Frequency-specific projections of individual neurons in chick brainstem auditory nuclei

Nucleus magnocellularis and nucleus laminaris in the avian brainstem contain second- and third-order auditory neurons thought to be homologous to the mammalian anteroventral cochlear nucleus and medial superior olivary nucleus, respectively. Nucleus laminaris in the chicken is a tonotopically organized sheet of bipolar neurons; each of these neurons receives spatially segregated bilateral innervation from the two magnocellular nuclei. In the present study, this projection was studied at the single cell level by analyzing the pattern of terminal arborizations of individual horseradish peroxidase-filled axons. Reconstruction of the terminal arborizations of nucleus magnocellularis axons revealed that each axon forms an elongated band of endings, the long axis of which is parallel to the physiologically defined isofrequency bands. Within a band, the individual terminal collaterals form distinct patches separated by areas without endings. We suggest that the elongated terminal fields provide the basis of the tonotopic organization observed in nucleus laminaris and that the trajectories of the ipsilateral and contralateral axons may provide differential conduction delays that are important for binaural integration of acoustic information.