LTP. A strong candidate for memory storage

This is the second part of a study of the article :

Is plasticity of synapses the mechanism of long-term memory storage? By Wickliffe C. Abraham, Owen D. Jones and David L. Glanzman

We have seen how LTP works. Now the author shows and explains the hypothesis that uses LTP for memory storage.

Part 2

The Synaptic plasticity and memory hypothesis (SPM).

This hypothesis is the most commonly known concept for memory.

In this hypothesis, synapses become stronger/induce LTP or weaker/ LTD in response to experience and this change is considered the mechanism of retention.

The two kinds of plasticity have 7 properties making it a good candidate for memory storage.

1. It is rapidly induced.

This is necessary as a memory cannot take too long to be stored. An experience that is or not to be memorised happens in an instant and no longer.

2. It is maintained for a long term but at varying degrees.

Memories are not all equal, some can be remembered for a lifetime while others only stay for days or even minutes. The properties of memory need to appear in the mechanism behind it. 

3. It is specific to an input.

This means it only reacts to certain information related or similar to the stored content.

This is the same to how memory may be reinforced when the same information or something near it appears, while anything unrelated will not do anything to the memory.

4. It becomes more persistent with spaced induction over mass induction

This property is a well known one of human memory, meaning it should be found in the biological mechanism of it.

5. Associativity

This means that a strong signal to a neuron causing LTP can also cause LTP in a nearby neuron.

Neurons close to each other usually share common points and hold similar information.

The Associativity of LTP can be seen in memory as one memory or thought can cause another similar one to appear or to be better remembered.

6. Regulation by neuromodulators

This may be how memories are modified over time with experiences or emotional states.

7. Expressed throughout the whole nervous system

Memories can take various forms such as knowledge/ facts, past experiences or movements.

Having LTP/LTD systems in many parts of the nervous system makes it possible for all of these different types of information to be stored.

Scientists have proposed 4 aspects needed for a neural mechanism to be convincinig enough.

For 3 of them, the SPM hypothesis works very well, however, the last one doesn’t seem to work out yet.

These 4 are :

1. Detectability

For this hypothesis, this is the need of a memory to cause observable change in the synapses. This aspect has been observed as learning changes the synaptic weight.

2. Anterograde intervention

This is the need of preventative manipulations to cause changes in the results.

3. Retrograde intervention

On the other hand, changing synaptic weight after the formation of memory should change or even erase the memory. This has also been found to be true.

4. Mimicry

This last aspect is the one the SPM hypothesis has an issue with.

This implies in the idea that recreating the synapses with identical synaptic weigths should recreate the memory.

This seems logical but is something that is hard to prove and hasn’t been proven yet.

Neurons don’t independently store a full memory. They need other neurons associated to different sources of information ( sensual, visual, auditory…).

Thus, a change in synaptic weight needs to be considered with all the other neurons related in the learning.

For example a change in synaptic weight of one cell may not change the memory at all because of a counteractive change in several other synapses.

This shows the possibility of multiple combinations of synaptic weight and synapses for a single memory.

In addition to this, knowledge and stored information can be updated by new learning while keeping the older information.

This will still cause the overall synaptic weight to change.

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