Action potential propagation in dendrites and axons of SOM-expressing (RS) and FS interneurons. A, Confocal z-projection of a VSD-filled SOM interneuron. Colored boxes and traces represent dendritic regions used for assessing action potential propagation. Below z-projection, Example of SOM spiking properties in response to a square pulse injected into the soma. Right, Colored traces represent average fluorescence changes (4 trials) recorded optically in interneuron dendrites and black trace shows an average of four trials recorded electrically at the soma. Gray boxes and traces represent regions outside of the dendritic arbor and were used to determine light scatter from the processes. B, Confocal z-projection of a VSD-filled SOM interneuron. Colored boxes and traces represent axonal regions used for assessing action potential propagation (average of 5 trials). Bottom, Overlay of voltage and fluorescence traces from above cell to demonstrate spike latency between the axon and soma. C, Scatter plot for latency-distance along the axon of action potential half-height time relative to the somatic action potential half-height (n = 8 cells). Half-height occurs first in the proximal axon. D–F, Same as in A–C, but for VSD-filled FS interneuron. D, Average traces are taken from 5 trials. E, Average traces taken from 5 trials. F, Latency-distance plot includes 10 cells.