Learning causes structural changes In affected neurons

When a laboratory rat learns how to reach for and grab a food pellet – a pretty complex and unnatural act for a rodent – the acquired knowledge significantly alters the structure of the specific brain cells involved, which sprout a whopping 22 per cent more dendritic spines connecting them to other motor neurons. 

The finding, published in the journal Proceedings of the National Academy of Sciences by Mark H. Tuszynski, MD, PhD, professor of neurosciences and colleagues at the University of California, San Diego School of Medicine, underscores the brain's remarkable ability to physically change as it learns (not just in rats, but presumably in humans too), but also reveals that the effect is surprisingly restricted to the network of neurons actually involved in the learning.  

''I think it's fair to say that in the past it was generally believed that a whole cortical region would change when learning occurred in that region, that a large group of neurons would show a fairly modest change in overall structure,'' said Tuszynski, who is also director of the Center for Neural Repair at the UC San Diego and a neurologist at the Veterans Affairs San Diego Health System.

''Our findings show that this is not the case. Instead, a very small number of neurons specifically activated by learning show an expansion of structure that's both surprisingly extensive – there's a dramatic increase in the size and complexity of the affected neurons – and yet highly restricted to a small subset of cells. And all of this structural plasticity is occurring in the context of normal learning, which highlights just how changeable the adult brain is as a part of its normal biology.''

Tuszynski said the new work improves science's basic understanding of how the brain learns. ''This tells us that learning may be mediated by relatively few cells, but that these few cells exhibit a substantial or extensive change in structure.'' Notably, the impacted cells in the rat study were not clustered together, but widely distributed over the motor cortex of the rat brain, suggesting that learned behaviors create expansive networks of distant cells.

Whether these new connections and changes are permanent is the subject of continuing research. For a rat, reaching for and grasping food is a learned behavior that takes time and repetition to master, not unlike a person learning to ride a bike or play the piano. If the behavior isn't regularly practiced, it becomes rusty, though it may be later resumed and remembered.