Innervation of the Cochlea

Recent physiological studies have raised questions regarding the validity of previous anatomical information on the afferent innervation of the organ of Corti. Important questions are: What is the relative number of nerve fibers to inner and outer hair cells? How long a segment of basilar membrane is innervated by a single fiber? Is there any dichotomy of fibers to inner and outer hair cells? These and other questions are considered in the light of past and recent studies. The pathways and terminals of the efferent olivocochlear tract have been well defined in some animals. For example, the well-documented concentration of efferents in the basal two-thirds of the guinea pig cochlea has been confirmed in the chinchilla but the number of efferents per outer hair cell is less in the latter animal. Our (Iurato and Smith) recent studies have shown that the outer hair cells are innervated for the most part by the crossed efferents whereas the inner spiral bundle is mostly, if not entirely, derived from the homolateral tract. An analysis of the sensory structure and innervation of the bird's basilar papilla (Takasaka and Smith) have revealed some parallels in terminal efferent fibers between pigeon and rodents. The large efferent findings associated with well-developed subsynaptic cisternae are found only in a particular location and on a differentiated type hair cell.