Fig. 2. A spontaneous oscillating current is activated during osteoclast spreading. Osteoclast background current was continuously monitored by voltage ramps applied every 2 seconds. The spontaneous outward oscillating current (measured at 0 mV) exhibited two patterns, depending on the cells, a periodic rhythmic activation (a) or variable inter-peaks intervals (b). The periodicity of the current signal was tested by the auto-correlation function. (c) Periodic function resulting from a simple periodic signal. (d) Complex pattern resulting from a non-periodic complex signal. (e) 19 out of 20 spreading cells showed a positive oscillating current during spreading, whereas 16 out of 20 non-spreading cells did not exhibit any current.