Hippocampus is a gray matter area of the brain responsible for the consolidation and processing of short term memories so that they can be stored in the long term memory of our brain.
It receives the neuronal connections from the primary sensory areas of the cortex, after which it translates the stimuli and relates them to similar information from past knowledge that has been previously stored in the brain. The information received from the sensory stimuli can be encoded as semantic (meaning), visual (images), as well as acoustic (sound) memories.
After being consolidated, the processed information from the hippocampus is integrated into the long term memory, i.e. the structures of the cerebral cortex.
What Is the Location of the Hippocampus?
Located in the medial temporal lobe, the hippocampus supports the temporal lobe’s functions and processes. Thus, it aids the recognition of sounds, as well as their tones and loudness. In addition, it also supports the process of facial recognition.
Due to its location in the temporal lobe, the hippocampus plays an important role in the process of generating and storing long-term memories, as well as translating and remembering our emotions.
What Is the Structure of the Hippocampus?
The hippocampus is, in fact, a formation composed of two hippocampi, just like most of the paired structures of the cerebral cortex.
There are three interlocking parts of this hippocampal formation:
- The hippocampus proper (also: Amons horn)
- The dentate gyrus
- The subicular cortex
The Hippocampus Proper (also: Amons horn)
Hippocampus proper refers to the very structure of the hippocampus, which can be subdivided into the following structures:
- A large-celled proximal region
- A smaller-celled distal region
The special regions of the hippocampus proper are called fields, and are labeled as:
- CA1
- CA2
- CA3
- CA4
These hippocampal fields support the functional differentiation of the pyramidal cells and their dendrites. The structure of the CA regions of the hippocampal formation can be further divided into distinct layers.
The CA areas of this brain structure are made of pyramidal cells that constitute the following layers of the hippocampus:
- An external plexiform layer – It contains the Alvear neuron pathway comprised of the axons of the pyramidal cells and hippocampal fibers.
- Stratum oriens layer – It comprises both basal dendrites and basket cells.
- Stratum pyramidale (pyramidal cell layer) – This layer contains only pyramidal cells (as the name implies)
- Stratum radiatum layer and stratum lacunosum-moleculare layer – Both layers contain the Perforant neuron pathway, and are comprised of apical dendrites of pyramidal cells and hippocampal afferent fibers.
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- Stratum luciderm layer – It’s the thinnest layer of the hippocampus, comprised of Schaffer collateral and Perforant path fibers.
- Hippocampal sulcus (fissure) – An area of the hippocampus that serves as a border between the C1 field and the dentate gyrus.
The Dentate Gyrus
This part of the hippocampus is composed mainly of granule cells that make a polymorphic cell layer. It also contains mossy fibers of the granule cell axons, that create synapses with the pyramidal cells in the C3 field of the hippocampus.
The dentate gyrus is also comprised of distinct cell layers:
- The polymorphic layer – A layer containing interneurons (neurons that connect and enable communication between two areas of the brain).
- Stratum granulosum – A layer comprised of dentate granule cells.
- Stratum moleculare, inner third – The layer where the contralateral dentate gyrus form synapses.
- Stratum moleculare, external two thirds – The deepest layer, whose cell fibers create synapses with the granule cell dendrites.
The Subicular Cortex
The subicular cortex is a transitional pyramidal cell layer that is thicker than the one in the hippocampus. It functions as a primary source of efferent fibers from the hippocampus.
What Is the Function of the Hippocampus?
This part of the brain is also the primary brain area where the declarative (explicit) memory is formed. The memory is, in fact, an electric impulse that carries the sensory information transmitted between two neuronal cells via their axons.
The neuronal cells form synapses with their axons that relay the signal received from the sensory stimuli to the other information-processing centers of the brain.
The hippocampus is in charge of both spatial recognition, and the merging of both short-term and long-term memories. It’s responsible for the transformation of recently memorized experiences into long-term memories by strengthening the synapses between the neuron cells.
In addition, it relays the information to the other regions of the cerebral cortex which are responsible for giving meaning to the received sensory signals. By conducting complex neurological processes and connecting them with other related memories, this brain structure interacts mostly with the amygdala and the hypothalamus.
How Does the Hippocampus Support Formation and Storage of Memory?
