One of the goals of neuroscience is to understand how neurons interact with each other to detect external stimuli and respond to it. In other words, how do brains process information and generate appropriate behaviour?
Neuroscientists have investigated this question at the level of individual neurons for some time. One technique that has made this possible is optogenetics—the ability to insert genes into neurons that fluoresce when the neuron is active.
To do this, researchers simply illuminate the modified neurons with laser light and wait to see which ones fluoresce. In theory this should allow neuroscientists to see how neurons interact and co-ordinate in response to various stimuli.
The problem is the sheer density of neurons in the brain—there are too many, too closely packed into too great a volume. This has prevented neuroscientists from resolving the individual activity of all of them in real time. At least until now.
Today, Tina Schrödel, Robert Prevedel and pals at the Research Institute of Molecular Pathology in Vienna, Austria, say they’ve solved this problem. And to prove it, they have filmed the simultaneous activity of the neurons in the entire brain of a nematode worm for the first time.
The nematode worm (C elegans) is a good subject for this kind of work. It has only 302 neurons that make 8000 connections with each other. What’s more, researchers known exactly how these neurons are arranged within the worms, giving them the entire ‘wiring diagram’ for the creature.
Nevertheless, the neurons are small and so densely packed into the head and tail regions of the worms—their ‘brains’—that resolving their activity has never been possible.
The problem breaks into two parts. The first is that fluorescent genes are expressed throughout the entire volume of a neuron. So when two neurons are close together, it’s hard to see where one starts and the other finishes.
To solve this, Schrödel, Prevedel and co developed a way to ensure that the genes only fluoresce in the nucleus of each neuron. That makes active neurons much easier to tell apart.
The second problem is the sheer size of the brain. The genes fluoresce when they are illuminated with laser light of a certain frequency. So researchers generate a spot of laser light and scan it across a brain sample to monitor activity in that area.
An important limit here is the speed at which the spot can be scanned across the sample. And this becomes a critical limiting factor when the spot has to be scanned through the entire volume of a sample. It’s just not possible to do this quickly enough to capture the simultaneous activity of neurons in the volume of an entire brain.
Schrödel, Prevedel and co have an ingenious solution to this too, called light sculpting. This works by bouncing the spot of laser light off a grating that stretches it out. This creates a disc of light that images an area of the brain in one go rather than a single point. In affect, it produces a cross-sectional image of brain activtiy.
The advantage is that the light disc need only be scanned in one direction to capture the whole volume of the brain. And this can be done at a rate that allows the team to film the neuronal activity of the entire brain at a rate of 80 frames per second.
“Using this approach, we demonstrate brain-wide and near-simultaneous imaging of 70% of the neurons contained in the head ganglia,” they say.
They go on to study the brain activity of paralysed nematode worms as the oxygen levels in the environment are changed. This clearly shows chemosensory circuits that other researchers have already identified. The team also show how the activity in different parts of the brain is co-ordinated when the worms are stimulated in different ways.
That’s fascinating work with significant future potential. For example, the technique allows significantly higher frame rates. “Our current instrumentation allows imaging speeds of up to 200 fps,” Schrödel, Prevedel and co.
And that should make it possible to extend the technique relatively easily. “By increasing the laser power and modifying the transversal shape of the beam, it would be straightforward to record from the entire anterior nervous system of C. elegans or brains of larger animals,” they say.
So nematode worms are obviously just the start. The filming of brain movies at single neuron resolution is set to to become a standard laboratory technique. And when that happens, neuroscientists will finally be able to study brain-wide neural activity at their leisure.
That should allow them to join the dots in ways that haven’t been possible before. Expect to see a lot more brain movies in the not too distant future.
Ref: arxiv.org/abs/1406.1603 : Brain-Wide 3d Imaging Of Neuronal Activity In Caenorhabditis Elegans With Sculpted Light