Scientists have discovered neuronal connections which are formed in the brain when learning occurs, and which ensure the precision of memory. This work represents an important step on the path towards an improved understanding of how learning and memories are stored in the brain.
How are experiences and learning stored in the brain? This "simple" question has intrigued scientists for several generations. Since the visionary neuroscientist and Nobel laureate Santiago Ramón y Cajal first postulated (at the end of the nineteenth century) that structures in the brain change during learning, and that what is learned - or the memory of what has been learned - is represented in neuronal connections, researchers have sought to detect structural changes of this kind. They have been searching, as it were, for nerve cells which encode the Pythagorean theorem or the memory of a red dress. So far, their efforts have not been successful, as the sheer number of neuronal connections, or synapses, in the brain has proved an insuperable obstacle.
Neurobiologists have now demonstrated a direct link between the formation of new synapses in the brain and a learning process, plus the quality of the associated memory. A team studied neurons in the hippocampus of mice learning to navigate a water maze. The hippocampus is a region of the brain which is essential for learning and recall.
During learning, the number of synapses onto interneurons was found to double along hippocampal mossy fibers. These new synapses indirectly inhibited other cells, known as pyramidal neurons, thereby contributing to the precision of what was learned. They thus laid down, for example, that at a certain point in the maze, the mouse should go to the right, but not to the left, straight ahead, back, up, or down. When the formation of these synapses was inhibited by the administration of a drug, the mouse would still find its way, but with less precision - it did not take the shortest route to the goal.
Pico Caroni, responsable of the project, explains: "Our experiments have, for the first time, demonstrated a clear association between the formation of new synapses and behavior after learning. So we’ve shown a specific structural change in the brain induced by learning - and also that this change is required for the precision of learning."
The researchers also found that the newly formed synapses often disappeared again several days after the end of the learning process. However, even weeks later, this did not prevent the mice from navigating the maze successfully, although precision was diminished. "What we’ve discovered," says Caroni, "are not structures which are indicative of memories, but structures which are necessary for precise recall of what’s been learned. If these mechanisms are disrupted, behavior can be diverted by more dominant stimuli, which could play an important role in many memory disorders."