Scientists have identified a unique form of cellular messaging occurring in the human brain, revealing how much we still have to learn about its mysterious inner workings.
Excitingly, the discovery suggests that our brains may be even more powerful computing units than we thought.
Back in 2020, researchers from institutes in Germany and Greece reported a mechanism in the outer cortical cells of the brain that produces a new “graded” signal all by itself, one that can provide individual neurons with another way to perform their logical functions.
By measuring electrical activity in sections of tissue removed during surgery from epilepsy patients and analyzing their structure using fluorescence microscopy, neuroscientists discovered that individual cells in the cerebral cortex use more than the usual sodium ions to “fire” , but also calcium.
This combination of positively charged ions initiates never-before-seen voltage waves called calcium-mediated dendritic action potentials, or dCaAPs.
Brains – especially those of the human variety – are often compared to computers. The analogy has its limits, but on some levels they perform tasks in a similar way.
Both use the power of electrical voltage to perform various operations. In computers, this takes the form of a fairly simple flow of electrons through junctions called transistors.
In neurons, the signal takes the form of a wave of opening and closing channels that exchange charged particles such as sodium, chloride, and potassium. This impulse of flowing ions is called an action potential.
Instead of transistors, neurons conduct these messages chemically at the end of branches called dendrites.
“Dendrites are central to understanding the brain because they are at the heart of what determines the computational power of individual neurons,” neuroscientist Matthew Larkham of Humboldt University told Walter Beckwith of the American Association for the Advancement of Science in January 2020.
Dendrites are the traffic lights of our nervous system. If the action potential is significant enough, it can be transmitted to other nerves that can block or transmit the message.
This is the logical basis of our brain – waves of tension that can be transmitted collectively in two forms: or And message (if x and y are triggered, the message is transmitted); or an OR message (if x or y is triggered, the message is transmitted).
Perhaps nowhere is this more complex than in the dense, wrinkled outer part of the human central nervous system; the cerebral cortex. The deeper second and third layers are especially thick, full of ramifications that perform the higher-order functions we associate with sensation, thought, and motor control.
It was the tissue from these layers that the researchers looked at closely, connecting the cells to a device called a somatodendritic clamp to send action potentials up and down each neuron, recording their signals.
“There was a ‘eureka’ moment when we saw dendritic action potentials for the first time,” Larkham said.
To make sure the findings weren’t unique to people with epilepsy, they tested their results in a handful of samples taken from brain tumors.
Although the team performed similar experiments in rats, the types of signals they observed buzzing through human cells were very different.
More importantly, when they dosed the cells with a sodium channel blocker called tetrodotoxin, they still detected a signal. Only by blocking calcium did everything subside.
Finding a calcium-mediated action potential is interesting enough. But modeling how this sensitive new kind of signal works in the cortex revealed a surprise.
In addition to the logical And and OR-type functions, these individual neurons can act as “exclusive” OR (XOR) junctions that allow a signal only when another signal is graded in a certain way.
“Traditionally, XOR The operation is believed to require a network solution,” the researchers wrote.
More work needs to be done to see how dCaAPs behave in whole neurons and in a living system. Not to mention whether this is a human thing or whether similar mechanisms have evolved elsewhere in the animal kingdom.
Technology also looks to our own nervous system for inspiration on how to develop better hardware; knowing that our own individual cells have a few more tricks up their sleeve could lead to new ways to network transistors.
Exactly how this new logic tool squeezed into a single nerve cell translates into higher functions is a question for future researchers to answer.
This research was published in Science.
A version of this article was originally published in January 2020.
