European scientists believe that they discovered the explanation of why it takes some time for us to learn new things. In a study published in the Journal of Neuroscience, investigators from the Max Planck Institute of Neurobiology describe a fairly slow continuous remodeling involved in synaptogenesis:
The Martinsried-based neurobiologists, in cooperation with colleagues in Zurich, have been investigating the relation between the development of new cell contacts, called “spines”, and the creation of functional synapses. Synapses enable the transfer of information between cells. The scientists have been focusing their experiments on nerve cells from the hippocampus, the brain region that is essential for learning and memory processes. In order to intentionally cause the nerve cells to react, the scientists stimulated a group of neurons via a short electrical impulse of high frequency. It is a known fact that this type of electrical stimulation causes the formation of new spines – similar to what happens during learning processes. The key question, however, whether and when these new spines actually form functional synapses and thus play a role in memory functions has, thus far, remained unanswered.
Using time-lapse two-photon microscopy, the scientists were able to follow the outgrowth of spines in the immediate area of the stimulated area. Further analysis with an electron microscope enabled the detection of functional synapses in the newly developed spines. The observed changes in neuronal connections and their dynamics surprised the scientists: new spines began to sprout from the stimulated nerve cells within minutes of the stimulation. The growth of these thin spines was initially not random, but directed toward a potential contact site. However, despite the quick connection of these spines to new contact sites, their further differentiation seemed to follow the motto “haste makes waste”: the transfer of information through the newly established contact was not possible within the first eight hours. It took another few hours before it could be established whether the connections would degenerate or thrive, thereby forming synapses. All of the contacts that still persisted after 24 hours had fully-functional synapses and a good chance for continued existence.
The unraveling of the time-scale and functional relationships were not the only exciting observations that the scientists were treated to. When a new spine made contact with a site already hosting a contact, the new spine was highly likely to displace the old connection. “We are not yet completely sure what this means,” said Valentin Nägerl from the Max Planck Institute of Neurobiology. “But it could indicate, for example, that newly learned information might lead to a fading of older information.” That it is easier to retrieve information which has been learned previously could also be related to spine modifications: the displaced connections might not disintegrate completely, but can perhaps be reactivated again at a later time. If this is true, and whether repeated learning impulses have an effect on the development and longevity of synapses, are some of the questions now being pursued by the scientists.
Press release: The building blocks of memory
