Where is the hippocampus. Emotional and declarative memory

The hippocampus is a paired part of the brain, located in its depths on both sides in the temporal regions.

It was formed at the very beginning of the evolutionary process, but still remains the most mysterious and unexplored area for science.

The hippocampus is an important part of one of the very first and ancient systems of the brain -.

We invite you to familiarize yourself with this important part of the brain.

General information

The hippocampus owes its name to the ancient Greeks - translated from their language, it means "seahorse". The basis for this was the similarity of the outlines of a marine animal and an organ in the human brain.

Apparently, this explains such a variety of functions assigned to it at the time of formation. However, their number has practically not decreased to this day.

The hippocampus has played an important role in mental activity since ancient times. But only modern achievements of science and medicine make it possible to identify new qualities and capabilities of this organ.

Region structure

In the body of the brain, the hippocampus looks like two arcuate structures, consisting of cells tightly fitted to each other. Cells form repeating modules that interact with each other and with other parts of the brain.

These arcs are located symmetrically in the temporal regions of both hemispheres. They are part of the cerebral cortex, or more precisely, its folds. Therefore, there is an extensive connection with various brain departments. This also explains its versatility.

The specific pyramidal cells that form the basis of this part of the brain are arranged in three layers. Each of these layers performs a specific function in common work brain activity.

We can say that a person has two hippocampus: left and right. The interaction between them occurs with the help of commissural nerve fibers. With their help, there is a distribution (and sometimes redistribution) of functions.

In critical cases, the healthy part of the organ can take over the function of the affected one.

In addition, this structure actively interacts with many areas nervous system, and especially powerfully with the associative cortex.

What is responsible for

People have been studying this mysterious organ for centuries. In the beginning, he was assigned the role of being solely responsible for the perception of smells. With the development of scientific and medical research, the functions of the hippocampus have expanded significantly, or rather, radically changed.

The discoveries of recent decades have made it possible to look not only into the human brain, but also into each of its cells. This has changed the way we look at the role of the hippocampus in the body.

Today, the main functions of this body are strongly associated with various types human memory. There are several main areas of responsibility:

Emotional and declarative memory

The hippocampus helps recognize people and objects; navigate events; experience a whole range of emotional feelings associated with them.

Stimulation or damage to these areas can cause the most unexpected behavioral response: a fit of rage, pleasure, lethargy, and others.

Often this provokes the appearance of various hallucinations: auditory, visual, tactile. Moreover, it is impossible to stop them or manage them, even realizing the unreality of what is happening.

Memories and past events are transmitted by the hippocampus to other parts of the brain. There they are stored until they are in demand.

Therefore, in most cases, memories of past years are more distinct. In other words, short-term memory is converted into long-term memory. True, the principle of such a "conversion" has not yet been fully studied.

Spatial orientation

With its help, a person has the ability to physically and emotionally exist in space and interact with the environment. We can say that this body is the internal navigator or compass of a person. Interestingly, people whose profession implies a good orientation (taxi drivers, travelers) have a larger part of the brain compared to other parts.

The ability to neurogenesis

The hippocampus is one of the few areas of the brain that can form new neurons and interneuronal connections. Moreover: this ability lasts all life cycle a healthy organ, unless as a result of any circumstances a malfunction occurs in its work.

The logical continuation of this feature is the leading role of this body in the learning process. When an organ loses this property, a person loses the ability to perceive and retain new information. Therefore, the mental abilities in humans largely depend on the condition and size of the hippocampus.

Pathologies and symptoms of pathologies

Like many other areas of the brain, the hippocampus is a very sensitive organ. Any aggressive changes in a person's lifestyle can affect his functioning. Emerging pathologies, even in the initial stages, have certain symptoms.

