Each afferent auditory fiber and the hair cell with which it is connected have a minimum threshold at one frequency (“characteristic” or “best” frequency). The basilar membrane vibrates at different frequencies throughout its length, according to the frequency of the sound stimulus. In this way, the fibers of the cochlear nerve respond to the full range of audible sound and can differentiate and resolve complexes of sounds.
The inner hair cells, numbering about 3,500, are of particular importance, because they synapse with approximately 90 percent of the 30,000 afferent cochlear neurons. The central processes of the primary auditory neurons constitute the cochlear division of the eighth cranial nerve. In addition, the nerve contains approximately 500 efferent fibers, which arise from the superior olivary nuclei (80 percent from the contralateral nucleus and 20 percent from the ipsilateral one) and synapse with the afferent neurons from the hair cells (Rasmussen). The function of this efferent pathway is not clear. It is thought to play some part in the auditory processing generated in the ear itself, possibly to enhance the sharpness of sound perception by a feedback mechanism. The eighth nerve also contains adrenergic postganglionic fibers that are derived from the cervical autonomic chain and innervate the cochlea and labyrinth. Their function has been the subject of investigation but remains unknown.
The semicircular ducts, utricle, and saccule, collectively comprising the vestibular apparatus, contain the sense organs for the detection of angular and linear acceleration. They are filled with an intracellular fluid, endolymph, and are surrounded by cerebrospinal fluid (perilymph) within excavated spaces of the temporal bone, the semicircular canals. The latter term, canal, is used interchangeably with the proper description, ducts, to describe the vestibular apparatus.
The vestibular division of the eighth nerve arises from cells in the vestibular, or Scarpa ganglion, which is situated in the internal auditory meatus. This ganglion is also composed of bipolar cells, the peripheral processes of which emanate from hair cells of the specialized sensory epithelium of the labyrinthine apparatus. The sensory epithelium is located on hillocks (cristae) in the dilated openings or ampullae of the semicircular ducts, where they are called the cristae ampullaris, and in the utricle and saccule, where they are called maculae acusticae. The hair cells of the maculae are covered by the otolithic membrane, or otolith, which is composed of calcium carbonate crystals embedded in a gelatinous matrix. The sensory cells of the cristae are covered by a sail-shaped gelatinous mass called a cupula (see Fig. 14-1). The labyrinthine semicircular ducts transduce angular acceleration of the head, and the otoliths transduce linear acceleration, including the effects of gravity.