Fiber Track Length Gradients in the Avian Sound Localization Circuit Require Conduction Velocity Gradients to Maintain Isochronicity.

The chicken brainstem sound localization neural network exhibits a case where fiber length varies continuously across the fiber arrays connecting n. magnocellularis and n. laminaris. Detailed morphometric measurements were taken to quantify these length gradients. For both ipsilateral and contralateral projections, the path lengths connecting the caudal low-frequency segments are longer than those at the rostral high-frequency end, with a linear path length gradient between them. The length and the length gradient of the contralateral projection increase from hatchling to 2-4 weeks. Synchronous transmission of information is achieved if there is a conduction velocity gradient across both the ipsilateral and contralateral fiber tracts. Adjustments of conduction velocity are required at both the cellular and population levels. A model is proposed that balances conduction distances and velocities by varying the number of nodes of Ranvier along a gradient to achieve isochronicity across the entire network. Continued differential growth between fiber arrays requires a continuous timing adjustment during development and maturation. Control is assigned to the oligodendrocytes that produce and maintain myelin.