Neuroscientists have observed for the first time how muscle memory is formed

(ORDO NEWS) — In a new study, for the first time, American scientists were able to observe the formation of motor memories in real time and find out why the so-called muscle memory is so resistant to forgetting.

The study could help identify the underlying causes of movement disorders that occur in neurodegenerative diseases such as Parkinson’s disease, and thus find the best way to treat them.

Memories are now thought to be stored in the brain as the activity of networks of hundreds or even thousands of neurons, sometimes distributed over very distant regions of the brain. Sometimes such a network is called a memory engram.

Although the concept of engrams explaining memory has evolved over a century, it has proven extremely difficult to pinpoint exactly what an engram is and how it is formed.

Previous research has shown that certain forms of learning activate certain networks of neurons, which then fire again when stored memories are recalled.

However, until now it was not known whether engrams are formed during learning of motor skills, and if so, what changes neurons and their networks undergo to form engrams. Scientists from Stanford University (USA) decided to answer these questions.

The authors trained mice to reach food pellets with their paws through a small slit in the cage. Using state-of-the-art neuroimaging techniques (c-Fos immunostaining), scientists were able to identify the neurons that fired during the learning process in the primary motor cortex, the area responsible for controlling movement.

The potential engrams were fluorescently labeled to see what role these networks of neurons play during recall of the desired movement.

Neuroscientists have observed for the first time how muscle memory is formed 2
Brief review of the experimental design and study results

A few weeks later, the researchers tested the animals’ memory and found that mice that still remembered the skill and were able to complete the task with ease showed increased activity in the same neurons that were first identified and tagged during the learning period.

This means that it is these neurons that make up memory engrams and are responsible for the formation of a skill. What’s more, the scientists observed in real time how “engram neurons” reprogram themselves as the rodents learned.

Thus, these neurons of the motor cortex acquired new input synapses (points of contact between two neurons), through which information is received about the success of the movement in order to correct it, and they themselves formed new output connections in a remote area of ​​the brain called the dorsolateral striatum (DST).

This is a key area of ​​the brain through which engram neurons can exercise control over the animal’s movements.

Thus, for the first time, the researchers observed the creation of new synaptic pathways on the same population of neurons, both at the input level (in the primary motor cortex) and at the output level (in the DPT).

Another important question that scientists tried to answer was whether the activation of only certain engram neurons is required to perform already learned motor tasks.

By suppressing the activity of neurons that are identified as part of the motor cortex memory engram, the authors of the work made sure that mice are still able to perform the motor task.

Consequently, motor memories are not only extremely dispersed, but also very redundant, which makes it possible to better preserve memories even with the loss of part of the neurons in the network.

By repeating learned skills over and over again, the researchers say, we continually develop motor engrams, creating new synaptic connections, improving the skill, and reinforcing the memory of it.

This is exactly what is meant by the term “muscle memory” a redundant network of motor engrams used so often that the skill associated with it seems automatic (cycling, playing the piano and guitar, ice skating). It is the constant repetition that is considered one of the main reasons for the resistance of muscle memory to forgetting.

Going forward, the scientists plan to find out whether Parkinson’s disease is the result of a blockage of these motor engrams or their complete loss. In the first case, patients should be able to improve their motor skills by practicing and strengthening muscle memory.

However, if the disease destroys motor engrams and prevents the creation of new ones – by affecting neurons with motor engrams and their synaptic connection, observed in the new study – then an entirely different approach will have to be used for effective treatment.

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