Scientists have recently identified a unique form of cell messaging occurring in the human brain that's not been seen before.
Excitingly, the discovery hints that our brains might be
even more powerful units of computation than we realized.
Back in 2020, researchers from institutes in Germany and
Greece reported a mechanism in the brain's outer cortical cells that produces a
novel 'graded' signal all on its own, one that could provide individual neurons
with another way to carry out their logical functions.
By measuring the electrical activity in sections of tissue
removed during surgery on epileptic patients and analyzing their structure
using fluorescent microscopy, the neurologists found individual cells in the
cortex used not just the usual sodium ions to 'fire', but calcium as well.
This combination of positively charged ions kicked off waves
of voltage that had never been seen before, referred to as a 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 similar ways.
Both use the power of an electrical voltage to carry out
various operations. In computers it's 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 manage these messages
chemically at the end of branches called dendrites.
"The dendrites are central to understanding the brain
because they are at the core of what determines the computational power of
single 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 passed on to other nerves,
which can block or pass on the message.
This is the logical underpinnings of our brain – ripples of
voltage that can be communicated collectively in two forms: either an AND
message (if x and y are triggered, the message is passed on); or an OR message
(if x or y is triggered, the message is passed on).
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 especially thick, packed with
branches that carry out high order functions we associate with sensation,
thought, and motor control.
It was tissues from these layers that the researchers took a
close look at, hooking up cells to a device called a somatodendritic patch
clamp to send active potentials up and down each neuron, recording their
signals.
"There was a 'eureka' moment when we saw the dendritic
action potentials for the first time," said Larkum.
To ensure any discoveries weren't unique to people with
epilepsy, they double checked their results in a handful of samples taken from
brain tumors.
While the team had carried out similar experiments on rats,
the kinds of signals they observed buzzing through the human cells were very
different.
More importantly, when they dosed the cells with a sodium
channel blocker called tetrodotoxin, they still found a signal. Only by
blocking calcium did all fall quiet.
Finding an action-potential mediated by calcium is
interesting enough. But modelling the way this sensitive new kind of signal
worked in the cortex revealed a surprise.
In addition to the logical AND and OR-type functions, these
individual neurons could act as 'exclusive' OR (XOR) intersections, which only
permit a signal when another signal is graded in a particular fashion.
"Traditionally, the XOR operation has been thought to
require a network solution," the researchers wrote.
More work needs to be done to see how dCaAPs behave across
entire neurons, and in a living system. Not to mention whether it's a
human-thing, or if similar mechanisms have evolved elsewhere in the animal
kingdom.
Technology is also looking to our own nervous system for
inspiration on how to develop better hardware; knowing our own individual cells
have a few more tricks up their sleeves 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.