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Diameter growth drives changes to conduction speeds along myelinated axons over time. A Schematic overview of the electrophysiological set-up for measuring conduction velocity along the Mauthner axon. B Representative super-resolution confocal images of Mauthner axons labelled using the transgenic line Tg(hspGFF62A:Gal4); Tg(UAS:mRFP) alongside whole-cell patch clamp traces showing action potentials generated in response to extracellular stimulation of the same axons (C). D Axon diameter measurements for Mauthner axons at 5 dpf at somite 15 in neuron-specific ipo13b mutant animals and control siblings (two-tailed unpaired t-test with Welch’s correction, n = 8 control and 10 neuron-specific mutant axons from individual animals, p < 0.0001). E Conduction velocity measurements for the same axons as in (D) (two-tailed unpaired t-test with Welch’s correction, n = 8 control and 10 neuron-specific mutant axons from individual animals, p < 0.0001). F Conduction velocity along Mauthner axon plotted against axon diameter for control axons (3–5 dpf, n = 18 animals) and neuron-specific ipo13b mutant axons (5 dpf, n = 10 animals). Control points are fitted with a linear regression line (R = 0.4959). Neuron-specific ipo13b mutants are not significantly different from controls by simple linear regression test (slope: p = 0.5931, intercepts: p = 0.6215). G Resting membrane potential in control and neuron-specific ipo13b mutant Mauthner neurons at 5 dpf (two-tailed unpaired t-test, n = 12 control and 10 neuron-specific ipo13b mutant Mauthner cells from individual animals, p = 0.2581). H The success rate of action potential firing by the Mauthner neuron in response to 10 stimulations at 300 Hz, 500 Hz and 1000 Hz (two-way RM ANOVA with Tukey’s multiple comparisons tests, n = 5 control 3 dpf, 5 control 4 dpf, 8 control 5 dpf, 10 neuron-specific mutant 5 dpf axons from individual animals with repeated measures at each frequency, no significant differences between the controls of different ages, or between any of the controls and neuron-specific ipo13b mutants, p = 0.9667). I Depiction of three consecutive action potentials, which have slight variations in their latency of arrival. This variation is referred to as jitter. J The precision of action potential arrival (jitter) along Mauthner axon (One-way ANOVA with Tukey’s multiple comparisons test, n = 5 control 3 dpf, 5 control 4 dpf, 8 control 5dpf, 10 neuron-specific mutant 5 dpf axons from individual animals, p = 0.2699). Scale bars = 10 µm. All data are presented as mean values ± SD. ****p < 0.0001. Source data are provided as a Source Data file.
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