Artificial electrical stimulation of the nervous system is one of the foundations of neurotechnology. However, the use of electrical currents to stimulate the nervous system presents several challenges. It is difficult to achieve selective stimulation of only the targeted neurons without activating neighboring neurons. Further, electrochemical reactions at the electrode-tissue interface may lead to electrode dissolution or tissue damage.
As an alternative, magnetic stimulation can be used to stimulate non-invasively. However, the power requirements for magnetic stimulation are high and the resulting stimulation is non-selective. Two recent publications may be harbingers of a future where light—another form of electromagnetic energy—is used to stimulate the nervous system.
Hirase and colleagues from Rafael Yuste’s lab at Columbia University applied laser illumination to depolarize and excite single neurons in vitro. This work follows on the classic work of Fork who, in the early 1970s, demonstrated that laser illumination could produce excitation of molluscan neurons through a reversible, but unknown mechanism. Hirase et al., reporting in the Journal of Neurobiology, used modern two-photon techniques that enabled the laser light to be focused much more precisely than the technology used by Fork. They demonstrated that excitation of pyramidal neurons in brain slices from mouse visual cortex required the illumination to be applied tangential to the membrane of the cell, and that excitation was ineffective if the laser was focused below or within the cell.
Although not entirely clear, the experiments suggested two mechanisms. First, the data support that light-induced membrane depolarization resulted from a photochemical reaction that produced reactive oxygen species adjacent to the cell. The second mechanism was a transient perforation of the membrane that quickly re-sealed after the light was discontinued.
Illumination was able to excite neurons at...
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