Figure 2.

Imaging AP initiation with VSDs. A, Left, High-magnification image of the axon of a layer 5 pyramidal neuron filled with VSD. Right, Average fluorescence change (ΔF/F) of 130 individually aligned APs recorded at the indicated axonal locations. APs were evoked by somatic current injection. B, Onset latency of axonal VSD signals relative to the somatic response (0 μm) plotted against distance from the axon hillock for the data in A. The site of AP initiation is indicated by a black arrow. Gray arrows indicate location of axonal branch points. C, Average onset latency of axonal VSD signals relative to the somatic response plotted against distance from the axon hillock for orthodromic (filled circles; n = 34) and antidromic APs (open circles; n = 6). Data for orthodromic APs were pooled from experiments in which APs were evoked by brief somatic current injection (n = 27) and synaptic stimulation (n = 7).