In a mere two days, I will be turning in my Master's thesis. (Thank goodness.) I figured now was the moment to share exactly what adventure has been the source funding all the other adventures I've been sharing with you since I arrived in France nearly a year ago.
In one sentence, for my Master's thesis, I made two optical systems. And now for some follow-up sentences...
Why did I make these optical systems?
To answer this, we should first review how neurons communicate. Inside a single neuron, the signal that travels down the length of the neuron is electrical, but when signals travel from one neuron to another, these signals are almost always chemical. If you can get your hands on these chemicals--called neurotransmitters-- and throw them at some neurons, those neurons will think they are getting a signal from another neuron, but it’s you who’s controlling how much signal, and exactly what neuron gets the signal, and when. The problem you’ll have now is how to deliver these neurotransmitters to the neurons to carefully control all these details of who sees what signal when.
Fortunately, some chemical engineers have worked out all the particulars of designing a special modified version of neurotransmitters called caged neurotransmitters. When you expose a bunch of neurons in a slice of brain tissue to these chemicals, NOTHING happens. (Great, huh? Actually yes) [Side note: it’s actually possible to slice up a brain, put it in the right kind of well-oxygenated sugar-salt water, and keep your brain slice alive for 6 to 8 hours after an animal dies.] So you’ve got a slice and you put this special chemical all over your slice. Then you shine a tiny spot of a very particular color light on whatever spot you so choose in your brain slice. Let the magic begin. When these caged neurotransmitters are exposed to a very particular color of light, the "cage" falls off and now you have your naked neurotransmitter, ready to start activating neurons.
System 1. This system focused a point of light on a sample that we were already able to image using a different laser that had been installed before I arrived.
 |
System 1: the system that focuses a laser down to a single spot. The blue line traces the early part of the optical path of the light from the fiber optic into the microscope head. |
System 2. This system I built entirely from scratch using super expensive toys to focus shapes of light on a sample. It’s actually very hard to make incredibly tiny shapes of light. The most common way is to scan a focused beam really fast across the shape you wanted to illuminate. You can scan so fast that the naked eye never tell the difference. However, sometimes the shapes you want to illuminate are so big or so specially shaped or the chemicals you are illuminating are so sensitive that scanning just won’t cut it. That’s when my system becomes useful: true simultaneous illumination of whatever shape you want.
 |
| System 2: the system that shapes a beam of light into whatever pattern you desire. The pink line traces the light's path from the laser off the spatial light modulator and onto the sample. The yellow line traces the light from the sample onto a camera. |
How does system 2 shape light?
We start with a laser that shoots out a circular beam of light. We shine this laser on a special device called a spatial light modulator (very expensive toy) that basically acts like a pixelated mirror, and at each pixel light travels a different depth into the spatial light modulator before reflecting. In this manner, after reflecting off of the spatial light modulator, the phase of the "beamlets" of light across your beam have been modified with respect to each other so that the light is no longer circular but will be shaped into the pattern of your design when you couple this spatial light modulator with an odd number of lenses. This kind of light shaping is called holographic shaping. And so we can shape a beam of light to make details as small as 1 or 2 microns! That’s one ten-thousandth of a centimeter! In the words of my friend, I have a ridiculously tiny projector.
System 1 lets me send a signal to a tiny spot on any individual neuron wherever I choose. This lets me study individual synapses. System 2 lets me send signals to several different synapses on one or several nearby cells at exactly the same time. (That is, when I’m not just making cool holographic shapes.)
The holographic shapes I made here really gave me a run for my money. After months of designing this optical system, constructing it, and carefully aligning all the lenses, I couldn’t figure for the life of me why I just couldn’t get a clean picture. It the nick of time, my professor spotted the problem just a few days before my Master’s thesis was due: I simply put the camera a couple centimeters too far away from my sample. After all the hours I’d toiled over this system I felt pretty dumb, but was ridiculously pleased to get some better images after staying in lab until after 1am this Monday. See the difference it made!
 |
| Holographic shaped illumination of Institut Pasteur logo BEFORE properly positioning camera... |
 |
| ...and AFTER |
No comments:
Post a Comment