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The internal resistance of axons to ionic current flow affects the speed of action potential propagation. As biological cables, axons contain mitochondria which are necessary to support axonal function with energy supply. Although we would expect mitochondria to increase the internal resistance to current flow, their impact on the conduction velocity of action potentials has remained elusive. To investigate the impact of mitochondria on action potential propagation in the small non-myelinated fibers found in the vertebrate brain, we combined computational modeling and electron microscopy from the axons found in the premotor pathway that controls the production of birdsong with submillisecond precision. Mitochondria occupancy of axonal cross-sections ranged from 5 to 73% (average: 29%) in the ∼ 0.2-0.7 μm diameter non-myelinated axons connecting song premotor nuclei HVC and RA in canaries. Interestingly, this occupancy depends on axonal diameter: axonal cross-section occupancy by mitochondria was larger in small axons, with an average occupancy of ∼46% for axons with diameters smaller than 300 nm and ∼21% for larger diameters. Computational modeling showed that when the propagating action potential meets a mitochondrion, the conduction velocity decreases and the action potential is delayed by tenths of microseconds to microseconds. This effect is stronger in small axons given their larger cross section mitochondrial occupancy and cumulates delays of tenths of milliseconds along the whole pathway linking HVC and RA. Finally, we modeled the impact of varying densities of mitochondria on action potential propagation along the songbird premotor pathway. In summary, our model shows that axonal mitochondria induce the anisotropic propagation of action potentials, and that this effect cumulates a typical delay in the order of tenths of milliseconds over distances of mms. By partially occupying axoplasm, mitochondria constitute a biological design constraint that delays information processing in the small-diameter unmyelinated axons found in the vertebrate brain. |
