Axons from anteroventral cochlear nucleus that terminate in medial superior olive of cat: Observations related to delay lines

The differences in path length of axons from the anteroventral cochlear nuc lei (AVCN) to the medial superior olive (MSO) are thought to provide the an atomical substrate for the computation of interaural lime differences (ITD) . We made small injections of biotinylated dextran into the AVCN that produ ced intracellular-like filling of axons. This permitted three-dimensional r econstructions of individual axons and measurements of axonal length to ind ividual terminals in MSG. Some axons that innervated the contralateral MSO had collaterals with lengths that were graded in the rostrocaudal direction with shorter collaterals innervating more rostral parts of MSO and longer collaterals innervating more caudal parts of MSG. These could innervate all or part of the length of the MSG. Other axons had restricted terminal fiel ds comparable to the size of a single dendritic tree in the MSG. In the ips ilateral MSG, some axons had a reverse, but less steep, gradient in axonal length with greater axonal length associated with more rostral locations; o thers had restricted terminal fields. Thus, the computation of ITDs is base d on gradients of axonal length in both the contralateral and ipsilateral M SG, and these gradients may account for a large part of the range of ITDs e ncoded by the MSG. Other factors may be involved in the computation of ITDs to compensate for differences between axons.