Fig. 1: Mitochondrial occupancy of unmyelinated axonal cross-sections between nuclei HVC and RA decreases with axon diameter.

A Schematic representation of the axons running between HVC and RA in the song motor pathway. The song motor pathway originates in HVC within the nidopallium, continues to the downstream nucleus RA in the arcopallium, whose neurons project onto nucleus nXII in the brainstem, which in turn projects to the vocal muscles that form the songbird vocal organ, the syrinx. Modified from⁵⁰. B Example of an axon bundle (yellow arrows) formed by axons from the pathway between HVC and RA, imaged with transmission electron microscopy. Axons comprise myelinated axons (white arrowhead) and unmyelinated axons (white arrow). C Representative examples of axonal cross-sections of different sizes containing mitochondria. D1 Occupancy of axonal cross-sections by mitochondria, measured as the ratio of mitochondrial to axonal areas, plotted as a function of measured axon area (n = 69). An exponential fit (±95% C.I.) of the data is shown in red. Spearman correlation coefficient ρ is reported in the figure. D2 Occupancy of axonal cross-sections by mitochondria, measured as the ratio of the area estimated from a disk whose diameter was set to the minor axis of an ellipse fitted to the mitochondrion to the area estimated analogously for the corresponding axon, as a function of axon diameter (n = 66). Spearman correlation coefficient ρ is reported in the figure. An exponential fit (±95% C.I.) of the data is shown in red. E. Mitochondrion diameter plotted as a function of axon diameter (n = 66). A linear fit (±95% C.I.) of the data is plotted in red. Pearson correlation coefficient r is indicated.