Innervation of the Apical Turn of the Human Cochlea: A Light Microscopic and Transmission Electron Microscopic Investigation

Abstract

A light and transmission electron microscopic investigation of the apical turn of a freshly fixed human cochlea. Our knowledge about the human cochlea rests to a large extent on animal species research. An opportunity to obtain tissue from normal-hearing persons occurs during surgery for life-threatening petroclival meningioma. This study presents detail on the morphology and innervation of the apical part of the human cochlea using light microscopic and transmission electron microscopic level sectioning. The tissue was histologically processed after removal during petroclival meningioma surgery. The cochlea was serially sectioned perpendicularly to its long axis, and at regular distances semithin sections were reembedded and prepared for transmission electron microscopy. Nerve fibers/fascicles were traced from the area of the spiral ganglion to the level of the inner hair cells, and a cochleotopic "map" of the cochlear nerve supplying the apical portion was constructed. The apical turn was found to be innervated by 3,694 myelinated nerve fibers representing approximately 10% of the total number of fibers innervating the cochlea. The total number of unmyelinated nerve fibers was 513. The majority belonged to the efferent olivocochlear system and the intraganglionic spiral bundle or represented Type II afferent neurons innervating outer hair cells. The significance of the anatomic findings in relation to cochlear implantation is discussed.