Cochleotopy of human cochlear nucleus

Introduction: The human peripheral auditory pathway in infants is a relatively less explored domain in auditory research. In the present study, the cochlea, auditory nerve (AN) and the cochlear nucleus (CN) were examined using immunohistochemistry and light and electron-microscopy. Methods: The study was performed on specimens retrieved from a 40-day-old infant who could not survive after corrective surgery for congenital cyanotic heart disease. The donation was done in accordance with the protocol approved by the Human Ethics Committee, All India Institute of Medical Sciences (AIIMS), New Delhi, India. Results: The AN revealed degeneration of nerve fascicles originating from the spiral ganglion neurons of the middle and basal turns of the cochlea. The proximal and middle segments of the nerve showed varying degrees of degenerating fibers with macrophages whereas the most distal part of the nerve showed dilated endoneurial capillaries without any degeneration. On electron-microscopy, the nerve revealed features of acute myelinopathy. The differential perisomatic synaptophysin immunoreactivity was observed in the CN. The unaffected neurons showed strong microtubule-associated protein 2 (MAP-2) immunoreactivity, while those denervated revealed decreased staining. The VCN showed varying degree of parvalbumin (PV) and calbindin (CB) immunostaining with AN fibers and terminals showing strong immunoreactivity for PV. The AN revealed strong immunoreactivity for CB on the affected side. The spiral ganglion neurons did not reveal any pathological changes. Discussion: The results provide a cochleotopic map of the human CN that maybe relevant in better understanding of the tonotopic organization and will be helpful during auditory implants in children.