Last updated: August 21, 2025

Synaptic Plasticity

Synapse

The release of neurotransmitters from the presynaptic terminal induced by single action potentials is called basal synaptic transmission.

Random notes

Synaptic plasticity is the phenomenon of modification of synaptic connections. The brain is formed of billions of neurons (in humans the estimate is 86 billions). Each neuron forms thousands of connections with other neurons, for a total estimate of 100 trillion synapses in the human brain.

Granule cells receive information from the mossy fibers and project the information to Purkinje cells. Granule cells and Purkinje cells (two neuron types found in the cerebellum) have a very different dendritic structure (see ref 1.)

The brain is a complex natural system which has an equally complex phenomonelogy: the animal behavior. To explain behavior, one must assume that the brain is able to perform so many functions. Let us try to see what I mean with the simplest behavior.

If you point a light in the eyes of an animal, it will close them. This is clearly a protective behavior. The brain must detect that the light stimulus is too strong, and should signal to some structure to close the eyes. At the same time it might want to put an arm in front of them for better protection. It wants to understand what is the source of light and get away from it.

There are behaviors that require memory of the past. Memory of events, or memory of sensations?

As we wake up in the morning, we start recording! In any moment, we might decide to stop and recall in our mind what happened in the previous hour. Or 2 hours. Or 2 days! We can be very precise for what happened in the previous hour, and it is very easy to recall. For what happened in the last two days we only remember some events. For sure, we remember those that were unusual. We can also recall what happened in the last 5 years actually, and in our whole life! Is it through. Well we store enough information to reconstruct some part of our life for sure. Is reconstructing still a form of memory? For example, I know that I am in the office now and this morning I woke up in my bed, so I know that I took the bus. Does it count? But actually I can recall taking the bus. For example I remember it was the 800, and it arrived suprisingly even if it was not on the schedule. The driver said hi. I seated in the four seats place (but I am not sure!) facing forward. I deduce this from the fact that I remember looking back for women. For sure I remember there was no particularly cute woman, and if there was I would remember.

But both the 1 hour old and 2 days hold memories require some effort to recall. It really feels like an action that I am doing: recalling, fishing for the memories.

When trying to remember events in the remote past, the easier things to recall are images, flashes of the moments. I read something about hippocampus and image memory. But images are formed in the visual cortex right? Are then such images stored in the visual cortex as a modification of synaptic connections?

List of brain functions

  • Memory
  • Language
  • Movement
  • Perception

Learning and Memory from Kandel

Memory can be classified along two dimensions: (1) the time course of storage and (2) the nature of the information stored.

We classify memory in short-term memory (or working memory) and long-term memory. We subdivided working memory in verbal memory and visuospatial memory.

Areas of brain involved in memory

Kandel’s book poses natural but important questions: “Are there discrete neural systems concerned with memory? If so, is there a “memory center” or is memory processing widely distributed throughout the brain?”

The answer is a bit yes a bit no.

There are several fundamentally different types of memory, and certain regions of the brain are much more important for some types of storage than for others.

Dopamine

Dopamine can be release by stressful as well as by rewarding stimuli. Rodents lacking dopamine can still exhibit hedonic response to sucrose.

Dopaminergic neurons do not simply fire when there is a reward. Instead, when reward is presented during learning experimental paradigms, their response have complex and changing patterns. Of course here we are referring to the experiments of Schultz on monkeys, which suggested the role of dopamine as a prediction-error signal. This is because dopaminergic neurons increased firing above basal levels when a reward arrived unexpectedly, and decreased their firing below basal levels when an expected reward did not arrive. During the experiment, the monkey learned something: it learned that a visual/auditory stimulus preceeds the delivery of a reward. Was dopamine released involved in the learning? Of cours we think so. We think its release has precisely the function of learning what behaviors/situations lead to that positive reward (positive survival value).

Dopamine has been implicated in the formation of long-term memories in hippocampal and cerebral cortical circuits.

Dopamine action at the D1 receptors leads to the activation of a protein called CREB, which influences transcription of genes. CREB has been implicated in diverse memory processes in a variety of species. See figure 49-8 of ref 2.

AMP is an important molecule: adenosine monophosphate. Many proteins depends on AMP. For example, protein kinase A (PKA). This protein can enter the nucleus, and together with CaMKIV activates CREB.

Not sure, but it does not seem like dopamine can act at a specific synapse. It rather have a global effect.

Learning can be defined as the process of acquiring new memories. So learning is definitely a behavior related to memory.

But not all memories are there to ‘learn something’, right? For example the information of what I ate this morning at breakfast does not represent really soemthing I have learned. However, it might still be useful: let us say my stomach hurts this afternoon, maybe I can trace back the cause to something I ate this morning, and thus learn to avoid it. That is clearly a form of learning. Quite analogous to the paradigms of ‘avoidance of aversive stimuli’ they use for assess learning in mice. They do something or something happens, they feel bad, they learn to avoid the situation. But would they be capable of doing what I did for the breakfast? I would say no, but who knows.

Among behaviors there is of course movement. Voluntary movement and involuntary movement. I would voluntary movement involves planning of it, calculations of spaces and a lot of things. It requires the some sort of memory in the sense of ‘calibration’, but I would not say it requires real memory of how you did that movement in the past. Altought one could define calibration exactly like that lol. Involuntary movements are automatic, with no planning or complex calculations. They are therefore learned, memorized, stored in the brain, and triggered under certain conditions.

Principles of memory formation

Survival value

Principle 1

There exist one or more mechanisms that are triggered by the perceived value of the situation, and have the effects of enhancing the formation of memory during the time period.

References

  1. https://hms.harvard.edu/news/new-field-neuroscience-aims-map-connections-brain
  2. Kandel, Schwatz, Jessel, Siegelbaum, Hudspeth, 2012