LONG-TERM MEMORY: TYPES, NEURONAL BASES AND DISORDERS - NEUROPSYCHOLOGY - 2025

The long - term memory is a memory store very durable and seemingly limitless capacity; a long-term memory can last from several hours to several years. As for the physiological plane, this type of memory carries with it a process of physical changes in the structures and connections of neurons, the cells of our brain.

Memories that come to short-term memory can be turned into long-term memories through a process called "consolidation." Repetition, meaningful associations and emotions intervene in it.

Depending on these factors, the memories may be stronger (your date of birth) or weaker or difficult to retrieve (a concept you learned years ago in school).

Short-term memory is generally more acoustic and visual. While in long-term memory, information is encoded, above all, visually and semantically (more linked to associations and meanings).

How is long-term memory formed?

The process of long-term memory in which changes occur in the structures and connections of neurons is known as long-term potentiation (PLP). It implies that, when we learn something, new neural circuits are created, modified, strengthened or weakened.

In other words, there is a neural reorganization that allows us to store new knowledge in our brain. In this way our brain is constantly changing.

Hippocampus

The hippocampus is the brain structure where information is temporarily stored, and serves to consolidate memories from short-term to long-term storage. It is believed that it can participate in the modulation of neural connections for periods of more than 3 months after the first learning.

The hippocampus has connections to multiple areas of the brain. It seems that for the memories to be fixed in our brain, this part of the brain transmits the information to cortical areas where they are stored in a lasting way.

Obviously, if these brain structures were damaged in any way, some form of long-term memory would be impaired. This is what happens in patients with amnesia. In addition, depending on the area of ​​the brain damaged, some types of memory or memories would be affected, but not others.

On the other hand, when we forget something, what happens is that the synaptic connections responsible for that knowledge weaken. Although it can also happen that a new neural network is activated that overlaps the previous one, causing interference.

Therefore, there is a debate about whether we erase information permanently in our memory or not. It may be that the stored data is never completely erased from our long-term memory, but becomes more difficult to retrieve.

Neural bases

For any information to reach long-term memory, it is necessary that a series of neurochemical or morphological changes take place in the brain. Memory has been found to be stored across multiple synapses (connections between neurons). When we learn something, certain synapses get stronger.

On the other hand, when we forget it, they become weak. Thus, our brain is constantly changing, acquiring new information and discarding what is not useful. These synapse gains or losses influence our behavior.

This connectivity is remodeled throughout life thanks to mechanisms of synaptic formation, stabilization and elimination. In short, there are structural reorganizations in neuronal connections.

In research with patients with amnesia, it was shown that short-term and long-term memory were found in different stores, having different neuronal substrates.

Long-term potentiation

As has been discovered, when we are in a learning context, there is an increased release of glutamate. This produces the activation of certain families of receptors, which in turn causes the entry of calcium into the nerve cells involved. Calcium penetrates mainly through a receptor called NMDA.

Once such a high amount of calcium builds up in the cell that it exceeds the threshold, what is known as "long-term potentiation" is triggered. Which means that more lasting learning is taking place.

These calcium levels cause the activation of different kinases: Protein kinase C (PKC), calmodulin kinase (CaMKII), mitogen-activated kinases (MAPK) and Fin tyrosine kinase.

Each of them has different functions, triggering phosphorylation mechanisms. For example, calmodulin kinase (CaMKII) contributes to the insertion of new AMPA receptors into the postsynaptic membrane. This produces a greater strength and stability of the synapses, maintaining learning.

CaMKII also causes changes in the cytoskeleton of neurons, influencing the active one. This results in an increase in the size of the dendritic spine that is linked to a more stable and durable synapse.

On the other hand, protein kinase C (PKC) establishes bridges between presynaptic and postsynaptic cells (Cadherin-N), producing a more stable connection.

In addition, early expression genes involved in protein synthesis will participate. The MAPK (mitogen-activated kinases) pathway is what regulates gene transcription. This would lead to new neural connections.

Thus, while short-term memory involves the modification of existing proteins and changes in the strength of pre-existing synapses, long-term memory requires synthesis of new proteins and growth of new connections.

