INTRACELLULAR LABELING OF AUDITORY-NERVE FIBERS IN GUINEA-PIG - CENTRAL AND PERIPHERAL PROJECTIONS

Auditory-nerve fibers (ANFs) in the cat have been subdivided according to spontaneous rate (SR), with high-SR fibers showing the lowest thre sholds. Cochlear terminals of the three SR groups differ in caliber an d synaptic position around the inner hair cell(liberman [1982b] Scienc e 216:1239-1241); central terminals differ in degree of branching and in which subregions of the cochlear nucleus (CN) are targeted (Liberma n [1991] J. Comp. Neurol. 313:240-258). The present study investigates whether these SR-bascd differences in ANF connections are unique to t he cat. Thirty ANFs from 15 guinea pigs were intracellularly labeled a fter measuring characteristic frequency, threshold, and SR. Labeled co chlear projections showed significant SR-based differences in axonal c aliber, with low and medium-SR fibers 20-40% thinner than those of hig h-SR fibers for both peripheral and central (modiolar) axons. Spatial segregation in the inner hair cell area could not be assessed; however , the peripheral axons in the osseous spiral lamina showed the same SR -based organization reported for the cat (Kawase and Liberman [1992] J . Comp. Neurol. 319:312-318). Labeled central projections also showed significant SR-based differences. Low: and medium-SR fibers: 1) were m ore highly branched, 2) sent significantly more terminals to the small -cell cap region of the CN, and 3) produced endbulb terminals (on sphe rical cells) that were significantly more complex than high-SR fibers. All of these SR-based trends for both central and peripheral projecti ons are analogous to those reported in the cat, and, thus, may represe nt a fundamental organizational principle of the mammalian ear. (C) 19 97 Wiley-Liss, Inc.