Recent research has shown a hitherto unseen type of cell signalling taking
place in the human brain.
The discovery is exciting because it raises the possibility that our brains
may possess greater computational capability than previously thought.
In 2020, scientists from German and Greek institutions described a process
in the brain's outer cortical cells that creates a fresh, self-generated
"graded" signal. This mechanism may give individual neurons an additional
means of doing their logical tasks.
The neurologists discovered that individual cells in the cortex used not
just the typical sodium ions to "fire," but also calcium, by detecting the
electrical activity in slices of tissue taken during surgery on epileptic
patients and examining their structure using fluorescence microscopy.
This confluence of positively charged ions sparked previously unseen
voltage waves known as calcium-mediated dendritic action potentials, or
dCaAPs.
Brains, particularly human ones, are frequently compared to computers.
Although the comparison has its limitations, they carry out jobs similarly
on certain levels.
Both rely on the strength of an electrical voltage to perform various
tasks. In computers, it takes the form of an extremely basic flow of
electrons through transistors.
The signal in neurons takes the shape of an oscillating wave of channels
that open and close in order to exchange charged ions like sodium, chloride,
and potassium. An
action potential
is a pulse of moving ions.
Neurons handle these signals chemically at the ends of branches known as
dendrites instead of using transistors.
At the American Association for the Advancement of Science in January 2020,
Humboldt University neuroscientist
Matthew Larkum told
Walter Beckwith that "the dendrites are crucial to understanding the brain
because they are at the center of what defines the processing capability of
single neurons."
The traffic lights of our nervous system are dendrites. A sufficiently
enough action potential can be transmitted to other nerves, which can either
block or transmit the signal.
Our brain's reasoning is based on voltage ripples that may be sent in one
of two ways: either as an AND message (the message is passed on if both x
and y are activated), or as an OR message (if x or y is triggered, the
message is passed on).
Maybe nowhere is this more intricate than in the cerebral cortex, the
cerebral nerve system's thick, wrinkled exterior. Particularly dense and
full of branches that perform higher order tasks including cognition, motor
control, and sensation are the second and third deeper layers.
The scientists closely examined tissues from these layers by attaching
cells to a tool known as a somatodendritic patch clamp to transmit active
potentials up and down each neuron while recording their responses.
We had a "eureka" moment when we first noticed dendritic action potentials,
according to Larkum.
They verified their findings in a few samples collected from brain tumors
to make sure any discoveries weren't specific to persons with
epilepsy.
Even though the scientists had conducted analogous studies
on rats, the signals they saw zipping through human cells were quite
different.
More crucially, they continued to detect a signal after giving the cells a
dose of the sodium channel blocker tetrodotoxin. All of this noise was only
silenced by inhibiting calcium.
It's intriguing enough to discover an action potential mediated by calcium.
Yet, a surprise emerged when the behavior of this delicate new type of
signal in the cortex was modelled.
These individual neurons might perform 'exclusive' OR
(XOR)
intersections, which only allow a signal when another signal is graded in a
certain way, in addition to the logical AND and OR-type operations.
The
researchers stated
that "traditionally, the XOR operation has been assumed to need a network
solution."
To understand how dCaAPs function across whole neurons and in a biological
system, more research is required. Not to mention if it's unique to humans
or if similar systems have developed in other animal species.
In order to create better hardware, technology is also looking to our own
nervous system for inspiration. By learning that each of our unique cells
has a few extra tricks up their sleeves, new techniques to network
transistors may be developed.
Future researchers will need to determine precisely how this novel logic
tool, which may fit inside a single nerve cell, translates into higher
functions.
This research was published in
Science.