If the hippocampus is damaged, amnesia (memory loss) may occur. It can be complete, partial (memory retains fuzzy images and segments of events), temporary. The clinical course of amnesia occurs in two forms:

• anterograde amnesia. Events following the moment of illness disappear from memory. More precisely, the patient cannot reproduce them in a logical sequence. At the same time, everything that preceded this period, the memory retains;
• retrograde amnesia. In this case, everything happens the other way around: the brain fixes the events following the defeat of the hippocampus; everything that happened before is erased from memory. Most often, the hippocampus blocks the periods of time associated with difficult events. This is a kind of protection of the body from traumatic memories of the psyche;

The relationship between pathologies of the hippocampus and many known diseases has been well established. It is still difficult to say what is the cause and what is the effect. But it is already known that among the diseases that cause changes in the hippocampus, or that can affect the work of this organ, include:

• Alzheimer's disease. It is one of the most serious diseases in violation of brain activity. Its progression causes a decrease in individual parts of the brain. The hippocampus loses its capacity to function properly. The first symptoms of Alzheimer's disease are disorientation in space and a decrease in the ability to memorize.
• epilepsy. Medical practice shows that 75% of these patients had pathologies of the hippocampus. Usually they looked like sclerosis of one or two lobes of the organ (unilateral or bilateral sclerosis of the hippocampus). Causes can be: head injury, infection, genetic predisposition.
• stress. The state of prolonged stress is now becoming the norm for many people. The body releases the hormone cortisol in response to stress. It has a devastating effect on many parts of the brain, leading to the death of a certain number of neurons.

Therefore, it is important to understand that maintaining endurance under any circumstances at the same time means maintaining healthy functions for a long time.

Schizophrenia is often diagnosed in patients who have an abnormally small organ in question. It cannot be confidently asserted that there is a dependence of one on the other. But medical statistics show that such a connection does exist.

Normal aging is not necessarily related to disease. But practice shows that most elderly patients have memory problems (as a rule, short-term memory suffers). The reasons are the death of a certain number of neurons or a decrease in the size of the hippocampus.

Not always, and not in everyone, but the natural aging of the body can cause changes in the functioning of the hippocampus.

Obviously, further research by scientists and doctors will reveal many new, perhaps unexpected, properties of this organ. Most likely, this knowledge will allow you to find more effective methods and opportunities in the treatment of these diseases.

One thing is certain: if a person seeks to live a long, fulfilling life, he needs to be attentive to his body. Most likely - he will answer him the same.

hippocampus (hippocampus; Greek, hippokampos, a sea monster with a horse's body and a fish's tail; syn.: ammon's horn, cornu Ammonis) - paired formation - part of the old cerebral cortex; located on the medial wall of the lower horns of the lateral ventricles. G. is the central structure of the limbic system (see).

The question of G.'s functions is very complicated and has not been fully resolved. Previous ideas about G.'s participation in the function of smell as part of the "olfactory brain" (rhinencephalon) are rejected. On the basis of consequences of G.'s removal and stimulation at animals assumptions were made that G. participates in the organization of an orienting reflex and attention, regulation of vegetative reactions, motivations (see) and emotions (see), control of arbitrary movements, mechanisms of memory (see. ) and learning. At the same time, after G.'s removal, the conditioned connections developed before G.'s destruction and the possibility of developing new simple conditioned reflexes remain in animals. However, the formation of more complex forms of behavior (chain, delayed conditioned reflexes, conditioned reflexes for time, complex differentiations, labyrinth skills) is greatly hampered. Particularly affected are the forms of behavior associated with the need for active inhibition - the extinction of the orienting reflex, unreinforced conditioned reflexes. Alteration of previously developed systems of conditional connections becomes impossible. In general, behavior becomes much less flexible, stereotyped, and difficult to adapt to changing environmental conditions.

When stimulated by G. electric shock with a physiologically adequate frequency and strength, it remains so-called. dumb structure. The current intensities, which, when acting on the hypothalamus, cause extensive complexes of somatic and visceral reactions, do not cause any external effects, except for the reaction of "calming" the animal. With an increase in the frequency and strength of the current that irritates G., a wide range of various somatic and vegetative manifestations can be obtained, which, apparently, is a consequence of the spread of convulsive discharges through the system of structures associated with G. or lying close to it, as well as patol, of the state of G. It has been established that G. has the lowest threshold for the occurrence of epileptic discharges in electrical activity, although external manifestations extended convulsive seizures with clonic and tonic phases occur only with a significant increase in the parameters of electrical stimulation. The application of moderate (not causing motor convulsions) stimulation of the brain immediately after the development of the conditioned reflex leads to the disappearance of traces of learning. A similar effect is given by the introduction into G. of a number of pharmacol, substances, in particular anticholinergics.

