Some Unresolved Problems concerning the Cochlear Nerve

Three types of afferent fibers innervate the hair cells of the organ of Corti: 1) specific radial fibers which establish contacts with a very few neighboring internal hair cells; 2) spiral fibers, each one of which establishes contact with a number of external hair cells distributed throughout long segments of the cochlea; and 3) unspecific radial fibers which are collaterals arising radially at irregular intervals from fibers of the ganglionic spiral bundles and which establish contact with internal hair cells. The existence of spiral ganglionic bundles of fibers oriented apicalward has long been described, and the fact that a number of ganglionic spiral fibers give off radial collaterals to innervate internal hair cells was illustrated by Cajal and by Lorente de Nó. However, those structural details are not mentioned in the modern literature. In the ventral nucleus there are neurons with efferent axons which join the trapezoid body and cells with short axons ramified within the ventral nucleus itself. Two types of cells with efferent axons are illustrated and described, the spherical or bushy cell and the basket cell; and it is shown that branches of division of the two types of efferent axons form association tracts which end in the tuberculum acusticum. Also, the fact is illustrated that fibers having their cells of origin in that tuberculum form association paths which end in the ventral nucleus by means of extensive ramifications which form numerous synaptic endings. The dendritic and fibrillar plexuses in the ventral nucleus are described, an analysis is made of the relationships between the two plexuses and of the synaptic junctions that mediate transmission of nerve impulses. The synaptic junctions belong to a considerable number of types and in all illustrations the important fact repeatedly appears that one and the same fiber may form synaptic endings of widely different sizes and shapes located either on the same neuron or, more frequently, on different neurons. The intimate structure of each type of synaptic ending cannot be revealed by light microscopy, but only light microscopy can reveal to which kind of fiber the synaptic endings do belong. The presentation is concluded with a brief and preliminary discussion of physiological corollaries of certain general features of the anatomy of the acoustic system.