Dendrites key to memory formation

Why do we remember some things but forget others? Northwestern University researchers may have found the answer to this enigmatic question through demonstrating a mechanism by which neurons in the brain store some experiences but not others. Their key discovery was that memory formation is in fact dependent on the function of dendrites, the tree-like structures of neurons.

Daniel Dombeck and Mark Sheffield investigated the issue by peering into the activity of neurons in the hippocampus known as place cells. There are hundreds of thousands of place cells in the hippocampus and they are essential in forming the brain’s ‘GPS system’. Place cells store information about particular positions in an environment and it was the formation of this type of memory the researchers focused on. Through the design of a novel laser microscope and the use of a genetically encoded calcium indicator, electrical activity in neurons could be detected.

What they discovered was remarkable. Their work showed that, contrary to current dogma, the activity of neuron cell bodies can be different to that of its dendrites. When the cell body is excited without activity in the dendrites, no lasting memory of the experience is formed. This suggests that it is the cell body which codes for ongoing experience, but the dendrites which form that experience into a memory. Prior to this, it was long believed that the mechanisms of neuronal computation and storing information were highly connected; when neurons compute information, they were also storing it, and vice versa. The Northwestern study provides evidence against this classic view.

‘A daily commute to work for example requires the activity of millions of neurons, but you would be hard pressed to remember what was happening halfway through your commute last Tuesday.’ said Sheffield, a postdoctoral fellow in Dombeck’s lab and first author of the study. ‘How is it then that the neurons could be activated during the commute without storing that information in the brain? Now we may have an explanation for how this occurs.’

Their findings not only increase our understanding of how the brain represents the world we live in but also points out dendrites as a potential therapeutic target for memory deficit neurological disorders such as Alzheimer’s Disease (AD). Disruption to the brain’s GPS system is one of the first symptoms of AD, with many patients unable to find their way home. Understanding how place cells and their dendrites store these memories could help to treat these debilitating symptoms. However, the researchers fail to show a direct link between dendritic activity and synaptic plasticity, a well documented mechanism required for memory formation, and further work is necessary in order to clarify this.