Thus, the most likely function of G. is to participate in the registration of new information. At the same time, already formed traces of memory are not stored in the G., but the recording of new traces significantly depends on its normal functioning. A number of researchers believe that G. compares newly incoming information with existing traces, on the basis of which the signals to be recorded are identified, and the conditions necessary for the formation of long-term memory are provided.

In phylogenesis, true, relatively differentiated G. first appears in reptiles. Initially, G. is located on the mediodorsal surface of the hemispheres, but with the subsequent development of the neocortex and its commissure (G.'s corpus callosum), it is pushed into the depths of the hemisphere. Part of G. undergoes reduction, turning into G.'s rudiment (indusium griseum). In rodents and carnivores, the G. occupies a dorsoventral position and is accordingly divided into dorsal and ventral sections. With further growth of the neocortex, the dorsal part of the G. is reduced. However, the remaining part of G. is a progressively developing structure. In the course of evolution, there is a qualitative differentiation and a quantitative increase in the number of nerve elements and fibers of the brain and structures directly related to it (in comparison with the nuclei of the thalamus and hypothalamus). The greatest increase in the number of cellular elements of G. (5 times) occurred in humans. In humans, G. occupies a position in the depth of the temporal lobe, where it forms the medial wall of the lower horns of the lateral (lateral) ventricles (Fig. 1). The development of the city is in close connection with the growth of the neocortex ( new cortex), and at each stage of phylogenetic development, the brain receives projections from the regions of the cortex that are higher for a given level of evolution, in particular, in primates and humans, connections come from the frontal lobes and the lower parietal lobule.

Embryology

Embryol, the study shows that the main structural features of G. are detected quite early (in a rabbit - by the end of the 4th week, and in humans - by the 4th month of intrauterine development). However, the bulk of G.'s neurons and especially the dentate fascia is formed postnatally. In the rat, the exit and proliferation of neuroblasts in the brain continue for two weeks of postnatal development, while in the dentate fascia this process does not end at 3 weeks, when the formation of cell layers in the neocortex is already completed. The final differentiation of cellular elements and the cessation of G.'s growth in rodents occurs simultaneously with the neocortex, at 40 days. In humans, the most intensive increase in the mass of fibers of the G.'s fornix, composed by the axons of its cells, occurs at 3-7 years, but the increase also occurs after 12 years.

Morphology

G. of animals and humans is part of a larger area - the hippocampus formation. It includes: the entorhinal region (area entorhinalis), which forms the parahippocampal gyrus of primates (gyrus parahippocampalis), a number of complexly organized transitional areas (parasubiculum, presubiculum and subiculum), as well as the dentate fascia (fascia dentata; its free part, facing the cavity of the ventricle, forms gyrus dentatus). The entorhinal region in animals (field 28) has a complex six-layer structure and is considered as a transition region between the neocortex and the more primitively organized paleocortex (ancient cortex) of the piriform lobe (gyrus piriformis). It is divided into the medial part, the most characteristic feature a cut is the presence of large cells in the II layer, and lateral, where the cells of the II layer are small. In the parasubiculum (field 49), the cell layers present in the entorhinal region expand and merge. The border with the presubiculum (field 27) is very sharp, pyramidal neurocytes (pyramidal neurons) disappear here, which are replaced by granular neurocytes (granular cells). A small additional zone is wedged between the para- and presubiculum (field 29 e, area retrosplenialis e). In the subiculum, large, loosely located pyramidal neurocytes reappear, which, during the transition to G., are collected in a narrow compact layer.

