The Nervous System – A Master of Adaptation

After reading this text, you truly won’t be the same person anymore. Sounds pretty immodest? Certainly. But it’s true. Scientifically true. For a long time, researchers considered the adult brain to be a rather rigid entity. Just like a computer, our Gray matter was thought to be firmly connected. But it has long been clear: Our brain is not rigidly wired like a computer. It is constantly being restructured and adapted to new demands, for example when new information flows into it and we learn something new, as is the case when reading this article. Each of the roughly 86 billion nerve cells in our brain is connected to its neighboring cells via thousands of contact points – the synapses. Absorbing new information, learning, and remembering are only possible because these connections are plastic and are constantly being formed and remodeled. That is why synapses have increasingly come into the spotlight for researchers.

During learning, nerve cells grow extensions a few thousandths of a millimeter long, known as Dendritic spines These are structures that receive stimuli from upstream nerve cells. To establish a synaptic connection, these very fine extensions develop into mushroom-like structures consisting of a stem and a Terminal button These terminal buttons enable the exchange of information between nerve cells.

In addition to the formation of new synapses, another mechanism plays a very important role: “We believe that learning and Memory occur primarily through changes in the transmission strength of synapses,” says neurobiologist Tobias Rose of the Max Planck Institute for Neurobiology in Martinsried (now the MPI for Biological Intelligence; Rose is now at the University of Bonn). A Synapse becomes stronger when the upstream nerve cell releases more neurotransmitters or when more receptors for neurotransmitters are formed in the dendritic spines. “If a synapse suddenly becomes stronger, it’s possible that a cell that previously had no influence on a second cell suddenly gains a major influence on that second cell,” says Rose. “And this strengthening allows information to be stored.” 

The most important principle of Plasticity is Hebb’s learning rule: neurons that fire together wire together. “We know that the simultaneous activation of cells strengthens the synapses between them,” says Rose. In recent years, the neurobiologist has repeatedly observed just how plastic the adult brain still is. Together with Mark Hübener, Rose has been closely examining the Visual cortex in adult mice in Martinsried. It has long been known that if you cover one Eye in an animal for a certain period of time and then uncover it again, the visual cortex’s response to light stimuli from the eye that remained uncovered is strengthened. The cells thus respond more strongly to the eye that remained open – and at the same time more weakly to the eye that was covered for a while. 

As Hübener and Rose observed through the microscope, new Dendritic spines – and thus new synapses – formed on pyramidal cells, a type of particularly large nerve cell. “We suspect that these additional neural connections were the reason for these stronger responses,” says neurobiologist Hübener. One thing was particularly surprising to the researchers: “In the cortex, the cells returned to their original activity as soon as they began receiving information again from the Eye that had previously been covered for an extended period.” It almost seemed as if the individual cells could “remember” which connections they had formed before the eye was covered, in order to reestablish precisely those connections. In this way, the adult brain might be able to adapt to changing environmental conditions without completely altering its “basic wiring.”

So it is clear, in any case, that synapses are remodeled during learning. But how is it that a specific Synapse of a nerve cell is altered in the process, while others remain unchanged? After all, every cell has dozens of extensions, each of which has 100 to 1,000 of these contact points. Let’s take Pavlovian conditioning as an example. In this process a Neuron learns to associate a sound with food. More precisely, a specific synapse learns this association. Once this happens a few times, the specific sound alone is enough for that synapse to associate the sound with food. As part of the learning process, the synapse is restructured in both function and structure with the help of new proteins.

To produce the necessary proteins, sections of DNA must be translated into proteins. What is unique about nerve cells is that a portion of the RNA – the transcript of the DNA – is translated into proteins not only in the cell body but also directly on-site, that is, in the dendrites. To do this, special protein molecules attach themselves to a portion of the RNA and package it into specific particles, known as RNA granules, which are then transported to the dendrites. 

Michael Kiebler, LMU Munich, likes to refer to this as “Running Sushi.” “With Running Sushi, the food from the kitchen passes by every seat in the restaurant,” says the biochemist. “In much the same way, the granules carrying the RNA travel from the cell body to all the synapses and offer the ‘sushi’ – the RNA.” In a restaurant, you can’t just reach for the sushi right away because it’s often covered by a lid. It’s similar at the cell’s extensions, says Kiebler. “Only when a Synapse signals that it is currently learning can it ‘take the RNA off the conveyor belt’; the RNA is unpacked on the spot and translated into a protein.” This ensures, at the molecular level, that only the learning synapse is remodeled and not other synapses of the nerve cell as well.

Whether RNA is translated into proteins depends on specific binding proteins contained within the granules. One of these, called Staufen 2, apparently binds to the RNA to prevent it from being translated while it is being transported to the synapses. In the future, Kiebler hopes above all to answer one question: “How can an actively learning synapse influence the binding protein Staufen 2 in such a way that it detaches from the RNA, allowing the RNA to be unpacked from the RNA granules and subsequently translated into proteins directly at the learning synapse?” 

It is quite clear to Kiebler that Staufen 2 plays a role in learning. In an experiment, he and his colleagues genetically engineered rats so that they lacked Staufen 2 in the nerve cells of the Hippocampus. This region plays a key role in Long-term memory The result: The general long-term Memory of the genetically modified rats still functioned. For example, the rats learned where a food source was located. But when the information they had learned was retrieved only after longer waiting periods, the mutants’ memory performance was significantly worse than that of the wild-type rats. “These experiments thus demonstrate the importance of Staufen 2 in certain long-term memory processes and provide initial insights into the underlying mechanisms of learning,” Kiebler concludes.

The nervous system is thus a true master of transformation – whether on a small scale, at the level of synapses between individual neurons, or on a large scale, at the level of entire networks. And reading this article has certainly led to some restructuring in your own brain as well. 

Further reading

• Jaepel J et al.: Lateral geniculate neurons projecting to primary visual cortex show ocular dominance plasticity in adult mice. Nat Neurosci.2017 Dec;20(12):1708-1714. doi: 10.1038/s41593-017-0021-0. Epub 2017 Nov 13.
   
• Berger SM et al.:.Forebrain-specific, conditional silencing of Staufen2 alters synaptic plasticity, learning, and memory in rats. Genome Biol.2017 Nov 17;18(1):222. doi: 10.1186/s13059-017-1350-8.

First published on April 30, 2020
Last updated on May 5, 2026