Every time your heart beats, a wave of pressure pulses throughout the body as blood vessels expand and contract. And now, researchers think that even individual brain cells jiggle with each beat.
The shift, which is less than the width of a human hair, can be made visible by tracking the electrical firings of a single cell. As each heartbeat shakes up the brain cells, the neurons shift slightly in their relationship to nearby electrodes. And, according to research published Tuesday in Cell Reports, what those electrical firings look like on paper changes whenever your heart pulses. Beyond that, the study authors think these recorded signaling shifts might present entirely new ways for scientists to categorize brain cells.
A Lucky Recording
Even the project’s origins were somewhat unexpected — at the time, one of the paper’s co-authors happened to be monitoring both heart and brain activity in epilepsy patients. By embedding electrodes in the brains of people with this condition, Ueli Rutishauser, a neurologist at Caltech, was hoping to pinpoint the brain region where the seizures started. But during periods when the patients were seizure-free, Rutishauser noticed that the brain electrodes were detecting neuronal activity synced to their heartbeats.
Since each wire implanted in the brain tracks the activity of multiple neurons at once, the researchers had to tease apart the data. Computer models helped the team identify which electrical bursts belonged to which cell, says Costas Anastassiou, a study co-author and neuroscientist at the Allen Institute.
The team concluded that neurons deep in the brain jiggle the most, probably because they’re the ones closest to blood vessels delivering the bursts of pressure. In one brain region called the hippocampus, which is important for learning and memory, the team classified neurons into one of three kinds of cells whose electrical signals changed on screen.
This animation shows how the brain jiggles when the heart beats. (Credit: Mosher et al./Cell Reports)
Researchers already knew that the recordings of brain cells firing can mysteriously change over time, says Anastassiou. Peaks and troughs in the waveforms — the squiggly line on the screen tracking the brain’s electrical activity — would spread out or condense. Those observations were always written off as a simple margin of error — previously, researchers didn’t know the heart was involved. “They were considered a ‘biological complexity’ in the way that we scientists call anything we don’t understand,” he says.
But now, the research team thinks they can categorize cells based on how their recorded signals change with each shake. With new subtypes of cells to work with, Anastassiou thinks researchers could start to build a more granular understanding of the different neurons in the brain.
More Pieces for the Puzzle
It’s this ability to sort cells by their heartbeat-induced jiggle that has Anastassiou excited. Brain research tends to take one of two approaches, he says: Neurologists either study how an entire living brain functions or they examine bits of preserved tissue on a microscopic level. Though researchers might know a lot about how a brain cell behaves under a microscope, Anastassiou says, it’s hard to apply that information to a functional brain. “No one can tell you what each of the cell types do when forming a memory or have any other real function,” he adds.
Tying heartbeat to brain activity “gives us more than one or two [neuron] cell types, and is a way to think of human brain circuits as puzzles that consist of pieces,” Anastassiou says. It could even help with the study of brain diseases that seem to only affect a select population of neurons.
So far, the study authors say that the work is a proof of concept, showing just how much more we have to learn about the brain. If our understanding of the brain’s circuits is akin to assembling a puzzle, Anastassiou says, we’re only just beginning to collect the pieces required. “We’re certainly at the start of it,” he adds.