Brain's rhythm signals may help memory circuits communicate

by University of California, Irvine

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A new study by researchers at the University of California, Irvine, suggests that slow electrical rhythms traveling along individual nerve fibers may do more than reflect brain activity: They may help coordinate communication between key memory centers in the hippocampus.

The study, "Axonal theta oscillations evoke bursting in target hippocampal subregions," published in the Journal of Neural Engineering, reports that theta-band signals—brain rhythms in the range of about 4 to 10 cycles per second—were detected in individual axons connecting hippocampal subregions. These rhythms were linked to stronger bursts of activity in downstream neurons, pointing to a possible mechanism by which memory-related brain regions time and route information.

"The brain is often described as a network of cells that send spikes, but our findings suggest there may be another layer of communication riding along the axons," said corresponding author Gregory J. Brewer, professor in the Department of Biomedical Engineering at UC Irvine and affiliated with the MIND Institute and the Center for Neuroscience of Learning and Memory. "These slower theta rhythms may help determine when and how strongly one hippocampal region influences another."

The hippocampus is a small but essential brain structure involved in learning and memory. Scientists have long known that hippocampal theta rhythms are associated with memory encoding, retrieval and coordination across brain regions. The new work suggests that some of these rhythms may travel through axons—the long, cable-like projections neurons use to send information—and may influence the strength of communication at their targets.

That possibility is important because local field potentials, the slow electrical signals commonly recorded in brain tissue and in some clinical settings, have often been debated: Are they merely a byproduct of neuron firing, or do they actively shape information processing? This study supports the idea that at least some slow voltage rhythms may play a functional role.

To study these signals in a controlled way, the team built a microfluidic device that lets neurons from hippocampal subregions grow and connect through tiny channels. The design allowed researchers to isolate activity traveling through single axons between regions such as CA3 and CA1, two hippocampal areas known to work together in memory circuits.

Recordings showed spontaneous theta-band activity in a sparse set of axons. These theta signals were not simply explained by simultaneous spiking activity. Instead, their timing and strength correlated with activity in the target region, especially the length of brief bursts of neuronal firing.

The researchers propose that theta oscillations in axons may help activate slow voltage-gated calcium channels, which could increase neurotransmitter release at synapses. In everyday terms, the rhythm may help make a message more likely to be heard by the next group of neurons.

The work also raises the possibility of "multiplex" communication in the brain: Fast electrical spikes may carry one kind of information, while slower theta rhythms may help regulate when, where or how strongly that information is delivered. Such a mechanism could have implications for understanding memory, attention, brain disorders, electroencephalography and future artificial neural networks.

"If confirmed in further studies, this could broaden how we think about neural communication," Brewer said. "The axon may not be only a wire for spikes; it may also carry slower signals that help organize activity across brain regions."

Publication details

Samuel Brandon Lassers et al, Axonal theta oscillations evoke bursting in target hippocampal subregions, Journal of Neural Engineering (2026). DOI: 10.1088/1741-2552/aea2d7

Journal information: Journal of Neural Engineering

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AxonsAnalytical Device, Microchip

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Neurology Provided by University of California, Irvine Who's behind this story?

Gaby Clark

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