Scientists have recently identified a unique form of cell transmission in the human brain that has never been observed before.
Intriguingly, the discovery suggests that our brains may be even more powerful computing engines than we thought.
Back in 2020, researchers from institutes in Germany and Greece reported a mechanism in the brain’s outer cortical cells that inherently generates a novel “graded” signal that could provide individual neurons with a different way to carry out their logical functions.
By measuring electrical activity in tissue sections taken from epileptics during surgery and analyzing their structure with fluorescence microscopy, the neurologists found that individual cells in the cerebral cortex used not only the usual sodium ions to “fire” but also calcium.
This combination of positively charged ions triggered voltage waves that had never been seen before, called calcium-mediated dendritic action potentials, or dCaAPs.
Brains – especially those of the human kind – are often compared to computers. The analogy has its limitsbut at some levels they perform tasks in a similar way.
Both use the power of an electrical voltage to perform various operations. In computers it takes the form of a fairly simple flow of electrons through intersections 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 pulse of flowing ions is referred to as on action potential.
Instead of transistors, neurons chemically manage these messages at the end of branches called dendrites.
“The dendrites are central to understanding the brain, because they are the core of what determines the computing power of individual neurons,” says the neuroscientist from Humboldt University Matthew Larkum told Walter Beckwith at the American Association for the Advancement of Science in January 2020.
Dendrites are the traffic lights of our nervous system. If an action potential is strong enough, it can be passed to other nerves that can block or relay the message.
This is the logical basis of our brain – waves of tension that can be collectively communicated in two forms: either as AND message (if x And y are triggered, the message is forwarded); or a OR message (if x or y is triggered, the message is forwarded).
Arguably nowhere is this more complex than in the dense, wrinkly outer portion of the human central nervous system; the cerebral cortex. The deeper second and third layers are particularly thick, packed with branches that perform higher-order functions we associate with sensation, thought, and motor control.
It was tissues from these layers that the researchers looked at closely, connecting cells to a device called a somatodendritic patch clamp to send active potentials up and down each neuron and record their signals.
“There was a ‘eureka’ moment when we first saw the dendritic action potentials,” said Larkum.
To make sure the findings didn’t just apply to people with epilepsy, they double-checked their findings in a handful of brain tumor samples.
While the team had been conducting similar experiments on ratsthe types of signals they saw buzzing through human cells were very different.
Even more important when they dosed the cells with something called a sodium channel blocker tetrodotoxin, they still found a signal. Only by blocking calcium did everything become quiet.
Finding a calcium-mediated action potential is interesting enough. But modeling the way this sensitive new type of signal worked in the cortex revealed a surprise.
In addition to the logical AND And OR-like functions that these individual neurons could function as ‘exclusive’ OR (XOR) crossingswhich only allow a signal if another signal is graded in a certain way.
“Traditional is that XOR It was assumed that the operation required a network solution”, the researchers wrote.
More work needs to be done to see how dCaAPs behave across whole neurons and in a living system. Not to mention whether it’s a human thing or whether similar mechanisms have evolved elsewhere in the animal kingdom.
technology is too Look at our own nervous system for inspiration to develop better hardware; Knowing that our own individual cells have a few more tricks up their sleeve could lead to new ways of connecting transistors.
Exactly how this new logical tool squeezed into a single neuron is translated into higher functions is a question future researchers will need to answer.
This study was published in Science.
A version of this article was originally published in January 2020.
