocampus called the ventral hippocampus.
In the investigation of this small region, it has been postulated that the hippocampus could play an important role both in behavioral inhibition and in the development of anxiety.
The most important study on these functions were made a few years ago by Joshua A. Gordon.
The author recorded the electrical activity of the ventral hippocampus and the medial prefrontal cortex in mice by exploring different environments, some of which elicited anxiety responses to the animals.
The study focused on seeking synchronization of brain activity between regions of the brain, as this factor constitutes a singo of information transfer.
As the hippocampus and prefrontal cortex are connected, synchronization became apparent in all settings in what was exposed to mice.
However, in situations that produced anxiety to animals, it was observed that synchronization between the two parts of the brain increased.
Also, it was demonstrated how the prefrontal cortex experienced an increase in theta rhythm activity when the mice were in environments that produced responses of fear or anxiety.
This increase in theta activity was associated with a marked decrease in the exploratory behavior of the mice, so it was concluded that the hippocampus is the region responsible for transmitting the information necessary to inhibit certain behaviors.
Hippocampus and memory
Unlike the role of the hippocampus in inhibition, there is now a high scientific consensus that this region is a vital structure for the functioning and development of memory.
Mainly, it is defended that the hippocampus is the brain structure that allows the formation of new memories of the experienced events, both episodic and autobiographical.
In this way, we conclude that the hippocampus is the area of the brain that allows learning and retention of information.
These hypotheses have been amply demonstrated by multiple neuroscientific investigations, and especially by the symptomatology that produces lesions in the hippocampus.
In this sense, it has been shown how severe injuries in this region cause profound difficulties in the formation of new memories and often also affects the memories formed before the injury.
However, the main role of the hippocampus in memory lies more in learning than in retrieving previously stored information.
In fact, it is maintained that when people form a memory, it is first stored in the hippocampus but over time the information accesses other regions of the temporal cortex.
Likewise, the hippocampus does not seem to be an important structure in the learning of motor or cognitive competences (how to play an instrument or solve logical puzzles).
This fact reveals the presence of different types of memory, which are governed by different brain regions, so that the hippocampus does not cover all the mnesic processes in full but a good part of them.
Hippocampus and spatial orientation
Certain research in rat brains has shown that the hippocampus contains a number of neurons that have"field of place."
This means that a group of hippocampal neurons trigger action potentials (transmit information) when the animal passes through a specific site in its environment.
Also, Edmund Rolls described how certain neurons of the hippocampus are activated when the animal focuses its eyes on certain aspects of its surroundings.
Thus, studies with rodents have revealed that the hippocampus could be a vital region in the development of guiding ability and spatial memory.
In humans the data are much more limited due to the difficulties that this kind of research has.
However,"place neurons"were also found in subjects with epilepsy Who underwent an invasive procedure to locate the source of their attacks.
In the study electrodes were placed in the hippocampus of the individuals and later they were asked to use a computer to move in a virtual environment that represented a city.
Hippocampus and related diseases
As we have seen, lesions in the hippocampus produce a number of symptoms, most of them related to memory loss and above all to decreased learning ability.
However, memory problems caused by severe injuries are not the only diseases related to the hippocampus.
In fact, 4 major diseases seem to have some kind of linkage to the functioning of this brain region. These are:
Brain degeneration
Alzheimer's patient's brain.
Both the normal and pathological aging of the brain appear to be closely related to the hippocampus.
Thus, memory problems related to age or the decline in cognitive abilities experienced during old age are related to a decrease in the hippocampal neuronal population.
This relationship becomes much more noticeable in neurodegenerative diseases such as Alzheimer's , In which a massive death of the neurons of this cerebral region is observed.
Stress
The hippocampus contains high levels of mineralocorticoid receptors, making this region very vulnerable to stress.
Stress can affect the hippocampus by reducing excitability, inhibiting genesis and producing atrophies of some of its Neurons .
These factors explain the cognitive problems or memory failures that we may experience when we are stressed, and become especially noticeable among people who suffer from Posttraumatic stress disorder .
Epilepsy
The hippocampus is often the focus of epileptic seizures. Hippocampal sclerosis is the most commonly seen type of tissue damage in temporal lobe epilepsy.
However, it is unclear whether epilepsy is due to abnormalities in the functioning of the hippocampus or if epileptic seizures produce abnormalities in the hippocampus.
Schizophrenia
Schizophrenia Is a neurodevelopmental disease that involves the presence of numerous abnormalities in the brain structure.
The region most associated with the disease is the cerebral cortex; however, the hippocampus could also be important, as it has been shown how many subjects with schizophrenia show a noticeable decrease in the size of this region.
References
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