Fig. 3: Uncoded Diff-CUP imaging of propagating internodal current flows in myelinated axons.

a Setup for imaging propagating internodal current flows in the sample arm of Diff-CUP’s Mach–Zehnder interferometer. \(E(t)\) is the transient field stimulation applied to the parallel bipolar microelectrodes. b Schematic of the equivalent double cable internodal circuit. IN internode, PN paranode, NOR node of Ranvier, MS myelin sheath; \({{{{{{\rm{R}}}}}}}_{{{{{{\rm{a}}}}}}}\), axonal axial resistivity; \({{{{{{\rm{R}}}}}}}_{{{{{{\rm{pa}}}}}}}\), periaxonal resistivity; \({{{{{{\rm{R}}}}}}}_{{{{{{\rm{pn}}}}}}}\), paranodal resistivity; \({{{{{{\rm{R}}}}}}}_{{{{{{\rm{m}}}}}}}\) and \({{{{{{\rm{C}}}}}}}_{{{{{{\rm{m}}}}}}}\), specific membrane resistance and capacitance; \({{{{{{\rm{R}}}}}}}_{{{{{{\rm{my}}}}}}}\) and \({{{{{{\rm{C}}}}}}}_{{{{{{\rm{my}}}}}}}\), specific myelin sheath resistance and capacitance. c, Spatiotemporal interferograms of a propagating internodal current flow in a myelinated axon captured by uncoded Diff-CUP (400 interferograms) under different conditions. The horizontal and vertical axes of the interferogram denote \(x\) and \(z\), respectively, where \(z={nd}+y\), \(x\) and \(y\) are the two spatial axes, \(n\) stands for the \(n\)-th axon image in the time series, and \(d\) denotes the FOV in the \(y\) dimension. From left to right: interference, unprocessed interferogram; LN, interferogram captured with a synchronous EMP induced in the LN crystal, where the strong Pockels effect caused by the EMP is used as a time reference for locating the starting time point of the current flow; control, interferogram captured without field stimulation; stimulus, interferogram captured with field stimulation; simulation, interferogram overlaid with a “virtual” stimulus generated by the NEURON simulation environment. Black box denotes the FOV of the axon. Gray electrode symbols denote when the stimulation is applied. Horizontal scale bars, 25 µm. Vertical scale bars, 3 µs. d–g Reconstructions based on the LN (d), control (e), stimulus (f), and simulation (g) interferograms in (c). Each reconstructed correlation curve corresponds to a segment of the FOV, labeled with numbers 1–8. Scale bars show the normalized correlation values. The regions corresponding to these segments are marked in the axon’s CCD image on the right of (f). Black dashed lines indicate the peak time of the synchronous EMP (d) or the signal region of the internodal current flow (e–g). T is the propagation time of the internodal current flow within the FOV of the axon.