The shape of action potentials invading presynaptic terminals, which can vary significantly from spike waveforms recorded at the soma, may critically influence the probability of synaptic neurotransmitter release. Revealing conductances that determine spike shape in presynaptic boutons could bear insights into how dynamic axonal electrochemical milieux modulate synaptic strength, but the small diameter of CNS axon collaterals has prevented direct characterization with traditional electrode recordings. Voltage imaging has long shown promise for overcoming the low spatial resolution of patch electrode recording; however, low sensitivity previously precluded investigation of phenomena too rapid or too complex to be elucidated with extensive averaging. A recent modification to the fluorescence excitation approach (Holthoff et al., 2010) now enables high signal-to-noise ratio (SNR), single-trial recordings from axons and axon collaterals (Foust et al., 2010, 2011; Popovic et al. 2011). Utilizing the improved sensitivity of single cell voltage sensitive dye (VSD) imaging, we characterized action potentials propagating through small diameter axon collaterals of pyramidal neurons and interneurons in mouse cortical brain slices with high spatial (1-4 µm) and temporal (0.05-0.1 ms) resolution in single trials. Averaging small numbers of trials (4-8 trials) generated SNRs appropriate for comparing the kinetics of action potentials in different types of cortical neurons. Importantly, the action potentials in the axons of parvalbumin-positive fast spiking interneurons repolarized more quickly than in SOM-positive Martinotti interneurons or excitatory pyramidal neurons. Furthermore, we found that prolonged subthreshold somatic depolarization of layer 5 pyramidal neurons elicited action potential broadening in collaterals and boutons in a distance dependent fashion (Foust et al. 2011). Lastly, by combining single-cell VSD imaging with pharmacological manipulations, we discovered that Kv1 subunit containing ion channels are important for repolarizing action potentials in cortical pyramidal axon collaterals and en passant presynaptic terminals. Moreover, the spike broadening with depolarization was blocked with Kv1 antagonists, indicating that D-current could play an important role in local graded synaptic transmission (Foust et al., 2011). Together these findings represent a small sample of new insights to be gained into axonal action potential characteristics and plasticity with high sensitivity single cell voltage imaging.