Neural stimulation with optical radiation

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Neural stimulation with optical radiation.

Laser Photon Rev. Author manuscript; available in PMC 2012 October 16.

Claus-Peter Richter,1,3,* Agnella Izzo Matic,1,2 Jonathon D. Wells,4 E. Duco Jansen,5 and Joseph T. Walsh, Jr.2

Published in final edited form as:
Laser Photon Rev. 2011 January 1; 5(1): 68–80.
Published online 2010 June 7. doi: 10.1002/lpor.200900044
PMCID: PMC3472451

Pulsed mid-infrared lasers have been suggested as a method for neural stimulation [13]. The use of lasers has several appealing features when compared to electrical stimulation: no direct contact is necessary between the stimulating source and the tissue, fine spatial resolution of stimulation can be achieved, no stimulation artifact is generated to deter simultaneous recording of electrical responses from the neurons, and there is no electrochemical junction between the stimulation source and the tissue. Furthermore, at many of the radiation wavelengths used for infrared neural stimulation (INS), the radiation can be easily coupled to an optical fiber. Limitations of INS relate to the laser–tissue interaction. Irradiation of tissue with infrared wavelengths will result in a transient temperature increase, which can cause thermal tissue damage. Although it was possible to stimulate auditory neurons through thin bony structures in the cochlea, thick absorbing or scattering layers above the target structure may prevent neural stimulation.
2. Optical stimulation of neurons
…Neural stimulation may also occur via a photochemical reaction, which is initiated by the absorption of the radiation energy…
Photochemical neural activation can also involve the addition of a “caged” molecule that is released upon irradiation. Caged molecules are molecules that were rendered inert by chemically modifying the structure of a bioactive molecule. Irradiation transforms and/or cleaves the caged molecule to restore the biological activity, which is commonly referred to as “photorelease” or “uncaging” [22]. The resulting active molecules can be agonists or antagonists. Thus, photolysis of caged compounds is a method for using light to switch biological processes on or off…
…The authors suggested that the primary mechanism of stimulation was a thermal effect from the laser. Furthermore, they demonstrated that all of the laser-stimulated neurons were sensitive to capsaicin, which binds to a temperature-sensitive ion channel…
3. Neural stimulation with a pulsed infrared laser
3.1. Stimulation of peripheral nerves
…The upper limit for the repetition rate that ensures nondamaging laser stimulation in peripheral nerves occurred near 5 Hz. The maximum duration for constant low repetition rate stimulation (2 Hz) was approximately 4 min with adequate tissue hydration [50]. Note, these results differ from the findings for the auditory nerve (see below for details)..
3.2. Stimulation of Cranial nerves
3.3. Stimulation of the central auditory system in vivo

Lee et al. [68] have demonstrated that neurons of the central auditory system can be stimulated via INS.

4. Possible mechanisms for optical stimulation of neurons

Although some regimes of optical stimulation occur via photochemical interaction (see earlier sections), it is unlikely that pulsed, mid-infrared lasers, such as the FEL, Ho:YAG, or Aculight diode lasers, evoke neural responses via a photochemical reaction. The individual photon energies emitted by these lasers are significantly lower than the energies required to move an electron to an excited state, as is needed for a photochemical reaction.


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