According to gistol, G.'s criteria is divided into a number of fields. S. Ramon y Cajal divided G. into two sections: regio superior (attached to subiculum) and regio inferior (attached to fimbria hippocampi). This classification is applied mainly in neurochem. research. Rose (M. Rose) and I. N. Filimonov divide G. into five fields (hi-h5, starting from subiculum). The most frequently used (Fig. 2) is the division of G into four fields (CA1-CA4), introduced by R. Lorente de No. The field CA1(h1) in a wedge, researches sometimes call Sommer's sector, and other fields - a resistant sector. The correctness of G.'s division into fields according to gistol, criteria is confirmed by the difference in afferent and efferent connections, biochemical, and fiziol, characteristics and different sensitivity to a number of pharmacol, substances and patol, factors. So, in the CA1 field first of all patol are found. changes in anoxia, as well as in Alzheimer's disease (see Alzheimer's disease). Other fields together with a gear fascia degenerate at amaurotic idiocy (see), though Sommer's sector remains almost intact.

The main cellular element of G. are large pyramidal neurocytes, whose bodies form a single dense layer. The processes of these cells are strictly oriented perpendicular to the longitudinal axis of the G. As a result, the following layers are clearly distinguished in the G., corresponding to different levels of branching of their dendritic system (and not the location different types cells, as in the neocortex): alveus, containing mainly myelinated axons of the pyramids (pyramidal neurocytes); stratum oriens, where branching basal dendrites are located; stratum pyramidale, containing the bodies of pyramidal neurocytes; stratum radiatum, where unbranched trunks of apical dendrites pass; stratum molecularelacunosum - area of ​​preterminal and terminal branches of apical dendrites. In the regio inferior, an additional layer is distinguished - the stratum lucidum, where the axons of the dentate fascia end on the proximal segments of the apical dendrites. The rest of the afferent fibers entering the brain also terminate at certain levels of the dendrites of pyramidal cells (pyramidal neurocytes), as a result of which synapses of the same origin are concentrated in narrow zones.

The dentate fascia adjacent to G. in animals consists of a dense layer of granular cells (granular neurocytes). Their axons (mossy fibers) end in giant synapses on the pyramidal cells of the CA3-CA4 fields, without going beyond their side. Thus, the dentate fascia, to which afferents (mainly from the entorhinal cortex) are suitable, is an internal relay structure of the hippocampal formation. In the dentate fascia, 3 layers are distinguished: stratum moleculare, containing dendrites of granular neurocytes; stratum granulosum, containing their bodies, and stratum polymorphe, where polymorphic cells are located and the axons of granular cells pass.

Axons of pyramidal neurocytes of G. leave it, forming a fringe (fimbria hippocampi) and a dorsal arch (fornix dorsalis). G.'s commissural fibers pass through the fimbria, forming G.'s ventral commissure (psalterium ventrale, commissura fornicis, commissura hippocampi, David's lyre). The efferent descending fibers of G. form a compact bundle - the postcommissural fornix (fornix postcommissuralis) and the more diffuse precommissural fornix (fornix precommissuralis). The fibers that make them partially switch in the nuclei of the septum (septum, in humans - septum pellucidum). The postcommissural fornix mainly ends in the medial nuclei of the mastoid, or mamillary, bodies (corpora mamillaria). The subsequent links of this system [the mamillo-thalamic tract - the anterior nuclei of the optic tubercle (thalamus) - the cingulate bundle - the cingulate and entorhinal cortex] form the main limbic circle, or the so-called. Peips circle. The rest of the descending G.'s fibers, partially switching in the lateral preoptic region and the lateral hypothalamus, go to the nonspecific (reticular) structures of the midbrain. Afferent communications to G. ascend from the same departments of a brain of hl. arr. within the medial forebrain bundle. Before entering G., most of these fibers switch to the medial nucleus of the septum (nucleus medialis septi). The second source of afferent connections is the entorhinal cortex.

Physiology

Rice. 3. Electroencephalogram (EEG) of various fields of the hippocampus in a rabbit: at the first application of a sound stimulus (tone), irregular high-amplitude waves recorded in the hippocampus are replaced by a regular low-amplitude sinusoidal rhythm with a frequency of 3-6 Hz (“theta rhythm”); when the stimulus is repeated, the reaction fades away: 1-mark of the action of the stimulus; 2-5-EEG fields CA1, CA2, CA3, CA4 of the hippocampus; I-EEG at the first application of a sound stimulus; II-EEG during the fifth application of a sound stimulus; III-EEG during the fifteenth application of a sound stimulus.

