The Human Cochlear Nerve in the Basal Turn

Abstract

Cochlear nerve fibers were studied with regard to their course to lower basal turn and their relation to the surrounding bony tissue in the modiolus. Fifteen human temporal bones from persons ranging in age from 70 to 80 years were used. The cochlear nerve fibers run apically taking a spiral course from the internal auditory meatus to the inside of the cochlea, whereas the nerve fibers destined for the lower basal turn go in an opposite direction before entering the modiolus. The dividing point of the fiber arrangement corresponds to an area of the basilar membrane 9 to 10 mm from the basal end. The cochlear nerve passes through the tractus spiralis foraminosus and the modiolar longitudinal canals before reaching Rosenthal's canal. The diameters of the modiolar longitudinal canals directly connected to the spiral tract are smaller than those of the passing nerve bundles; thus, the nerve bundles are in contact with the surrounding bony wall. On the other hand, the nerve bundles destined for the upper turns have more space around the bundles. The vulnerability of the nerve fibers in the lower cochlear turn may be explained by this structural characteristic of the nerve bundle, i.e., its relationship to the bony canal. Methods for preparing a cochlear nerve specimen are presented in this paper. The nerve fibers in the lower basal turn were sectioned from the cochlear nerve trunk up to the fundus of the internal auditory meatus. This removed, quadrangular or triangular nerve bundle mass was first stained and then mounted on a slide in the usual manner. After 1 week, the flattened, stained nerve bundle was removed from the glass slide, and the tissue was embedded in paraffin and sectioned. Thus, individual nerve fibers can be readily evaluated in a thin specimen. An alternative method is to first make paraffin sections of flattened, unstained nerve fibers followed by staining of the sections on glass slides. These methods were useful in evaluating nerve fibers running to the lower basal turn.