Functional synapse elimination in the developing avian cochlear nucleus with simultaneous reduction in cochlear nerve axon branching.

We studied the chick auditory system to determine whether there is a normal developmental reduction in the number of cochlear nerve axons innervating individual cochlear nucleus (nucleus magnocellularis, NM) neurons. We also examined the preterminal branching patterns of cochlear nerve axons during development. The number of cochlear nerve axons innervating individual NM neurons was estimated by counting the increments in the postsynaptic response as the intensity of cochlear nerve electrical stimulation was varied gradually; this number fell from a mean of 4.0 on embryonic day 13 (E13) to a mean of 2.2 on E17 and E18 and the 4th day after hatching. This highly reliable decline in functional convergence was accompanied by a decrease in the number of preterminal branches of cochlear nerve fibers innervating the NM. On E13 and E14, most axons stained by iontophoretic injections of horseradish peroxidase showed two distinct preterminal branches in the NM. By E17 and E18 and thereafter, cochlear nerve axons were unbranched and terminated with a characteristic single large calycine ending in the NM. There are about twice as many cochlear nerve axons as neurons in the NM and the number of fibers in the nerve appears to decline only slightly between E13 and E17. The 50% decline in the number of cochlear nerve axons making functional synapses on individual NM neurons therefore is associated principally with the concurrent elimination of cochlear nerve axon branching in the NM.