When recording the total electrical activity of G. in animals at rest, irregular high-amplitude waves are recorded, which, under the action of sensory stimuli, are replaced by a special regular sinusoidal rhythm with a frequency of 3-6 Hz (theta rhythm). This rhythm is most clearly expressed in lower mammals (rodents). At higher stages of evolution, the severity of the theta rhythm in G. decreases, but in primates it can also be distinguished by the method of frequency analysis. Theta rhythm can be induced by electrical stimulation of the reticular formation of the midbrain, as well as the hypothalamus. A gradual increase in the frequency or strength of stimulation first causes an increase in the frequency of the theta rhythm (up to 8-10 Hz), and then leads to desynchronization of the activity of the heart. ). The theta rhythm in G. arises both under the action of any new sensory stimuli and during the development of various conditioned connections (regardless of the quality of the reinforcement and the nature of the response). The extinction of the orienting reflex and automation of conditioned connections is accompanied by a decrease in frequency, limitation and suppression of the theta rhythm (Fig. 3). Apparently, the theta rhythm is a special manifestation of the general activation reaction organized through the ascending reticular formation and reflecting an increase in the functional state of the brain, which is necessary for the analysis of new information and the development of new conditional connections.

Registration of the activity of single G. neurons reveals the high reactivity of the pyramidal neurocytes of the CA3-CA4 fields to various sensory stimuli. These cells respond to all stimuli with prolonged tonic reactions. With repeated stimulation, the responses of neurons decrease and even stop, but are restored again when the parameters of the stimulus change. The cells of the MAR field are more selective in relation to active stimuli, and their responses to various stimuli are different. Electrical stimulation of the G.'s communication systems during registration of the activity of its neurons reveals the features of the excitation of this structure. With low-frequency (up to 8 Hz) and high-frequency (over 30-40 Hz) stimulation, G.'s neurons are predominantly inhibited. Active excitation of G.'s neurons occurs only in a narrow frequency range of stimulation (approximately 8–30 Hz). Beyond these limits, G.'s stimulation can be equivalent to its functional shutdown. This phenomenon is called frequency or rhythmic potentiation.

Hippocampal dysfunction

In the clinic, the consequences of a bilateral lesion of G. (with tumors, strokes, "limbic" encephalitis caused by the herpes simplex virus), as well as its surgical removal (when excising the focus of epileptic activity in cases of temporal lobe epilepsy) are expressed in memory impairment. If damage to the hippocampus is not accompanied by general brain disorders and does not affect neighboring structures, there is complete preservation of sensory processes, motor and emotional spheres, intelligence and speech. The skills and knowledge acquired by patients before G.'s defeat remain safe. However, the ability to memorize any new information disappears (anterograde amnesia) and retrograde amnesia appears (see), with which the volume of short-term memory may remain normal, but its transition to long-term does not occur. The observed disturbances do not depend on the sensory modality of the input information (visual, auditory) or on its nature (words, pictures, motor skills). Thus, suffers from the so-called. the general factor of memory is the possibility of the transition of short-term memory into long-term memory. Similar phenomena - a violation of the memorization of the presented material and the forgetting of previous events - are observed in humans during electrical stimulation of the brain. Unilateral damage to the brain does not entail obvious consequences.

If it is necessary to remove the epileptic focus that captures one G., an amytal test is preliminarily carried out to find out if the opposite G. patol has been changed by the process so that convulsive discharges are not detected in it. At the same time, sodium amytal is injected into the G., which is subject to resection, temporarily turning it off, and a memorization test is given; if the memorization is not disturbed, the contralateral G. is preserved and the operation is possible. There are indications that unilateral G. damage in humans also affects memory, although it is more limited and specific - if G. is damaged in the dominant (left) hemisphere, the memorization of verbal material is somewhat worse, and if G. is damaged in the right hemisphere, the ability to memorize non-verbal material decreases (faces, combinations of lines, etc.).

