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S4 Implant hardware & new implant technology
Optical stimulation of the cochlea - electrophysiological responses of irradiated spiral ganglion neurons
in vitro
Rettenmaier A. , Lenarz T. , Reuter G.
MHH, ENT, Hannover, Germany
It has been shown that the inner ear can be stimulated in vivo by laser pulses resulting in cochlear potentials
corresponding to auditory evoked signals [1]. Optical stimulation can be very site specific which coincides, due to
the tonotopy of the cochlea, with a very frequency specific stimulation, possibly overcoming the limitations of
conventional hearing aids as well as of electrical cochlea implants. To investigate the basic effects and
mechanisms of the optical stimulation of the cochlea, single cell measurements were performed. Spiral ganglion
neurons, isolated from the cochleae of P3 - P6 Sprague Dawley rats, were stimulated with 5 ns laser pulses.
Their electrophysiological reactions on different laser parameters such as pulse energy and wavelength were
detected by means of the whole cell patch clamp technique. The irradiated cells show inward currents at resting
potential, depending linearly on the pulse energy of the laser light as well as the absorption coefficient of water.
These reactions are clearly elicited by the laser beam and can be observed in voltage clamp measurements as
current spikes on a timescale of less than 0.3 ms. Current clamp experiments demonstrate slight depolarizations
of the membrane potential due to the irradiation with laser light. With a magnitude of less than 2 mV the
depolarizations were not sufficient to generate action potentials. The laser-induced temperature change was less
than 1.5 °C. The results show that the thermal effects of laser irradiation with pulse durations in the nanosecond
range lead to cellular responses, but do not suffice to generate action potentials. In combination with in vivo
experiments demonstrating positive stimulation results, performed with similar laser parameters (1), this
suggests that direct stimulation of spiral ganglion neurons is not the main mechanism of optical cochlear
stimulation. The results rather support the theory that the optical stimulation of the cochlea is based on an
optoacoustic effect for the investigated laser parameters.
[1] Wenzel et al. Green laser light activates the inner ear. J. Biomed. Opt. 144, 044007 (2009)
Support provided by the Deutsche Forschungsgemeinschaft (DFG) SFB Transregio 37 and the Georg-ChristophLichtenberg scholarship of the Federal State of Lower Saxony.