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In order to perform all of our metabolic functions, to transmit a sensation, or to generate a mechanical response, our nervous system must constantly process a large amount of information. They take the form of nerve impulses (electrical impulses) and travel through our neurons at a very high speed. The speed is such that the phenomenon could never be observed directly. Using the latest advances in high-speed photography, Caltech engineers have developed an ultra-fast camera capable of capturing the movement of electrical impulses through neurons. Observing this previously elusive phenomenon could lead to a better understanding of the biology of the brain, which is fundamental in the search for neurological treatments. Electromagnetic signals traveling at the speed of light can also be captured.
In order to provide sensation (such as touch) through our peripheral nervous system, a whole cascade of information transmission takes place up to the central nervous system. The nerve impulse travels through the neuron cells of the spinal cord and reaches the thalamus, a sensory processing center located deep in the brain. The latter, thanks to its more than 100 billion neurons, then determines the appropriate response in accordance with the information received.
These complex interactions, which affect many neurological functions, happen very quickly. Nerve impulses transmitted through sensory nerves move, in particular, at a speed of almost 160 kilometers per hour. Sensations that require an immediate response (such as a burn) can generate even faster nerve impulses at speeds up to 300 miles per hour.
Medical imaging technologies such as functional MRI can indicate that areas of the brain are activated (by depolarization) when nerve impulses are stimulated. However, “the observation of nerve signals is fundamental to our scientific understanding, but it has not yet been achieved due to the lack of speed and sensitivity of existing imaging techniques,” said Lihong Wang, co-author of the new study, described in Nature Communications. and researcher at the Optical Imaging Laboratory at the California Institute of Technology.
For the first time, the movement of these nerve impulses through the axons could be captured using a camera using differentially enhanced compressed ultrafast photography (Diff-CUP) technology. In fact, Wang’s research team had previously developed the CUP image processing system to be able to capture laser pulses (moving at the speed of light) and record video at 70 billion frames per second. Diff-CUP combines this system with a device called a Mach-Zehnder interferometer to capture sensory nerve impulses.
“Visualization of signals propagating along peripheral nerves is the first step,” says Wang. “It would be important to depict live movement in the central nervous system, which would shed light on how the brain works,” he suggests.
Nerve impulses traveling at very high speeds through the axons are captured by the Diff-CUP camera. © California Institute of Technology
High speed imager connected to interferometer
Thanks to the Mach-Zehnder interferometer, the new Diff-CUP camera can capture fast-moving objects by splitting the light beam in two. Then only one of the two fragments passes through the object, to then recombine with the first and leave. Since light waves are affected by the materials that make up the objects they pass through, the beam that passes through the object becomes desynchronized with the one that does not pass through it (with which it recombines upon exiting). This desynchronization causes interference whose patterns reveal information about the object.
Namely, that this type of interferometry has also been used to detect gravitational waves, and its connection to CUP allows images to be captured at incredibly high speeds. To test their technology, the researchers filmed electrical impulses passing through the sciatic nerve of a frog (Xenopus laevis) at about 100 meters per second. The movement of electromagnetic pulses through a lithium niobate crystal (at the speed of light) has also been successfully recorded.