Bibliography: Vinogradova O. S. Hippocampus and memory, M., 1975, bibliogr.; Serkov F. N. To the physiology of the hippocampus, F1zyulogichn. journal, vol. 14, no. 6, p. 830, 1968, bibliogr.; Filimon about in I. N. Comparative anatomy of the cerebral cortex of mammals, Paleocortex, archicortex and interstitial cortex, M., 1949, bibliogr.; Douglas R. J. The hippocampus and behavior, Psychol. Bull., v. 67, p. 416, 1967, bibliogr.; The hippocampus, ed. by R. L. Isaacson a. K. H. Prilram, v. 1-2, N.Y., 1975; KimbleD.P. Hippocampus and internal inhibition, Psychol. Bull., v. 70, p. 285, 1968, bibliogr.; Lorente de No R. Studies on structure of cerebral cortex, continuation of study of ammonic system, J. Psychol. Neurol. (Lpz.), v. 46, p. 113, 1934; Milner B. Disorders of learning and memory after temporal lobe lesions in man, Clin. Neurosurge., v. 19, p. 421, 1972, bibliogr.; Ramon at Caja 1 S. Studies on the cerebral cortex, L., 1955; o h same, The structure of Ammon's horn, Springfield, 1968, bibliogr.

O. S. Vinogradova.

responsible for the consolidation of memories when they move from short-term memory to long-term memory, as well as for the creation of emotions and spatial orientation. The human brain has two hippocampi located in the temporal parts of the hemispheres. The connection between them is maintained with the help of nerve fibers that run in the commissure of the fornix of the brain.

Functions

In the past, scientists have put forward the version that the hippocampus is responsible only for the sense of smell. But Scientific research held modern specialists, proved its important role in the formation of orientation in space, perception and storage of information. Some neurons in the hippocampus, called spatial and cells, help a person or animal to determine in space and its location, to find a short path between two landmarks.

The normal operation of the hippocampus is very important in learning, but it is not the ultimate repository of knowledge. That's what the cerebral cortex is for. In the hippocampus, a memory of recent events is formed, and only after a while - hours, days and weeks - this new information is placed in the cerebral cortex. The structure of this body is heterogeneous, and consists of several specialized departments. Thanks to this, the hippocampus is able to instantly remember various events, but sometimes, in order for this to happen, it is necessary to repeat a particular situation that a person has already encountered.

Biased Decisions

Neuropsychologists from the USA explained the mechanism of decision-making by a person in cases of his encounter with unfamiliar circumstances, when it is not possible to rely on previous experience and calculate the final result of getting out of the current situation. In this case, a person associates the situation with cases that have already happened to him and, based on this, draws conclusions. This affects the adoption of certain decisions to get out of the current situation. The hippocampus plays a major role in making these decisions.

It has already been noted that people who, by the nature of their occupation, often have to deal with memorizing this or that information or finding a way out, for example, mentally plotting a route, have an increase in the hippocampus. In addition, this can be achieved by regular exercises - playing checkers and chess, memorizing poetry or foreign languages, solving crossword puzzles, developing your numerical sense and so on.

Hippocampal damage

In Alzheimer's disease, this organ first of all suffers, which leads to a decrease and loss of memory, disorientation. In addition, damage to the hippocampus can lead to anoxia, encephalitis, or medial temporal lobe epilepsy.

If both hippocampi are damaged, anterograde amnesia can occur, in which a person loses the ability to remember recent events, but his long-term memory does not suffer. He will be able to continue to live, talk, walk, hear - do everything that seems normal and natural, but will not be able to create new memories. That is, the information that he will receive after damage to the hippocampus will seem new to him every time, for example, each visit to the doctor will be like the first for him.

Scientists have proven that the normal functioning of the hippocampus depends on the duration and sleep of a person. A night without rest can affect the memory of information. So, in a person who missed a dream once, the impossibility of perceiving positive words by almost 60% was revealed, while he could not remember negative words by only 19%. From this it follows that the hippocampus of a sleepy person shows little activity.

Is an area in human brain, which is primarily responsible for memory, is part of the limbic system, and is also associated with the regulation of emotional responses. The hippocampus is shaped like a seahorse and is located in the inner part of the temporal region of the brain. The hippocampus is the main part of the brain for storing long-term information. The hippocampus is also believed to be responsible for spatial orientation.

