Optoakustische Stimulation der Cochlea-Einzelzelluntersuchungen an Modell- sowie Spiralganglienzellen

For patients suffering from sensorineural hearing loss due to damaged hair cells of the cochlea, a cochlear implant can restore auditory function. It electrically stimulates the spiral ganglion neurons of the auditory nerve, bypassing the no longer functioning hair cells. However, hearing performance is strongly dependent on the ambient conditions. In noisy environments, especially the capability to understand speech deteriorates significantly. This has been partially attributed to the low spatial precision of the electric stimulation since the electric field extends over a large area in tissue. Due to the tonotopy of the cochlea, low spatial specificity corresponds to low frequency selectivity. In contrast, laser light can stimulate tissue very site-specific promising higher frequency selectivity and thus an improved hearing performance. Therefore, the optical stimulation of the cochlea was investigated in the last years in a number of in vivo experiments. Although the feasibility of the optical stimulation of the cochlea was demonstrated, the stimulation mechanism is still unclear. Although they could provide insight into the mechanism of optical stimulation of the cochlea, in vitro studies concerning the reaction of spiral ganglion neurons to irradiation over a broad wavelength range are yet not available. Therefore, in this study single cell measurements on spiral ganglion neurons and model cells, which were irradiated over a wavelength range of 420 nm to 1950 nm with laser pulses of 5 ns duration, were performed using the patch clamp technique. It could be demonstrated that irradiation with laser light produces similar electrophysiological responses in different cell types. The laser-induced inward current responses at resting potential were linearly dependent on the pulse energy of the laser light and the absorption coefficient of water. The observed cell responses are consistent with the assumption that they are based on a common underlying mechanism which relies on a change in the capacitance of the cell membrane generated by a laser-induced temperature change or a laser-induced pressure pulse. Furthermore, for spiral ganglion neurons it could be shown that optical stimulation resulted in a slight depolarization which was not sufficient to generate action potentials. Since in this investigation much higher radiant exposures were used than in comparable in vivo studies, the results indicate that in a stimulation paradigm with nanosecond-pulses, direct stimulation of spiral ganglion neurons is not the main cause of optical cochlea 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. Thus, the absorption of the laser pulse in the cochlea would lead to the generation of pressure waves that finally stimulate the still intact inner hair cells. Therefore, an optical cochlear implant may substitute damaged outer hair cells, respectively their frequency selective gain, but requires, similar to conventional hearing aids, the presence of inner hair cells.