The Effect Of Electrical Stimulation On Excitability Of The Auditory Nerve In Guinea Pig

The effect of electrical stimulation on excitability of the auditory nerve in guinea pig Objective1. To develop a rectangular pulse stimulator for electrical stimulation of auditory nerve. 2. To clarify the correlation between EABRs and cABRs. 3. To assess the effect of high rate electrical stimulation on excitability of the auditory nerve by recording the change of amplitude of the electrically evoked auditory brainstem responses (EABRs) 4. To evaluate the effect of stimulus intensity on excitability of the auditory nerve via recording the change of amplitude of the electrically evoked auditory brainstem responses (EABRs) of guinea pig. 5. To investigate the influence of electrode position within scala tympani of the guinea pig on neural excitability while recording the change of threshold of electrically evoked auditory brainstem responses(EABRs). 6. To clarify the situation concerning the effects of less perilymph in the scala tympani of guinea pig on neural excitability via recording the change of threshold of electrically evoked auditory brainstem responses (EABRs). Methods1. A rectangular pulse stimulator for electrical stimulation of auditory nerve was developed using intelligence-controlled and practical Single Chip Micyoco system. 2. After click evoked auditory brainstem responses (cABRs) were recorded in the anesthetic guinea pigs, both bullaewere exposed and the round window membranes incised. A standard electrode array was carefully inserted a distance of approximately 4mm into the scala tympani of both the experimental cochlea and the control cochlea. The most apical electrodes were used as stimulating electrodes. The round window was then sealed with crushed muscle. The electrode arrays were maintained in the same position throughout the experiment. While the stimulus intensities were kept constant in the normal clinical level (6dB above EABR threshold), four stimulus rates of 200(n=14), 400(n=10), 1000(n=11), 2000(n=10) pulses/s(PPS) were each delivered to bipolar electrodes in scala tympani of 45 guinea pigs for 2h acute electrical stimulation.Threshold and wave I amplitudes of EABRs were recorded for periods of 3h following 2h of acute stimulation. By comparing the pre-stimulus amplitudes of wave I in EABRs with the post-stimulus one(t- test), the effect of high-rate electrical stimulation on the excitability of auditory nerve was assessed. 3. With acute electrical stimulation using stimulus intensities within normal clinical levels [6dB(n=35) or 12dB(n=32) above EABR threshold] or significantly above normal clinical levels[18dB above EABR threshold(n=30)] at different stimulus rate of 200, 400, 1000 pulses/s(PPS) the effect of different stimulus intensity on the excitability of auditory nerve was evaluated by recording the stimulus induced amplitude changes of wave I. 4. With the aid of an operating microscope, the standard electrode array was placed in the specific location adjacent to the modiolus under visual control. After perilymph was again observed at the round window, the electrode entry point was then sealed by gently placing crushed muscle around the electrode array. The stimuli used to evoke EABRs(the probe current) consisted of 50μs per phase biphasic current pulse presented at 30 PPS. The two most apical electrodes were used as stimulating electrodes.The threshold was defined as the average of two EABR threshold recordings. Following the completion of EABR threshold recordings, the crushed muscle seal was removed, perilymph was continuously aspirated from basal turn by placing the end of 1mm(?)×4cm(L)sponge in the scala vestibule , until the perilymph in the scala tympani was found disappearing under the operating microscope. Then the averaged EABR threshold was again recorded as done before. The situation concerning the effect of less perilymph in the guinea pig scala tympani on neural excitability was clarified by comparing the pre- and post- EABR threshold (t-test). 5. First, with the help of operating micros...