There are two main types of activity in the hippocampus: theta mode and great irregular activity(BNA). Theta modes appear mainly in the state of activity, as well as during REM sleep. In theta modes, the electroencephalogram shows the presence of large waves with a range frequencies from 6 to 9 Hertz. At the same time, the main group of neurons shows sparse activity, i.e. in short periods of time, most cells are inactive, while a small part of the neurons show increased activity. AT this mode an active cell has such activity from half a second to several seconds.

BNA-modes take place in the period long sleep, as well as during a period of calm wakefulness (rest, eating).

The structure of the hippocampus

In man two hippocampi one on each side of the brain. Both hippocampi are interconnected by commissural nerve fibers. The hippocampus consists of densely packed cells in a ribbon-like structure that runs along the medial wall of the inferior horn of the lateral ventricle in an anteroposterior direction. The bulk of the nerve cells of the hippocampus are pyramidal neurons and polymorphic cells. In the dentate gyrus, the main cell type is the granular cells. In addition to these types of cells, the hippocampus contains GABAergic interneurons that are not related to any cell layer. These cells contain various neuropeptides, calcium binding protein and of course the neurotransmitter GABA.

The structure of the hippocampus

The hippocampus is located under the cerebral cortex and consists of two parts: dentate gyrus and Hippocampus. Anatomically, the hippocampus is a development of the cerebral cortex. The structures lining the border of the cerebral cortex are part of the limbic system. The hippocampus is anatomically linked to the parts of the brain responsible for emotional behavior. The hippocampus contains four main zones: CA1, CA2, CA3, CA4.

The entorhinal cortex, located in the parahippocampal gyrus, is considered part of the hippocampus due to its anatomical connections. The entorhinal cortex is carefully interconnected with other parts of the brain. It is also known that the medial septal nucleus, the anterior nuclear complex that combines the nucleus of the thalamus, the supramammary nucleus of the hypothalamus, the raphe nuclei, and the locus coeruleus in the brainstem direct axons to the entorhinal cortex. The main exit path of axons of the entorhinal cortex comes from the large pyramidal cells of layer II, which perforates the subiculum and protrudes densely into the granular cells in the dentate gyrus, the superior dendrites of CA3 receive less dense projections, and the apical dendrites of CA1 receive an even rarer projection. Thus, the pathway uses the entorhinal cortex as the main link between the hippocampus and other parts of the cerebral cortex.

The dentate granulosa cells relay information from the entorhinal cortex on spiny hairs emerging from the proximal apical dendrite of the CA3 pyramidal cells. After that, CA3 axons emerge from the deep part of the cell body and form upward loops to where the apical dendrites are, then all the way back to the deep layers of the entorhinal cortex in the Schaffer collateral, completing the mutual closure. The CA1 area also sends axons back to the entorhinal cortex, but in this case they are rarer than the CA3 outputs.

It should be noted that the flow of information in the hippocampus from the entorhinal cortex is significantly unidirectional with signals that propagate through several densely packed cell layers, first to the dentate gyrus, then to the CA3 layer, then to the CA1 layer, then to the subiculum, and then from the hippocampus to the entorhinal cortex, mainly providing a route for CA3 axons. Each of these layers has a complex internal layout and extensive longitudinal connections. A very important large exit tract leads to the lateral septal zone and to the mammillary body of the hypothalamus.

The hippocampus receives modulating incoming serotonin, dopamine, and norepinephrine pathways, as well as from the thalamic nuclei in the CA1 layer. A very important projection comes from the medial septal zone, sending cholinergic and gabaergic fibers to all parts of the hippocampus. Inputs from the septal zone are essential in controlling the physiological state of the hippocampus. Injuries and disorders in this area can completely stop the theta rhythms of the hippocampus and create serious memory problems.

There are also other compounds in the hippocampus that play a very important role in its functions.. At some distance from the exit to the entorhinal cortex, there are other exits that go to other cortical areas, including the prefrontal cortex. The cortical area adjacent to the hippocampus is called the parahippocampal gyrus or parahippocampus. The parahippocampus includes the entorhinal cortex, the perirhinal cortex, which got its name from proximity with olfactory gyrus. The perirchinal cortex is responsible for visual recognition of complex objects. There is evidence that the parahippocampus performs a memory function separate from the hippocampus itself, since only damage to both the hippocampus and the parahippocampus leads to complete memory loss.

