Upward lightning happens predominantly from tall objects. The channel-base currents associated with upward negative lightning initiated from towers exhibit pulses that can be classified, based on their waveform features and on the concurrence with other processes in the flash, into four categories. Three of them (M-component-type pulses, M-component pulses, and Mixed-mode pulses) are always superimposed either on the continuous or on the continuing current, and the remaining type (Return Stroke pulses) is initiated during a zero-current interval. Simultaneous measurements of the channel-base current and electric fields 15 km from the tower were analyzed. Good agreement was found between measured and calculated electric field waveforms when simulations were carried out using the MTLE model for the return stroke and for the mixed mode pulses, and when the model of Rakov at al. was used for M-components and M- component-type pulses. The analysis validates the similarity between M-component-type pulses superimposed on the Initial Continuous Current during the initial stage of upward negative lightning and M-components pulses superimposed on the continuing current that follows some return strokes. In addition, the results indicate that mixed-mode pulses are similar to return strokes. In the two studied flashes, the initial stage was associated with a larger charge transferred to ground than the ensuing return strokes and their associated continuing currents and M components, the factor being almost four-to-one.