Dendritic Spine Clusters Physically Link Memories Formed Close in Time

According to a recent study, the reason why memories from the same day seem connected while events from weeks apart feel separate is as follows: Human brains use dendrites, which are spiny extensions of neurons, rather than the cell bodies to physically connect memories that happen in close succession.

This finding originates from research conducted on mice, where scientists utilized advanced imaging techniques, including miniature microscopes that provided single-cell resolution in live subjects, to observe memory formation.

The study demonstrates that memories are stored within dendritic compartments: When one memory is formed, the corresponding dendrites become primed to absorb new information that arrives within the subsequent hours, thereby linking memories that are formed in close temporal proximity.

If you think of a neuron as a computer, dendrites are like tiny computers inside it, each performing its own calculations. This discovery shows that our brains can link information arriving close in time to the same dendritic location, expanding our understanding of how memories are organized.”

Megha Sehgal, Study Lead Author and Assistant Professor, Psychology, The Ohio State University

The research was recently published in the journal Nature Neuroscience.

While most studies on learning and memory have concentrated on the formation of individual memories in the brain, Sehgal’s lab seeks to explore how multiple memories are organized.

The idea is that we do not form memories in isolation. You do not form a single memory. You use that memory, make a framework of memories, and then pull from that framework when you need to make adaptive decisions.”

Megha Sehgal, Study Lead Author and Assistant Professor, Psychology, The Ohio State University

Neurons, the primary cells in the brain, are recognized for their role in encoding and transmitting information. Dendrites the branch-like extensions from neurons play a vital role in processing information by receiving incoming signals and relaying them to the neuronal cell body.

However, dendrites are not merely passive conduits; each dendritic branch can function as an independent computational unit. Although dendrites have been acknowledged for their significance in brain function, their influence on learning and memory has remained unclear until now, according to Sehgal.

In experiments where mice were exposed to two distinct environments in quick succession, the research team discovered that memories of these environments became interconnected. When the mice experienced a mild shock in one of the environments, they exhibited freezing behavior in both settings, associating the shock from one room with the other.

The study concentrated on the retrosplenial cortex (RSC), a brain region essential for spatial and contextual memory. The researchers found that linked memories consistently activated the same groups of RSC neurons and their dendritic branches.

The team monitored these changes at the dendritic level by visualizing dendritic spines, which are tiny protrusions on dendrites where neuronal communication occurs. The formation of new memories led to the addition of clustered dendritic spines, a process crucial for enhancing communication between neurons and facilitating learning.

Clusters of dendritic spines that formed after the initial memory were more likely to attract new spines during a second memory formed closely thereafter, physically linking those experiences in the brain.

To validate the role of dendrites in memory linking, the team employed optogenetics, a method that enables researchers to control neurons using light. By reactivating specific dendritic segments that had been active during memory formation, they successfully linked otherwise unrelated memories, further underscoring the significance of dendritic changes in shaping memory networks.

In addition to revealing a previously unrecognized function of dendrites in memory linking, the findings pave the way for new insights into memory-related disorders, Sehgal noted.

Our work not only expands our understanding of how memories are formed but also suggests exciting new possibilities for manipulating higher-order memory processes. This could have implications for developing therapies for memory-related conditions such as Alzheimer’s disease.”

Megha Sehgal, Study Lead Author and Assistant Professor, Psychology, The Ohio State University