Hippocampal Functions

The very first theories about the role of the hippocampus in human life were that it is responsible for the sense of smell. But anatomical studies have cast doubt on this theory. The fact is that studies have not found a direct connection between the hippocampus and the olfactory bulb. Nevertheless, further studies have shown that the olfactory bulb has some projections to the ventral part of the entorhinal cortex, and the CA1 layer in the ventral part of the hippocampus sends axons to the main olfactory bulb, the anterior olfactory nucleus, and to the primary olfactory cortex of the brain. It still does not rule out certain the role of the hippocampus in olfactory responses, namely in remembering smells, but many experts continue to believe that the main role of the hippocampus is the olfactory function.

The next theory, which is currently the main one, says that the main function of the hippocampus is memory formation. This theory has been repeatedly proven in the course of various observations of people who were subjected to surgical intervention in the hippocampus, or were victims of accidents or diseases that somehow affected the hippocampus. In all cases, persistent memory loss was observed. A famous example of this is the patient Henry Molison, who underwent surgery to remove part of the hippocampus in order to get rid of epileptic seizures. After this operation, Henry began to suffer from retrograde amnesia. He simply stopped remembering the events that took place after the operation, but he perfectly remembered his childhood and everything that happened before the operation.

Neuroscientists and psychologists unanimously agree that the hippocampus plays an important role in the formation of new memories(episodic or autobiographical memory). Some researchers regard the hippocampus as part of the temporal lobe memory system responsible for general declarative memory (memories that can be expressed explicitly in words - including, for example, memory for facts in addition to episodic memory). The hippocampus of each person has a dual structure - it is located in both hemispheres of the brain. If, for example, the hippocampus is damaged in one hemisphere, the brain can retain almost normal memory function.

But if both parts of the hippocampus are damaged, there are serious problems with new memories. At the same time, a person perfectly remembers older events, which indicates that over time, part of the memory passes from the hippocampus to other parts of the brain. It should be noted that damage to the hippocampus does not lead to the loss of opportunities to master certain skills, such as playing a musical instrument. This suggests that such memory depends on other parts of the brain, and not just on the hippocampus.

The studies carried out over the years have also shown that the hippocampus plays an important role in spatial orientation. So it is known that in the hippocampus there are areas of neurons called spatial neurons that are sensitive to certain spatial locations. The hippocampus provides spatial orientation and memorization of certain places in space.

Hippocampal Pathologies

Not only age-related pathologies such as (for which the destruction of the hippocampus is one of the early signs of the disease) have a serious impact on many types of perception, but even normal aging is associated with a gradual decline in certain types of memory, including episodic and short-term memory. Since the hippocampus plays an important role in memory formation, scientists link age-related memory disorders with physical deterioration of the hippocampus. Initial studies found significant loss of neurons in the hippocampus in older people, but new studies have shown that such loss is minimal. Other studies have shown that older people experience significant reduction hippocampus, but again similar studies did not find such a trend.

Especially chronic, it can lead to atrophy of some dendrites in the hippocampus. This is due to the fact that the hippocampus contains a large number of glucocorticoid receptors. because of constant stress steroids caused by it affect the hippocampus in several ways: they reduce the excitability of individual hippocampal neurons, inhibit the process of neurogenesis in the dentate gyrus, and cause atrophy of dendrites in the pyramidal cells of the CA3 zone. Studies have shown that in people who experienced prolonged stress, atrophy of the hippocampus was significantly higher than other areas of the brain. Such n negative processes can lead to depression and even schizophrenia. Hippocampal atrophy has been observed in patients with Cushing's syndrome (high levels of cortisol in the blood).

Epilepsy is often associated with the hippocampus. With epileptic seizures, sclerosis of certain areas of the hippocampus is often observed.

Schizophrenia seen in people with abnormally small hippocampus. But to date, the exact relationship of schizophrenia with the hippocampus has not been established. As a result of sudden stagnation of blood in areas of the brain, acute amnesia can occur, caused by ischemia in the structures of the hippocampus.

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