Each action potential is a single nerve impulse, traveling through a neuron as chemically gated ion channels open and close with changes in electrical polarity. The quantum physical optics phenomenon, second-harmonic generation (SHG) is the first multiphoton technique capable of detecting action potentials. SHG is particularly useful for imaging these impulses because it picks out only the cell membrane where impulses occur and does not suffer from other background signals. This selectivity, combined with SHG's fast response to the electrical signals, allows for high signal-to-noise ratio recordings of neuron signaling.
SHG in light waves, Dombeck explains, is similar to the more familiar phenomenon in sound waves, such as those produced in the body of a guitar. The second-harmonic of a guitar string's vibration is twice as high in pitch when it resonates in the wood body of the guitar. Similarly, laser light striking molecules of uniform polarity produces a second-harmonic wave of twice the energy -- or half the wavelength -- which is easily detected by the microscope in the forward propagating direction.
Thus, every change in polarity and every action potential is optically imaged in submicrometer and millisecond spatiotemporal resolution. Previous attempts in other laboratories to record fast electrical signals in live cells with SHG had achieved, at best, about 1-second resolution and were not detecting impulses a few milliseconds in duration.
"Nevertheless, with all the advantages of second-harmonic generation, we still faced two obstacles in imaging action potentials in living tissue," says Webb, the co-inventor (with Winfried Denk) of multiphoton microscopy. "First, many dyes are chemically toxic to neurons of living animals
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Contact: Roger Segelken
hrs2@cornell.edu
607-255-9736
Cornell University News Service
15-Feb-2004