We often compare the brain to a computer
Think about the mechanics of yours for a moment.
It’s 1.3 kilograms of congealed porridge, held together by cling film, surrounded by fluid, enclosed in a spider web, wrapped in a leathery case and protected by your skull.
Although unimpressive to look at, its scale is mind-boggling.
You own approximately 100 billion brain cells. Your individual neurons are about 4 microns thick. 30,000 of them would fit on a pin head.
You have more than 125 trillion synapses, or connections between brain cells, in the cortex alone. The synapse, a tiny gap between the edges of two of your brain cells, is smaller than a thousandth of a millimetre.
You have more than 100,000 miles of blood vessels, capillaries and transport systems, through which passes nearly a litre of blood every minute.
Your brain is capable of 0.1 quadrillion computations per second which it performs almost effortlessly most of the time, and without you realizing how much work it’s actually doing.
Is it as good?
But for all that, the biggest meanest computers outdo the brain in pure computational power, able to generate calculating grunt more than ten times what the brain can do, without getting tired. They aren’t called supercomputers for nothing, and pack a formidable processing punch that they brain can’t compete with.
At least, not in that sense.
From out of the beginning of each brain cell come spindly projections, called dendrites, looking like roots of a bush. Each dendrite increases the surface area of the brain cell and, much like roots sucking up water for the benefit of a plant, dendrites receive information from other cells and relay it to their parent.
At least that’s what we thought
Rather than being bit-part actors in the brain, dendrites themselves do more than passively pass on information. They are active players, processing information as they go and, therefore, multiplying the brain’s raw computing power.
Add to the 100 billion brain cells the additional power of hundreds of billions, even trillions of dendrites, and we can grasp at what a machine the brain really is.
“Suddenly, it’s as if the processing power of the brain is much greater than we had originally thought,” said Spencer Smith, PhD, assistant professor in the UNC School of Medicine, and one of the team whose research was published in Nature, October 27. Already, the research has asked searching questions about long-held beliefs
The dendrites have their own computational resources, and contribute to information processing in a way we had not imagined, the implications of which are fascinating. Suddenly, there is a springboard for computing power as neurons can harness the capability of their dendritic spines.
Spines and spikes
It’s in the axon, the long fibre running from the cell body, where brain cells usually generate their electrical spike. Interestingly, molecules present there to support the axonal spike, are also present in the dendrites. We knew that dendrites can use such molecules to generate their own electrical spike, but didn’t know if this was a part of normal brain activity.
Take vision for example. We ‘see’ with the visual cortex, located at the back of the brain. A key question from this new development is, Might dendritic spines, and the spikes they create, be involved in how we see?
According to Smith, absolutely yes. Senior author Professor Michael Hausser adds: “This work shows that dendrites, long thought to simply ‘funnel’ incoming signals towards the soma, instead play a key role in sorting and interpreting the enormous barrage of inputs received by the neuron. Dendrites thus act as miniature computing devices for detecting and amplifying specific types of input”.
So how do you prove it?
Proving this took some time, and no small effort. The researchers were aiming to “listen in” to a single dendrite in a mouse’s brain, trying to hear the electrical signalling of the single dendrite.
“Attaching the pipette to a dendrite is tremendously technically challenging,” Smith said. “You can’t approach the dendrite from any direction. And you can’t see the dendrite. So you have to do this blind. It’s like fishing if all you can see is the electrical trace of a fish.” And you can’t use bait. “You just go for it and see if you can hit a dendrite,” he said. “Most of the time you can’t.”
So, he built his own two-photon microscope system to do it instead.
Once the pipette was reliably attached to a dendrite, the team began to record the electrical activity from single dendrites. The mice were anesthetized, but awake, which meant they could watch visual stimuli while the researchers recorded their brain activity. And as they watched, they noticed strange new things.
The dendrites themselves were spiking.
And not randomly, but selectively, and based on the visual stimulus they were encountering. This showed beyond doubt that the dendrites themselves were processing what the mouse was seeing.
To develop visual evidence of their discovery, Smith’s team filled neurons with calcium dye, which provided an optical readout of spiking. This showed clearly that dendrites themselves spiked while other neuronal parts did not, showing that the spikes were due to localized processing in the dendrites.
His team plans to explore what this newly discovered dendritic role may play in brain circuitry and particularly in conditions like Timothy syndrome, in which the integration of dendritic signals may go awry.
So here’s the take home bit
It’s an old chestnut to say that if the brain were so simple we could understand it, we would be so simple that we couldn’t.
What is clear, is that the brain has resources, capacity and systems still yet to be understood. It remains the most complex thing we know.
We build computers to do one thing, repeatedly, very fast. They aren’t a match for what you possess.
Impressive words to drop into the morning coffee chat
What do you think?
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