Triphasic extracellular waveform. A, The extracellular triphasic waveform (black) is commonly identified by its initial positive peak (prepeak, arising from a positive source: return/capacitive currents or a current source from the propagating action potential; Fig. 5), a large trough (depolarization), followed by a second peak (postpeak, posthyperpolarization). B, Triphasic waveforms are typically recorded away from the AIS, near the dendrites and axon (pink). C, Schematic of current sources and sinks on the triphasic waveform. As the action potential traverses from the soma and through the axon (or dendrite), the membrane under the electrode serves as a source (positive deflection; left inset), followed by a sink (negative deflection; middle inset), and finally a source again (positive deflection; right inset). As a result, the electrode (pink) records a triphasic waveform. D, Variations in triphasic waveforms. Common shapes include lower-amplitude prepeak or postpeak (left). Narrow and broad triphasic waveforms have been classified (middle). Triphasic waveforms characterized by very low amplitude and extended durations have been reported and are referred to as compound spiking (right).