What’s a neural circuit?

January 11, 2022

Written by: Joseph Stucynski

When you think of supercomputers, you might think of unimaginably complex and powerful machines designed for figuring out how things work, like particle physics, the randomness of weather patterns, or even the best way of beating players in a game of Jeopardy! if you recall IBM’s Watson. But what if I told you the most powerful supercomputer in the world is sitting right between your ears? Yes that’s right, your brain is accomplishing much more complex and demanding tasks in a much efficient manner than any computers available today. But why is that? What makes the brain that much better?

To answer these questions you have to examine what’s happening deep down at the level of individual neurons and how they work together to build circuits. The human brain has over 100 billion neurons and each neuron can electrically “talk” to many other neurons, both near and far throughout the brain. Neurons transmit signals via long branches on the cells, called axons, and connections between individual neurons, called synapses. Astonishingly, there are estimated to be 100-150 trillion synapses in the brain,¹ more than there are stars in the Milky Way galaxy. While supercomputers typically have a greater number, in the range of quadrillions,² of their “cells” called transistors, what they lack is flexibility and interconnectedness which are fundamental properties of brain circuits.

Neural circuits are unique in this way because individual neurons have their own distinct identities. Often times people think of the brain as being made up of different areas that talk to each other to control behavior. And while that’s at least superficially true, what the field of neuroscience has really been addressing for the past 10+ years is which different types of neurons in one brain area are talking to which other types of neurons in another brain area, down at the cellular level. You can imagine it like this: If you go to a party full of people, you might start looking at what they have in common. Maybe you notice that some people are wearing the same things, for instance the same red shirt, and that they only like to talk to each other or certain other people at the party, like those with blue shirts. And likewise maybe people with blue shirts only talk to people with green shirts, whereas the green shirt people only talk to each other and nobody else. Apart from being an awkward party in real life, this is similar to how neurons selectively communicate in the brain.

In a biological sense then, these neuronal identities often take the form of different genes they turn on and proteins they contain. Excitingly, neuroscientists can leverage these cell-type specific ‘marker’ molecules and put all sorts of experimental tools into those neurons to do different things.³ One common approach is to introduce a fluorescent tag to make specific types of neurons stand out under a microscope in order to see exactly where they are in the brain and where their connections travel. Other experimental manipulations involve introducing particular genes into specific cells to make them sensitive to laser light. By shining different types of light, researchers can either excite or inhibit them to test how they interact in circuits. To use the party metaphor again, it would be a bit like shouting through a megaphone to the whole room, but only the people wearing the red shirts can hear you and pass on the message to the blue shirt people they talk to. This technique, called optogenetics, has revolutionized neuroscience by allowing researchers to investigate the functions of specific neural circuits while not affecting the activity of other neurons nearby.³

Neural circuits are also exceptionally flexible and plastic. They self-organize to streamline communications and the same neurons can participate in a variety of overlapping circuits to perform different functions. Moreover, synapses between neurons are not fixed in place like the soldered connections on a computer chip, but rather increase or decrease in number and relative strength in response to use. These essential properties are what allow you to adapt, learn, and form memories, among many other things. And in fact because your brain is so good at doing these things, scientists have leveraged what we know about how the brain works to develop artificial neural networks which try to emulate these properties and allow computers to accomplish tasks that they would otherwise be unable to easily do, like object recognition or optimal robotic locomotion.

All this to say, when it comes to reverse engineering the brain neuroscientists have their work cut out for them. But we now have excellent tools which allow for the finely detailed analysis needed to really understand what brains are doing at the level of separate and distinct circuits. And research is well underway to determine how your brain is doing things that would be the envy of any computer on earth.

References:

1. Stanford University Medical Center. “Stunning details of brain connections revealed.” ScienceDaily. ScienceDaily,17 November 2010.
2. Wikipedia contributors. “Transistor Count.” Wikipedia, 22 Dec. 2021, en.wikipedia.org/wiki/Transistor_count.
3. Deisseroth, K. (2015) Optogenetics: 10 years of microbial opsins in neuroscience. Nature Neuroscience 18;1213-1225.

Cover image by GDJ via Pixabay.