Our brain contains tens of billions of nerve cells (neurons) which constantly communicate with each other by sending chemical and electrical flashes, each lasting one millisecond. In every millisecond, these billions of swift-flying flashes travel together in a giant starmap in the brain that lights up a complicated glittering pattern. These are the origins of all body functions and behaviors such as emotions, perceptions, thoughts, actions, and memories; and also, brain disorders, for example, Alzheimer’s and Parkinson’s diseases.
One grand challenge for neuroscience is to capture these complex flickering patterns of neural activities - the key to an integrated understanding of largescale brain-wide interactions. To capture these swift-flying signals live has been a challenge to neuroscientists and biomedical engineers. It would require a highspeed microscope to peer into the brain, which has not been possible so far.
A research team now offers a novel solution with their super high-speed microscope — a two-photon fluorescence microscope, which has successfully recorded the millisecond electrical signals in the neurons of an alert mouse.
The new technique is minimally invasive to the animal being tested compared to traditional methods that require inserting an electrode into the brain tissue. Not only is this less damaging to the neurons but also can pinpoint individual neurons and trace their firing paths, millisecond by millisecond.
At the heart of the high-speed microscope is an innovative technique called FACED (free-space angular-chirp-enhanced delay imaging). It makes use of a pair of parallel mirrors that generate a shower of laser pulses to create a superfast-sweeping laser beam at least 1000 times faster than the existing laser-scanning methods.
In the experiment, the microscope projected a sweeping laser beam over the mouse’s brain and captured 1000 to 3000 full 2D scans of a single mouse brain layer (of the neocortex) every second. To probe the genuine electrical signals that pulse between the neurons, the team inserted a biosensor (protein molecules) into the neurons of the mouse brain. The engineered proteins light up (fluoresce) whenever a voltage signal passes through the neurons. The emitted light is then detected by the microscope and formed into a 2D image that visualizes the locations of these voltage changes.
Apart from electrical signals, the team also used the microscope to capture the slow motion of chemical signals such as calcium and the neurotransmitter glutamate, as deep as one-third of a millimeter from the mouse brain’s surface.
A notable advantage of this technique is the ability to track the signals that do not trigger the neuron to fire — weak neuronal signals, called sub-threshold signals, that are often difficult to capture and detect. These often occur in disease conditions in the brain, but they have not yet been studied in detail because of the lack of a high-speed technique.
Another important feature of this technique is that it is minimally invasive. The classical method for recording electrical firing in the brain is to physically embed or implant electrodes in the brain tissue. However, such physical intrusion could cause damage to the neurons, and can only detect fuzzy signals from a couple of neurons.
The team is now working to further combine other advanced microscopy techniques to achieve imaging at higher resolution, wider view, and deeper into the brain in the neocortex, which is about 1 millimeter.