Thanks to the PKA, MAPK, CREB-1 and CREB-2 pathways, short-term memory becomes long-term memory. This as a result is reflected in changes in the size and shape of the dendritic spines. As well as an enlargement of the terminal button of the neuron.

Traditionally it was thought that these learning mechanisms only occurred in the hippocampus. However, it has been shown in mammals that long-term potentiation can occur in numerous regions such as the cerebellum, the thalamus, or the neocortex.

It has also been found that there are places where there are hardly any NMDA receptors, and even so, long-term potentiation appears.

Long-term depression

Just as memories can be established, other information that is not handled can also be “forgotten”. This process is called "long-term depression" (DLP).

It serves to avoid saturation and occurs when there is activity in the presynaptic neuron, but not in the postsynaptic or vice versa. Or, when the activation is very low intensity. In this way, the structural changes mentioned above are gradually being reversed.

Long-term memory types

There are two types of long-term memory, the explicit or declarative and the implicit or non-declarative.

Declarative or explicit memory

Declarative memory encompasses all knowledge that can be consciously evoked; it can be easily verbalized or transmitted to another individual. In our brain, the store seems to be located in the medial temporal lobe.

Within this subtype of memory is semantic memory and episodic memory. Semantic memory refers to the meaning of words, the functions of objects, and other knowledge about the environment.

The episodic memory, on the other hand, is one that stores the experiences, experiences and important or emotionally relevant events of our life. That is why it is also called autobiographical memory.

Non-declarative or implicit memory

This kind of memory, as you can deduce, is evoked unconsciously and without mental effort. It contains information that cannot be easily verbalized, and can be learned unconsciously and even involuntarily.

Within this category is procedural or instrumental memory, which involves the memory of abilities and habits. Some examples would be playing an instrument, riding a bike, driving, or cooking something. They are activities that have been practiced a lot and, therefore, are automated.

The part of our brain that is responsible for storing these abilities is the striated nucleus. In addition to the basal ganglia and cerebellum.

Non-declarative memory also encompasses learning by association (for example, relating a certain melody to a place, or linking a hospital with unpleasant sensations).

These are classical conditioning and operant conditioning. The first causes two events that have appeared several times jointly or contingently to be associated.

While the second involves learning that certain behavior has positive consequences (and therefore will be repeated), and that other behaviors produce negative consequences (and their performance will be avoided).

Responses that have emotional components are stored in an area of ​​the brain called the tonsillar nucleus. Instead, responses involving the skeletal musculature are located in the cerebellum.

Non-associative learning such as habituation and sensitization is also stored in the implicit memory, in the reflex pathways.

Long-term memory and sleep

It has been shown in several studies that adequate rest is essential to store memories in a stable way.

It seems that our body takes advantage of the sleep period to fix new memories, since there are no interferences from the external environment that hinder the process. Thus, in wakefulness we encode and retrieve information already stored, while during sleep we consolidate what we have learned during the day.

To make this possible, it has been observed that during sleep reactivations occur in the same neural network that was activated while we were learning. That is, long-term potentiation (or long-term depression) can be induced while we sleep.

Interestingly, studies have shown that sleeping after an apprenticeship has beneficial effects on memory. Whether it's during an 8 hour sleep, a 1 or 2 hour nap, and even a 6 minute sleep.

Also, the shorter the time between the learning period and sleep, the more benefits it will have in long-term memory storage.

Long-term memory disorders

There are conditions in which long-term memory can be affected. For example, in situations where we are tired, when we do not sleep properly or we go through stressful times.

Long-term memory also tends to gradually worsen as we age.

On the other hand, the pathological conditions that are most linked to memory problems are acquired brain damage and neurodegenerative disorders such as Alzheimer's disease.

Obviously, any damage that occurs in structures that support or participate in the formation of memory (such as the temporal lobes, the hippocampus, the amygdala, etc.) would produce sequelae in our long-term memory store.

There may be problems both to remember information already stored (retrograde amnesia), and to store new memories (anterograde amnesia).

References

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