How does the cochlear implant work instead of the cochlea?
The main function of the cochlea is to spatially separate the sound according to frequency and then convert it into a neuroelectric signal for transmission to the auditory nerve.
The spatial separation frequency is mainly achieved by the cochlear basement membrane. The basement membrane extends from the oval window to the top of the cochlea, and the hardness gradually decreases, the width increases, and the mass increases. The basement membrane has a high resonance frequency near the oval window and a low resonance frequency near the volute. This maximizes the amplitude of the low frequency sound at the apex, and the high frequency sound has a high amplitude at the oval window. The basement membrane of the cochlea is thus separated from high and low frequencies.
The basement membrane vibration causes mechanical stimulation of the hair cells that grow on the basement membrane, the ion channels open, the current flows, and the membrane potential of the hair cells changes. That is to say, the hair cells in the cochlea turn mechanical vibration into a change in nerve potential. After that, changes in the hair cell potential trigger the release of neurotransmitters, which act on the auditory nerve, causing the auditory nerve to release action potentials and transmit information to the auditory center.
The electronic cochlea, also known as the cochlear implant, is an implantable hearing aid that is suitable for patients with severe deafness who have a damaged cochlea but a normal auditory nerve. Cochlear implants include both in vitro and in vivo parts.
The extracorporeal part mainly converts the external sound into electromagnetic waves and sends them to the internal body. The body part is surgically implanted, and the transmitted signal is decoded and converted into an electrical pulse for transmission to the stimulation electrode. The stimulating electrodes are strip-shaped, inserted into the cochlea, spirally arranged along the cochlea, and electrical impulse stimulation is applied to the auditory nerve fibers of different parts according to the frequency of the sound, thereby allowing the patient to recover a certain degree of hearing.
The cochlear implant bypasses the natural peripheral channels of the sound conduction (the outer ear, the middle ear and the cochlea) and directly stimulates the auditory nerve.
The cochlear implant does not fully restore hearing because it is too rough compared to a sophisticated natural cochlear implant. After the patient’s adaptation and training to the electrical pulse stimulation, the patient can finally hear the ambient sound and understand the voice call.