Scientists have identified a unique form of information transmission in the human brain, revealing how much we still have to learn about its mysterious inner workings.
The discovery excitingly suggests that our brain may be an even more powerful computing unit than we thought. In 2020, researchers from German and Greek institutes discovered a mechanism in the brain’s outer cells that produces a new “gradual” signal on its own, which could provide individual neurons with another way to carry out their logical functions. By measuring electrical activity in sections of tissue removed during surgery on epileptic patients and analyzing their structure using fluorescence microscopy, neurologists found that individual cells in the cortex used not only the usual sodium ions to “fire” themselves, but also football. This combination of positively charged ions triggered 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 limitations, but at some levels they perform tasks similarly. Both use the power of an electrical voltage to perform various tasks. In computers it comes in the form of a rather simple flow of electrons through intersections called transistors. In neurons, the signal is in 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 called an action potential. Instead of transistors, neurons chemically handle these messages at the ends of branches called dendrites. “Dendrites are critical to understanding the brain because they are at the heart of what determines the computational power of individual neurons,” Humboldt University neuroscientist 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 significant enough, it can be transmitted to other nerves, which can block or relay the message. These are the logical foundations of our brain: ripples of tension that can be communicated collectively in two forms: either an AND message (if x and y are activated, the message is transmitted); or an OR message (if x or y is triggered, the message is transmitted). Arguably, nowhere is this more complex than in the dense, wrinkled outer section of the human central nervous system; the cerebral cortex. The deeper second and third layers are particularly thick, full of branches that perform high-order functions we associate with sensation, thought, and motor control. It was the tissues in these layers that the researchers examined closely, attaching the cells to a device called a somatodendritic patch clamp to send active potentials up and down each neuron, recording their signals.
To ensure the findings weren’t unique to people with epilepsy, they double-checked the findings in a handful of samples taken from brain tumors. Although the team had conducted similar experiments on rats, the types of signals they observed buzzing through human cells were very different. More importantly, when they gave the cells a sodium channel blocker called tetrodotoxin, they still found a signal. But modeling how this new type of sensitive signal works in the cortex revealed a surprise. In addition to AND- and OR-type logic functions, these individual neurons could act as “exclusive” OR (XOR) intersections, allowing a signal only when another signal is classified in a particular way. “Traditionally, XOR operation is thought to require a network solution,” the researchers wrote. There is still a lot of work to be done to see how dCaAPs behave on 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 is also looking to our nervous systems for inspiration on how to develop better hardware; Knowing that our individual cells have a few more tricks up their sleeves could lead to new ways to network transistors. How exactly this new logical tool packed into a single nerve cell translates into higher functions is a question for future researchers to answer.
This research was published in Science.