Located in the temporal lobe, the hippocampus (along with the entorhinal cortex and perirhinal cortex), is the place where the declarative (explicit) memories are processed, encoded, translated, recorded, and consolidated in the brain so that they can be retrieved at some future point in time.
However, the memories formed in the manner described earlier are stored and merged in the temporal cortex, as well as in other areas of the brain, because there is no single area of the brain dedicated to storing memories.
The formation of memories is a process comprised of 3 stages:
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The Stage of Encoding of Information
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The Stage of Consolidation of Memory
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The Stage of Recollection of Memory
The Stage of Encoding of Information
The hippocampus is the part of the brain that supports the process of giving meaning to the information gained from the senses.
The hippocampus transfers the information to regions of the cerebral cortex that give meaning to the declarative memories. Then, it connects them with our other memories which are related to them.
In other words, the first stage of the formation of memory is the encoding of information by this structure of the cerebral cortex. After the memories are encoded in these brain structures, they are stored in the short-term memory of the cerebral cortex.
The Stage of Consolidation of Memory
The memories are further consolidated and integrated with the previously stored information. During this process, the brain structures and neural networks strengthen, stabilize, and store the traces of memorized information received via the sensory stimuli. This is done so that they can be easily retrieved at a future point in time.
The Stage of Recollection of Memory
A recollection of memory is nothing but a conscious retrieval of the information gained from the sensory stimuli stored in the brain. Several cues, such as emotions, smell, sound, visual representations, and semantic knowledge, can aid the retrieval of the memory.
How Does the Hippocampus Influence Emotions?
The hippocampus is also a part of the limbic system, the area of the brain responsible for the regulation and control of emotions.
Thus, it not only encodes the emotional context from the structures comprising the limbic system (particularly the amygdala) but also finds a strong emotional connection between the episodic memories and the locations that trigger emotional responses.
More specifically, the hippocampus and the amygdala function in mutual collaboration in order to consolidate out long term memories with the feelings they evoke.
Which Disorders Are Associated with the Hippocampus?
Trauma or any other form of damage inflicted to the hippocampus can result in some of the following medical conditions or diseases:
- Temporal lobe epilepsy
- Encephalitis
- Global cerebral ischemia
- Dementia
The affected individual can suffer from a wide range of symptoms, such as loss of memory, loss of ability to form new long term memories, as well as difficulty in remembering how to get from point A to point B.
To Sum Up
Learning and memory are two of the most distinguished functions performed by the hippocampus. By combining these two functions, this brain structure plays a crucial role in the process of forming, consolidating, and retaining both spatial and declarative memory, i.e. facts that we can consciously recall and use to describe the information processed and stored in our brain.
This structure of the brain is a part of the limbic system which is involved in turning short term memories into long term memories by integrating and transferring the information to the other dedicated areas of the cerebral cortex of the brain.
The hippocampus also supports the translation of emotions and their association with the previously stored information of an emotion-evoking event.
Fun Facts
Did you know?
- The hippocampus was formerly known as Cornu Amonnis, meaning ram’s horn. This is due to the C-shaped structure of this part of the cerebral hemisphere. The name honors the Egyptian deity Amun, who is depicted with the head of a ram.
- The contemporary name of this brain structure, hippocampus, originates from the Greek words ‘hippos’ (meaning: horse) and ‘Campos’ (meaning: sea monster).
- Concentration is one of the most crucial features for the creation of long-term memory.
- Neuroplasticity of the brain is a process that takes place when a change in both the neural pathways and the synapses they make occurs as a result of various factors. The brain removes the synapses which no longer serve a quality function in order to strengthen the important ones.
- Semantic memory is a type of explicit (declarative) memory that stores our general knowledge about the world that surrounds us.
- Episodic memory is a type of explicit (declarative) long-term memory that stores images and our personal feelings related to a past event in our life. It comprises 3 main elements: personal thoughts/feelings, awareness of the world around us, and subjective time. Hence, it’s impossible for the other people involved in the same experience to share an identical memory of it.
- The implicit memory is an unintentional long-term memory. It stores simple facts, our skills, as well as knowledge of how to perform a certain task.
- The C1 field of the hippocampus contributes to the retrieval of contextual memory.
- In the case of inflicted damage to the hippocampus, we might lose our memory, which could lead to the need of learning how to read, write, or speak anew.
- The perforant pathway connects the entorhinal cortex to CA1, CA2, CA3, and CA4 fields of the hippocampal formation.