A cochlear implant is an electronic device that is implanted into the inner ear to restore auditory perception, at least partially, to the deaf and hard-of-hearing.  Cochlear implants create auditory sensation by generating electric field gradients in the area of the peripheral nerve fibers of the auditory nerve bundle.  This bundle contains approximately 30,000 individual afferent nerve fibers normally linked to approximately 4,500 inner hair cells.  Sound signals are picked up by a microphone within the implant, converted into digital signals, and processed by a signal processor in order to activate different stimulation channels.  These channels, in turn, stimulate different groups of nerve fibers within the auditory nerve. 
The ear is composed of four main sections: the external ear, middle ear, inner ear, and the transmission pathway to the hearing center in the brain.  In normal hearing, sound waves travel along the external ear canal and cause the tympanic membrane (also called the ear drum) to vibrate.  The three small bones of the middle ear (the malleus, incus, and stapes) transmit these vibrations to the cochlea of the inner ear.  The cochlea is divided along its length by a basilar membrane that distributes vibrational energy longitudinally by frequency.  The lowest frequencies cause maximum membrane motion near the cochlea's apex, and the highest frequencies maximize motion near the base.  Four parallel rows of hair cells extend along the length of the basilar membrane and, when vibrated, transduce acoustic signals into electrical impulses carried to the brain by auditory nerve fibers (see FIGS.  1 and 2).