It’s surprisingly difficult to pinpoint what a given nerve is doing at any given moment. The electrochemical dance of neuronal function never stops, and it’s synced to the beat of the default mode network more strongly than to any external clock. Neurons are constantly receiving and transmitting information, but an electrical action potential is measured in millivolts and only changes the magnetic field around a neuron by a few picoTesla.
Up ’til now, this tiny variance has meant we needed invasive methods to get any information whatsoever about the function of living nerves. Now, scientists from Denmark have used a game-changing device for an all-new neuronal imaging technique: an optical magnetometer that can measure a single nerve’s function from outside the body — with quantum-level precision.
Today’s best methods of discerning a nerve’s function are still pretty invasive. Either we have to stick an electrode into a nerve, which is pretty disruptive, or we have to actually dissect a creature and thread one of its neurons through a tiny conductive coil on a machine called a SQUID. Either way, we’re not very good at getting information out of nerves in vivo. And neuroscience has needed another way of looking at nerves in real-time from outside the body, preferably at room temperature.
That’s where the optical magnetometer comes in. These devices work because they use a laser that detects the change in polarization of gaseous cesium atoms when they’re subjected to a changing magnetic field. The flux induced by an electric action potential causes a flutter in the polarized light, which the magnetometer can detect. And the sensitivity of these devices is unparalleled: Their resolution is limited only by quantum effects like the quantum shot noise of light.
The breakthrough here is in the application. This kind of magnetometry has never before been applied to living cells, in part because the magnetic flux generated by an action potential is so very small: nine orders of magnitude smaller than your average fridge magnet. That kind of precision is pretty hard to get at all, to say nothing of using it in vivo. But the combination of how it uses the laser and the tiny size of the sensor end means this device can point at a nerve and see what that particular nerve is doing, to the exclusion of fibers around it. Jensen and co. tried it out on a frog, and used the magnetometer to detect when its sciatic nerve was firing.
A discovery like this has the potential to change the entire brain-imaging field. The team that did this project notes “The magnetometer [is] perfect for medical diagnostics in physiological/clinical areas such as cardiography of fetuses, synaptic responses in the retina, and magnetoencephalography,” or presumably anything else that requires noninvasive brain imaging in the time domain. And it won’t be long until we’re using this technique on humans. This is a clear step forward for both basic research and the biomedical sciences alike.