A research team from the Palanker Lab at Stanford University has developed a new technique for visually monitoring nerve cells firing. The technology could one day allow doctors to observe nerve activity in the eye (Light: Science & Applications 10.1038/s41377-018-0107-9).

The method relies on shape changes that occur when a nerve cell fires or “spikes”. During a spike there is a change in the potential across the cell membrane, which increases its surface tension, resulting in the cell temporarily becoming more spherical. This means that light passing through the cell exhibits phase changes after the spike, which the researchers can then detect using a technique called quantitative phase microscopy.

Previously, watching the electrical activity of cells was limited to using electrical recordings or fluorescent probes. Electrical recordings require placement of invasive electrodes adjacent to cells and have poor spatial resolution, while fluorescent probes are susceptible to phototoxicity, photobleaching and heating.  In contrast, the new optical method is ideally suited for potential application in patients.

Developing new techniques

The researchers used cells engineered to spike in a similar way to nerve cells and examined these using an interferometric microscope. To confirm that what they were seeing was a spike, the researchers matched the images from the microscope with signals from an electrode array, which is a proven technique to identify nerve cells firing. They saw that when there was an electrical signal, there was also a change in the phase of the cells, attributed to the cells becoming more spherical.

The phase change, however, was smaller than the noise in the images, which made it impossible to detect a single spike. To circumvent this, the researchers used an ultrafast camera that collects 50,000 frames per second. They then combined 50 frames into one, which averaged out the noise and enabled them to see the cell deformation. The team also developed an algorithm — based on the basic template of a spike that they recorded previously using the electrode array — to identify regions where the signal was strongest and further increase the signal-to-noise ratio.

Using this method, the researchers could determine the extent of cell deformation and provide valuable insight into mechanical changes. They found that the cells deformed up to 3 nm and were able to test theories on cell deformation. As well as the clinical possibilities of the work, the researchers also hope that this study will provide a solid reference for understanding the mechanical effects in cells when they fire.

Eye opening possibilities

One key advantage of this technique is that it could be used in patients to image light-accessible parts of the body, such as the eye. This work is part of a larger collaborative project that aims to use the technology to detect signals passing through the optic nerve, or signals from individual nerve cells in the retina.

In the future, the team aims to use this technique in conjunction with optical coherence tomography — an imaging method commonly used to visualize the back of the eye. They hope that direct monitoring of individual cells will provide more information and allow researchers to better design new therapies for retinal diseases.

“These developments give promise for a day when we can study retinal diseases in humans on a cellular scale and evaluate the treatments to cure them,” comments the principal investigator of the collaboration, Austin Roorda from the University of California, Berkeley.