Every now and then, a result comes along that sounds like something out of science fiction. Nature Photonics played host to one of these this past weekend. In it, researchers built turn signals for neurons out of nothing more than a tiny bead and some light. It's not especially useful right now, but it's still an impressive bit of engineering.
In adult animals, neurons extend small, thread-like projections called axons for large distances away from the main body of the cell. These axons help form connections between places like the brain and the end of the spinal cord, or the spinal cord and the sensory neurons on the tips of your fingers. In the body, directing these axons to the right location is a very carefully controlled process that involves lots of signaling and adhesion molecules. Put a neuron in a culture dish, where none of these signals are present, and it will tend to grow an axon in a straight line. Not satisfied with this, the authors decided to give it a little nudge and see what would happen.
The axon's growth comes at a specialized structure at the tip called a growth cone. To redirect the growth cone, the team placed a special bead nearby, and held it in place by an optical trop created with a laser. The bead was made of materials that have two different indexes of refraction, which allowed a second laser to set it spinning (the second laser had circular-polarized light, which interacts with the two different refraction indexes on the bead to impart motion). Once the bead was spinning, it set fluid flowing near the growth cone, creating a small force that could deflect it.
Between 30 and 40 percent of the time, this was enough to cause the axon to shift direction by as much as 30 degrees from the straight line (the rest of the time, they kept going straight). It was also possible to create gates with two spinning beads that channeled the axon between them.
Being physicists, the authors created a model with shear forces and viscosity to explain the behavior they observed, and they speculate about possible future utility for doing things like spinal repair. What strikes me as a bit more likely is the prospect of using a system like this to control how multiple neurons in a culture dish form connections. It may be possible to build a neural network out of actual neurons.
Nature Photonics, 2011. DOI: 10.1038/nphoton.2011.287 (About DOIs).
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